What’s the Flightgear news?

Changes of various kinds affect FG – it seems this year more than in others. Please read here an overview.

Whatever happened to the 3.6 release?

The simple truth is – it didn’t work out. In the end, the release team had personal constraints, got as far as producing a release candidate but then the efforts stalled. Flightgear is developed by human beings, and sometimes things don’t go the way we’d like them to.

Ideally this should have been communicated earlier – we’re sorry for this, the final decision not to have a 3.6 release was made not so long ago.

At this point, let me take the opportunity to point out the links to the automated builds where you can find the (as we now know rather stable) 3.6 release candidate and an automated nightly build of the FG development version.

The matching aircraft collection can be obtained from the repository following these instructions.

What’s the future?

At this point, we’re not sure whether there will be a regular 3.8 release. Rather, the idea which will be tried is a series of more automated stable releases – about four per year. We hope that this will stretch bug reporting by the general audience from the current one-week period between release candidate and stable (which makes it very hard to act in time) over a wider period, giving us the opportunity to respond better. At the same time, it will decrease the waiting period for the next stable. Time will tell whether this works better.

In addition, there are multiple changes behind the scenes having to do with the server infrastructure for the FG scenery. Ideally these would not affect the end user, but they take time and effort nevertheless.

Development on the other side did not stop – we keep making FG more interesting.

More impressive weather

The weather system has received an update to simulate lightning strikes during thunderstorms. Each lightning strike will not only trigger a visible bolt, but also (in the Atmospheric Light Scattering (ALS) framework) at night illuminate the clouds in the vicinity for a split second. In addition, the position is registered in FG, allowing aircraft modelers to include ambient thunder sounds with the correct delay – this has for instance been done for the C-172p. Encountering a T-storm at night will be memorable now!

Improvements to aircraft interior rendering

The ALS framework offers a host of new options for aircraft developers, for instance a quick option to render panel backlighting using the so-called implicit lightmap technique, irradiance mappings giving a more faithful impression from where indirect light can fall into the cabin and the option to show reflections of the lit cockpit panels in the windows at night.

A fire-breathing dragon (and other impressive aircraft)

Among the most interesting aircraft to be added to the repository is the dragon. Demonstrating the versatility of FG, this reptile doesn’t fly on magic (like the Santa and his reindeer do, if you know the model…), instead it uses the aerodynamical data from attempts to reconstruct the flight dynamics of a Pterosaur. Let it slowly climb with mighty wingbeats, soar thermals to rise to high altitudes or swoop down in a steep dive – the dragon sure is fun to fly and at the same time instructive.

Substantial updates also have been done for other aircraft which have been under heavy development: Sounds and pre-flight inspection simulation of the C-172p have been improved. The F-14A variant has been added, making use of all recent rendering improvements and including a simulation of compressor stall. Lots of work have also gone into making the F-15 yet better – it now offers C and D variant, operational weapons and radar, detailed sound and simulation of hydraulical, electrical and ECS pressurization system. The Space Shuttle is now well on the way towards a photorealistic 3d cockpit. Substantial work has been done on the avionics, implementing automatic tracking and pointing routines and a realistic procedure to command OMS orbital insertion and de-orbit burns, as well as one of the most sophisticated failure meanagement simulations in FG.

Improvements to scenery rendering

The ALS rendering framework has added effects for improved rendering of runways. Paved runways are now shown with skid-marks whereas unpaved runways can be rendered with a multitude of materials, smoothly blending the runway into the surrounding airport green.

New terrain textures

The regional scenery texture project is progressing substantially – we have new runway textures, dedicated industrial and port textures as well as detailed customized definitions for South America, Africa, the Middle East and South Asia).

Behind the scenes work

Less visually, there is ongoing development on various fronts. The out of window GUI Phi continues to be expanded, providing support e.g. for an instructor station. Using Qt5, a common launcher for all OS is being developed which is expected to ultimately grow into a new in-window GUI option. Finally, the HLA effort aims at utilizing multiple CPUs better by running various subsystems in dedicated threads outside the FG main loop. Currently, only a very limited AI scenario can be run in this mode, but ultimately it is expected to provide improved framerate where poor CPU utilization is the bottleneck.

We hope to be back soon with details on the new release process.

A preview of features for Flightgear 3.6

Flightgear is constantly under development and as the feature freeze for the next 3.6 release approaches, it is becoming increasingly clear what the next version will have to offer to users:

(to avoid misunderstandings – this is a selection of features currently under development, not a release note, i.e. there is no guarantee that all items will appear in 3.6, nor is are the features of 3.6 limited to what is listed here)

A complete makeover for the default aircraft

The C-172p has always been Flightgear’s default plane, as it is easy to fly and great to learn the basics of aviation. Thanks to a joint effort of several gifted developers, it just got a lot better. The revised version offers improved flight dynamics with the ability to bring the plane into spins. It makes good use of the latest state of the art of Flightgear’s rendering frameworks, including hires textures, the new internal shadow effect for ALS, environment dependent fogging of the windshield and of course support for the Rembrandt rendering engine.

And it comes with a damage model, creating a very visual impression of what happens if you land too hard:

A new user interface / instructor station

Using Flightgear’s inbuilt web-server, Phi is a new way to access Flightgear from an external device. You can run a web browser on your pad, connect to a running FG instance on your PC and access everything you need from there. This makes for a great training setup in which the instructor can select challenging conditions for the student, monitor the flight or cause failures which need to be responded to. Phi supports pre-flight checklists, environment settings, a moving map widget and many features more.

‘Houston, the Atlantis has reached orbit.’

Launching vertically like a rocket, capable of limited maneuvering in orbit and entering the atmosphere again to land like a plane, the Space Shuttle is a truly unique flying experience. Based on a large body of public domain wind tunnel data by NASA, Flightgear now offers the possibility to take the Shuttle into low orbit and back in a highly realistic simulation.

Experience the strength of the aerodynamical forces during launch as thrust vectoring keeps the Shuttle on its ascent path, learn about the inherent yaw instability of the Shuttle during the hypersonic entry phase and the crucial role of the RCS jets and the body flap, explore how elevon deflection changes the airstream at the aft fuselage and alters roll and yaw stability, or simply start in orbit, enjoy the view or do a spacewalk.

The simulation includes all mission phases with many different digital autopilot settings to control thrust vectoring, RCS jets or airfoils, checks on aerodynamical and structural limits as well as damage and failure simulation in case of limit violations – basically the Shuttle can be flown by the Crew Operations Manual. A 3d cockit is already in place, and work is underway to provide the original avionics.

Rain on the windshield

The Atmospheric Light Scattering rendering framework is rolling out a new suite of effects to render the cockpit interior in more detail. These include a glass shader which renders interior reflection, damage and dirt, glare, raindrop splashes, frost and temperature-dependent fogging and an interior effect capable of drawing shadows, light filtering through colored glass or caustics as well as panel backlight illumination. Enjoy the enhanced immersion into the simulation these features provide!

Regional textures for Latin America

Thanks to local users, the whole of Latin America is receiving more realistic local textures. Look forward to the typical red roofs or urban terrain, to seeing dramatic changes in the water color where the dark Rio Negro meets the muddy Rio Solimoes close to Manaus and to many other nice touches in the area. If you haven’t done it yet, schedule a flight in South America after the next release, there’s lots to explore!

New and improved aircraft

The Citation II provides a new cockpit, making use of plenty of the new effects. Enjoy Flightgear in this nice business jet!

The new F-15 comes with a detailed JSBSim flight dynamics model with lots of wind tunnel data worked in as well as a detailed 3d cockpit with tons of functionality.

And many improvements more

* work on an integrated launcher, specifically making life easier on newer Mac OS distributions
* the aircraft center, a tool to download and manage aircraft in-sim
* expanded functionality of the Canvas 2d rendering framework
* …

Stay tuned as we fly towards our next release!

Simulating the ever-changing scenery

The secrets of the environment settings

If you look at aerial imagery of a region every day for a year, it never changes. Yet if you would fly over the same region in reality every day, it would almost never look the same twice. In reality, nature is a dynamically changing environment, and what you see from a cockpit reflects this.

Some of these changes have to do with weather – on a cloudy day, the light is different from bright sun, the shadows are muted, the amount of haze may change so that faraway terrain looks fainter… and these are readily captured by the weather simulation.

Yet there are more subtle effects. For instance, snow may linger on the ground even on a sunny day with temperatures above freezing if the original layer was thick enough. Snow may fall, but not remain on the ground if the ground is warm enough. In essence, whether you see snow or not depends not so much on how the weather is now, but how it has been the last days, weeks or even months.

Such changes to the scenery in FG are taken care of by the environment settings which control how the terrain is shown. You can find the menu as an entry under Environment.

Currently, the full range of environment effects is only implemented for the Atmospheric Light Scattering (ALS) framework starting from medium quality settings, however the snow effect is available for all rendering frameworks.

Let’s explore some of the things this can do:

Seasonal changes

This is how the default terrain is shown without any environment effects – a summer day in Grenoble:

Moving the season slider somewhat to the right brings autumn coloring into the scene – deciduous tree patches change colors to orange-red, fields and grass appear yellowish:

Changing to a yet later season causes deciduous trees to shed leaves and changes most of the vegetation to a dull brown:

Modifying the snow line and thickness allows to add a sprinkle of snow to the valleys, simulating the first snowfall of late fall:

Finally, adding more snow changes the whole scene into deep winter:

In coastal regions, the appearance of water can also be changed. Here is the coast of Norway near Bergen in summer:

Using the snow and ice sliders allows to simulate winter with lots of drift ice in the sea:

Using a combination of the season and snow settings, it is hence possible to simulate a lot of the seasonal changes during the year. But that’s not all.

Dust and greenery

Have you noticed how colors fade during a long spell of dry weather, to be restored only when rain washes the dust away? Or how a desert might look green for a few weeks after rainfall, to change to its usual dusty appearance later? The environment system also provides those options – let us take a look at the Sierra Nevada. This is how the chain appears from China Lake (with a good measure of snow added to the peaks):

Using the dust slider makes all the colors fade and lets the scene appear dry:

Using the vegetation slider instead gives a fresh green touch to the desert as if after a rainfall:

Changes may be subtle and affect more than just color. Consider this close-up of a dry runway:

The environment settings allow to make it wet (this will happen automatically when the weather predicts rain, but terrain can be wet without current rainfall). This creates puddles and alters the whole reflectivity of the surface – look at how the light changes:

Finally, adding snow covers the runway partially in snowdrifts:

Why can’t this happen automatically?

The environment subsystem just renders as it is told, it is hence easy to misuse it – think snowfall and ice cover on Caribbean islands for instance. Sometimes, the question gets asked why this is implemented that way, and why parameters aren’t just set automatically.

The answer to that is – based on what should they be set? Flightgear does not include a global climate simulation as would be needed to determine how likely it was that there was e.g. snowfall during the last days or weeks, or that there was a dry summer and hence everything should look dusty.

The idea is that the user can adjust these settings, either based on how the scene currently looks at a location, or based on what the user wants to experience (it’s a simulation after all – there’s nothing wrong with simulating a tropical day in Hawaii on a bleak winter day).

If used with some care, the environment settings offer a chance to experience the same scenery in a hundred different ways, each time subtly different.

If misused, the settings deliver weird to crazy results of course.

For the sake of completeness, for low-performance systems which are unable to run shader effects, using the commandline option –season=winter offers at least the choice between the default summer textures and a snow-covered set of textures, although no control over snowline and thickness.

FlightGear v3.4 Released

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The FlightGear development team is delighted to announce the v3.4 release of FlightGear, the free, open-source flight simulator. This new version contains many exciting new features, enhancements and bugfixes. Highlights in this release include frame-rate improvements on some systems, reduced memory usage and enhancements to the built-in web server.

Founded in 1997, FlightGear is developed by a worldwide group of volunteers, brought together by a shared ambition to create the most realistic flight simulator possible that is free to use, modify and distribute. FlightGear is used all over the world by desktop flight simulator enthusiasts, for research in universities and for interactive exhibits in museums.

FlightGear features more than 400 aircraft, a worldwide scenery database, a multi-player environment, detailed sky modelling, a flexible and open aircraft modelling system, varied networking options, multiple display support, a powerful scripting language and an open architecture. Best of all, being open-source, the simulator is owned by the community and everyone is encouraged to contribute.

Download FlightGear v3.4 for free from FlightGear.org

FlightGear – Fly Free!

Major enhancements in this release

Performance

  • Improved frame-rates on some systems from more efficient use of Uniforms
  • Reduced memory occupancy for scenery tiles
  • AI models are now rendered based on display size rather than range
  • AI/MP models may now define objects as being part of the interior, which will not be rendered at large distances

Usability

  • Built-in web server now includes a moving map, a screenshot grabber, and supports SVG-based panels
  • In-application launcher for Mac, based on Qt5

Graphics

  • Improved rendering of runway and other lights under ALS
  • Landing and spotlight support for ALS

Scenery

  • Improved materials XML format making customer material definition easier
  • Procedural rock material definition

JSBSim

  • Synchronization with latest JSBSim

Highlighted new and improved aircraft

  • Extra EA-500
  • North American P-51D Mustang
  • Cessna Citation II
  • F-14b

Other

  • Firewall exceptions are automatically added during setup on Windows systems
  • Aircraft moved to a SVN repository.

Bug fixes

  • See our bugtracker for an extensive, yet incomplete, list of the bugs fixed in this release.

Web Site Updates

December 30, 2014:

  • Upgraded to wordpress 4.1
  • Testing a new theme that is a bit less “bloggy”
  • Added a big fat “download now” button on the front page.
  • Fixed layout for small screens (like smart phones in portrait mode.)

January 24, 2014:

  • v3.0.0 release candidates are available for download and testing.  The official v3.0.0 release is scheduled for 17 Feb.
  • Upgraded to WordPress 3.8.1

December 20, 2013:

Upgraded to WordPress 3.8 and the Twenty Fourteen theme.

November 25, 2013:

FlightGear v2.12.1 (bug fix release) is now available for download.

October 3, 2013:

Upgraded to WordPress 3.6.1 and new TwentyThirteen theme.  The FlightGear web site server hardware has been relocated to a newer larger building.  And v2.12 has just been released!

February 13, 2013: Updated Scenery Download Path

The FlightGear scenery downloads has been updated to v2.10 in preparation for the 17 Feb v2.10 release.  The scenery content does not follow the same release schedule and has updates and improvements every few days.  Thus this is more of a name change formality, and the “v2.10” scenery will work fine with v2.8 and probably most v2.x versions of FlightGear.

January 12, 2013: New Wiki Server

The FlightGear Wiki (http://wiki.flightgear.org) has been moved from a shared hosting server to a new dedicated virtual private host.  The FlightGear wiki is *very* popular and generates a lot of traffic and server load so hopefully this will improve the performance and reliability of our wiki and at the same time help all the other services on the old shared hosting server.

The new wiki host has been donated to the FlightGear project by DigitalOcean. If you are searching for a good hosting service among an ocean of possible options, they are good guys.

December 18, 2012: WordPress 3.5 & New Theme

The FlightGear web site has been upgraded to the newest version of wordpress (3.5) and I am experimenting with a new theme.  We can always return to the old them if we decide we like that better, or we can more forward too.  The new theme has some better support for mobile devices.

December 29 update: when switching to the new 2012 theme, we ended up with comments enabled on all content pages.  This was unintentional.  The page comments were mostly support requests or the odd snarky comment.  I have removed the comments area from regular content pages, but comments are still allowed (and encouraged) for “post” pages.  However, comments will be filtered carefully for topic and usefulness.  Do they expand or clarify the conversation of the post topic?  Support questions will still be referred to the FlightGear forum.  Random positive/negative statements (like “I love flightgear” or “I hate flightgear” will generally be ignored.)  English is preferred for post comments, but exceptions have been made and probably will be made in the future.

October 24, 2012: Scenery Download Page updated

The World Scenery Download page is updated to SVN version 20579.  It may take a day or so for the updated files to flush through the mirror system.

September 7, 2012: WordPress 3.4.2

Upgraded to wordpress-3.4.2.

August 20, 2012: Updated Gallery

Featuring the winning entries of the 15th anniversary screenshot contest, we have added a new screenshot gallery to go along with the v2.8.0 release!

August 17, 2012: Version 2.8.0 Released

Yeah!  Look on the front page (or the recent posts list in the side bar) to read the official release announcement.  Better graphics, new aircraft, new visual effects, tons of new things to explore!

July 30, 2012: v2.8.0 Release Candidate “RC4” Available.

If you are interested in trying the next release of FlightGear ahead of time (and helping us sniff out any remaining bugs or packaging issues) then please take a look for download links in the release candidate section towards the bottom main download page.  Also notice that updated v2.8.0 aircraft are also available for download along with the pre-release.

June 28, 2012: WordPress 3.4.1

The FlightGear web site software has been updated to WordPress v3.4.1.

February 28, 2012: Version 2.6.0 Updates

Both Mac OS X and Windows have had small tweaks to follow up the v2.6 release.  For Mac OS X there is “r319” version of the 2.6.0 dmg which fixes a couple problems some Mac users were seeing.  For Windows there is a “Setup FlightGear 2.6.0.1.exe” which fixes one small 32bit vs. 64bit dll packaging problem some 64bit users were seeing.

February 17, 2012: Version 2.6.0 Released

There has been a large number of changes and updates to the download and information pages as part of the v2.6.0 roll-out.

Jan 29, 2012: New v2.6.0 Release Candidate Available

A complete test release for the upcoming FlightGear 2.6.0 version is available to try.  Follow this link to the FlightGear v2.6.0 Release Candidate page.

Jan 6, 2012: New Developer Snapshot Available

A new developer snapshot (v20120105) is available for download and testing.  This is a way to keep up with all the coolest new features and experimentation without needing to compile the code yourself from scratch.  You can find the download link on the main download page.

Dec 28, 2011: Contributors Section added

A new section has been added to the FlightGear web site: Contributors.  We plan to periodical add profiles of different contributors to this section.  If you’d like to be included here, or have corrections or updates to existing entries, please contact the web master!

Sep 27, 2011: Scenery Download Page added

A World Scenery Download page has finally been added to the new web site.  You can find the page in the main site menu.  The graphical download page has also been updated.  All the links should now point to the v2.4.0 version of the scenery (this corresponds to svn version 16700 from the terrascenery archives.)  Update: a small link error has been fixed so the download map should be working again.  Thanks to those who reported it!

Sep 27, 2011: New wiki and liveries server

The server hosting wiki.flightgear.org and liveries.flightgear.org has been upgraded and the content has been migrated over.  There shouldn’t be any problems, but of course if you spot something odd, please let us know.

The magic of light and haze

The ‘Atmospheric Light Scattering’ (ALS) rendering framework

Have you ever admired the beautiful colors of a sunset? Have you maybe wondered why sometimes sunsets show a fantastic palette of glowing red and golden colors in the sky and sometimes a rather muted blue-grey? Have you observed distant hills fade into blue haze while the glittering reflection of the sun on water shifts color to a yellow-orange and asked yourself where the difference comes from? Have you wondered why there’s sometimes a halo visible around the moon?

All these phenomena and more are related to light scattering in the atmosphere. Actually, most of what we see looking out of a cockpit from 36.000 ft is not scenery but light scattered somewhere on haze, clouds or air molecules. To create a realistic impression of a scene during flight, we can’t think of haze being something simple that obscures the scene, instead we have to invest as much attention to rendering haze properly as to the more prominent scene elements. In Flightgear, that’s what the ALS framework is doing.

A little bit of theory

To first approximation, the normal lighting situation of a scene during daytime is that the sun is high in the sky and illuminates an object, from which reflected sunlight falls into the eye. There are thus two light rays – the illumination ray (I-ray) goes from the sun to the object and the observation ray (O-ray) from object to the eye.

In vacuum, that’s all there is to it, and pictures from the surface of the Moon illustrate this – objects remain visible no matter how far away, and any surface which is not in direct light is pitch black.

In an atmosphere, light scattering can affect both the I-ray and the O-ray, and there can be in-scattering and out-scattering. In-scattering corresponds to light from somewhere else in the scene being scattered onto the object (or into the eye), out-scattering corresponds to light from the sun being scattered away from the object or light from the object being scattered away from the eye. I-ray in-scattering causes ambient (non-directional) light – shadows are no longer pitch black but receive still some kind of illumination. Under a thin overcast haze layer, there is for instance strong I-ray in-scattering – while there is lots of light available, it comes from almost everywhere in the sky and no shadows are cast onto the ground. O-ray in- and out-scattering both cause objects being shrouded by haze, but in-scattering causes a bright haze, out-scattering a dark haze effect.

To complicate matters, there are three basic physical scattering mechanisms which can take place: Rayleigh, Mie and diffuse scattering.

* Rayleigh scattering occurs on very small particles – the air molecules themselves or fine dust (‘dry haze‘). It has no preferred direction, but is much stronger for blue light than for red light.

* Mie scattering occurs on larger particles – usually water droplets (‘wet haze‘). It has no color dependence, but is much stronger at small angles than at large angles, i.e. Mie-scattered light almost keeps its original direction.

* diffuse scattering isn’t really a distinct elementary process but the effect of multiple scattering processes over which direction and color specific dependence is blurred, hence diffuse scattering has no color or directional dependence.

Any real scene is hence a mixture of Rayleigh, Mie and diffuse in- and out-scattering on O-ray and I-ray (which makes for a total of 12 scattering channels, out of which 11 are modeled in at least some approximation by ALS – only Mie in-scattering on the I-ray is not considered since it is not very important in practice).

Wet and dry haze

Since most flight-relevant fog is wet haze, in FG the amount of wet haze is directly linked to the reported visibility. In many weather situations, fog is densest in the lowest convective air layer and the air is much cleaner above. ALS hence allows to render a lower layer of volumetric fog in addition to much less dense haze in the upper atmosphere. Seen from above (as in the scene showing morning fog at the foothills of Nanga Parbat), wet haze appears a bright white during the day, but when entering the fog, its color gradually changes to a dark blue-grey as diffuse out-scattering blocks the light.

The amount of dry haze (or Rayleigh scattering) relative to the wet haze is controlled by the air pollution slider in the weather configuration. Since Rayleigh haze is stronger for blue light, at large visibility O-ray in-scattering dominates (driven by the strong light coming from above) and far objects appear shifted towards sky-blue in color (such as the Sierra Nevada chain seen from China Lake below):

However, if the visibility is poor and/or the incident light from above is blocked, O-ray out-scattering is dominant, and all objects appear shifted to a dirty yellow – in other words, high air pollution makes objects appear in smog (here, downtown San Francisco):

The sky in low light

During the day, the I-ray is typically much shorter than the O-ray because the sunlight crosses the atmosphere vertically. The density of the atmosphere is variable in altitude, but effectively the whole vertical extent correspondsto a length of perhaps 10 km, whereas on a clear day objects 200 km distant can easily be seen. This is why Rayleigh out-scattering for the incoming light is not dominant while the sun is high.

In low light however, the I-ray passes a long distance through the atmosphere, the blue light is scattered out, and hence the direct light of the scene illumination is shifted to red (the indirect light however is driven by Rayleigh in-scattering and hence is shifted to blue). This can be seen here where the sun is below the horizon and illuminates the clear air close to the horizon whereas the lower haze layer is only visible in blue indirect light:

Without a lower haze layer, the whole scene appears in blue indirect light as seen here in the predawn Himalaya

As soon as the sun comes above the horizon and touches the highest peaks, strikingly beautiful contrasts appear between the blue indirect and the red direct illumination, leading to the phenomenon known as Alpenglow:

Looking away from the sun in low light, the clear atmosphere takes a deep violet color:

At very early predawn, just the far fringes of the upper atmosphere are illuminated. In clear air, the colors of dawn are muted:

In contrast, here a strong dry haze component leads to a sizable shift of the light to a red-golden color which lets the low wet haze layer glow brightly in the early morning light. While the light illuminating the wet haze is driven by Rayleigh scattering, the wet haze itself is a Mie scatterer – it glows most close to the sun, and the colors get more muted away – this is most evident from the thin clouds in the scene:

The combination of wet and dry haze can lead to nice and subtle color variations in low light:

The atmosphere seen from above

The following screenshots have been rendered with the EarthView orbital rendering option of FG in combination with ALS.

The characteristic electric blue glow of the atmosphere which is so prominently visible from low earth orbit is predominantly driven by Rayleigh scattering in the upper atmosphere.

Where the bulk of earth blocks the incoming light, Rayleigh scattering can no longer take place and the glow of the atmosphere gradually fades out:

The terrain itself is illuminated by light which has suffered Rayleigh out-scattering. In the dawn zone, this gives it again a color shift, here just slightly towards the yellow in this late afternoon impression of clouds hanging above the coast of Florida:

It is quite possible to observe the shadow earth casts into the atmosphere from lower altitude – here is an impression of it from 36.000 ft above an overcast cloud layer:

Clouds

Although they are rendered with rather different techniques, clouds physically are wet haze – thin translucent clouds are Mie scatterers, and thick clouds are diffuse scatterers. Thus, thin clouds light up very brightly in a halo when the sunlight is seen through them due to O-ray Mie in-scattering, whereas thick clouds appear dark and hide the sun due to O-ray diffuse out-scattering.

In predawn light, low clouds appear dark since they are yet in the shaded part of the atmosphere, but high Cirrus cloulds can already receive some sunlight:

As the sun comes up, this can lead to a dramatic play of light and shadow, with bright high-altitude clouds seen through a dark lower layer:

Again, the light that reaches the clouds at low sun is subject to I-ray Rayleigh scattering and its color depends on the amount of dry haze. In clean air, the colors of a cloud layer appear more muted

whereas for a high air pollution value the colors are much more strongly shifted towards the red-golden.

However, faraway cloud banks at the horizon can also reduce or alter the incident morning light by I-ray scattering. For thin, scattered clouds, this reduction is small and mornings appear bright

but if the cloud cover gets stronger, the light is reduced

and color shifted

to the point that a sunrise appears no longer red-orange-golden but blue-violet underneath a thick layer:

Diffuse haze in the atmosphere acts on the incoming light just the same way as a well-formed cloud layer – the light illuminating the clouds is reduced, and as the direct light is filtered out, the blue indirect I-ray Rayleigh in-scattering becomes more important, shifting colors to violet.

Compare the subtle play of dawn light shining through a cloud for a clear day

with a very hazy day with poor visibility aloft to appreciate the flattening of the color distribution:

Artificial light

At night, artificial light sources contribute a lot to the illumination – think of the orange glow of haze above a well-lit airport or a city. In principle, artificial light follows the same principles as sunlight, except that the intensity is usually far less, and so the paths through the atmosphere are smaller and effects are only visible in fairly dense fog. Then, often Mie-scattering can be observed, creating halos around lights seen through the fog.

Most of these effects are currently not included in ALS, however the Mie-scattering halos for runway lighting and the illumination of dense fog by landing lights are features already implemented:

Final thoughts

All the different scattering phenomena described above only scratch at the surface of what nature really does. In a real sunrise, clouds may cast shadows onto each other. There’s multiple scattering processes – a brightly illuminated haze layer high above may scatter lots of light down onto lower layers. There’s genuinely colored hazes like dust in a sandstrom which change the colors of dawnlight in yet different ways. There are effects of the human perception which make the eye see very faint or very bright light in colors different from what they actually are (which is why moonlight, despite being actually white light, appears as blue). While ALS tries to capture some of these processes, nature still does infinitely more, and sometimes one wonders how nature manages to get it all done in real time.

But even thinking about some of the phenomena causing it, you will never look at the play of haze and light the same way as before – be it in Flightgear or in reality.

All the screenshots above are rendered with the current development version of Flightgear (FG 3.3) out of the box. On a modern gaming laptop, in flight they typically render with 30+ fps (mainly dependent on visibility and LOD settings and the usage of hires scenery).

The ALS framework itself takes some 10 atmosphere-related input parameters to generate the visuals of the sky and of hazes, and this leads to an almost infinite variety. Unfortunately the majority of parameter combinations can not occur on Earth (ALS as such is quite capable of rendering a Martian sky), hence the raw input parameters are largely not under user-control. What limits the visuals ALS generates out of the box in practice is the actual range of parameters passed to the renderer by the weather simulation. Here, Advanced Weather using the offline weather engine is somewhat more faithful in generating reasonable light propagation models in the lower atmosphere than Advanced Weather in METAR mode, which is in turn better than Basic Weather, but even Advanced Weather currently exhausts just a fraction of the possibilities ALS really offers.

Modeling a compelling haze distribution and the resulting light attenuation in real time is a genuine challenge, since it is impossible to actually do the scattering calculations (which involve nested integrals) in anything resembling real time, so in every case, fast yet faithful approximations have to be found.

To experiment some with sunrises, try various weather scenarios and play with the lower haze settings and the air pollution on the Advanced Weather options panel.

A preview of features for Flightgear 3.4

Flightgear is constantly under development and as the feature freeze for the next 3.2 release approaches, it is becoming increasingly clear what the next version will have to offer to users:

(to avoid misunderstandings – this is a selection of features currently under development and not a release note, i.e. there is no guarantee that all items will appear in 3.4, nor are the features of 3.4 limited to what is listed here)

Precipitation

Added realism for precipitation:

The precipitation system has been partially upgraded. The speed of falling raindrops now follows a physical scaling with droplet size, and the system renders now hail in addition to rain and snow. The correct dependence of lighting with illumination of the scene has been added. In Advanced Weather, droplet size and rain intensity are now set independently, allowing to realistically render fine spray as well as splattering rain in thunderstorms. A dynamical splash-pattern of raindrops added to the Atmospheric Light Scattering (ALS) runway effect completes the visual impression.

Advanced Weather clouds

Near photo-realistic 3d clouds:

The cloud rendering system has received an upgrade, allowing to let the edges of cloud patches gradually fade out. This makes several types of cloud formations appear even more realistic.

Haze

Have you ever wondered why the terrain appears bluish in the distance?

The Atmospheric Light Scattering (ALS) rendering framework has received a significant upgrade rendering the effects of Rayleigh scattering of light with air molecules and fine dust. This includes the in-scattering of light, resulting in the blue appearance of distant objects, as well as an out-scattering effect which makes colors seen through dry haze shifted towards the red. Combined with the already existing model for rendering hazes, this leads to truly impressive visuals.

Instrumentation

Improved instruments:

A new flexible CDU framework allows aircraft developers to set up extensive CDU pages with relatively little effort. The framework supports both 3D and 2D instruments, multipages, down selecting settings to the scratchpad and various input formats (e.g. FL115 or 11500). It comes with a 2D panel popup screen so you won’t have to pan around the cockpit all the time. The CDU is expected to be introduced on the Boeing 747-400, but the framework has been designed to be flexible and fit other airliners.

Aircraft

Enjoy the latest additions:

The Extra 500 introduces a luxury aircraft one of the most advanced glass cockpits to be simulated by Flightgear. The F-14b has received a significant upgrade with an added JSBSim flight dynamics model. There is also progress on a new version of the X-15 which might make it into the release, as well as the F-20.

Atmospheric Light Scattering

Enjoy yet more interesting visuals:

ALS continues to receive a host of additions, allowing for some stunning visuals:

* tree shadows, rendered using a very performance-friendly technique
* landing and search lights for better night flight experience
* improved implementation of Fresnel scattering on water surfaces, leading to more realistic water appearance
* a procedural rock effect, capable of rendering a large variety of different rock textures and colors across the world

And many improvements more!

Much work is done under the hood which is not all visible:

* improvements to the rendering framework, leading to better performance
* more applications utilizing the FG-internal webserver
* a canvas-based alternative GUI and aircraft center, allowing to manage installed aircraft inside FG

Stay tuned as we fly towards our next release!

The F-14b is back

Ready to launch?

Thanks to Alexis Bory and Enrique Laso, the F-14b has been for a long time one of Flightgear’s most impressive 3d models, with a highly detailed cockpit and a large number of modeled systems.

But it just got even better – are you ready for a ride?

New flight dynamics

Richard Harrison has added a detailed JSBSim model for the flight dynamics based on a number of aerodynamical data sources which makes especially the behaviour at low airspeed very close to the real airplane. This also includes an accurate modeling of stall and departure into spin or flat spin and high alpha control reversal. Wing sweep can be controlled manually and affects the behaviour of the plane,

All of the plane’s control systems are implemented in JSBSim rather than in Nasal (which means they are computed at a much higher rate than the framerate), making the response of the plane more fluid, especially at framerates below 30 fps. All in all, the detailed JSBSim FDM adds quite a lot to the flight experience,

Improved systems modeling

The 3d cockpit has received a number of additions, among them a master warning panel with working indicators, an engine control panel and a master generator control panel. Other switches, such as the fuel cutoffs on the glareshield panel, are now functional, such that an engine startup/shutdown procedure from the cockpit is now possible.

Here is an example of the cockpit view in low-level flight:

And the RIO view:

The full range of operations

Just like the previous YaSim version, the new JSBSim F-14b supports a full range of military operations. The plane is fully aircraft-carrier capable (due to the improved modeling of low airspeed behaviour, carrier landings are somewhat more difficult than with the YaSim version though).

The plane also has a detailed radar with several different modes, capable of tracking targets, and the operation of the AIM-9M sidewinder missile is modeled as well as the M61A6 Vulcan gun.

Full air-to-air refueling capability from e.g. the KA-6 is also modeled:

Enjoy the new F-14b along with many exciting new features on current GIT (3.3) or with the forthcoming stable release 3.4!

(All features presented in the screenshots (bluish atmosphere haze, details on the Vinson flightdeck, improved appearance of water,…) are available in the current development version and will be part of the 3.4 release. The screenshots have been taken off the coast of Corsica and over Nevada, both in the default 2.0 World Scenery.)

FlightGear v3.2 Released

The FlightGear development team is delighted to announce the v3.2 release of FlightGear, the free, open-source flight simulator. This new version contains many exciting new features, enhancements and bugfixes. Highlights in this release include an experimental aircraft manager allowing users to download and load aircraft in-sim, a very capable built-in HTTP server, built-in voice synthesizer for ATIS messages, and many improvements to the Canvas rendering framework.

Founded in 1997, FlightGear is developed by a worldwide group of volunteers, brought together by a shared ambition to create the most realistic flight simulator possible that is free to use, modify and distribute. FlightGear is used all over the world by desktop flight simulator enthusiasts, for research in universities and for interactive exhibits in museums.

FlightGear features more than 400 aircraft, a worldwide scenery database, a multi-player environment, detailed sky modelling, a flexible and open aircraft modelling system, varied networking options, multiple display support, a powerful scripting language and an open architecture. Best of all, being open-source, the simulator is owned by the community and everyone is encouraged to contribute.

Download FlightGear v3.2 for free from FlightGear.org.

FlightGear – Fly Free!

Major enhancements in this release

Aircraft Modeling

  • A Failure Management Framework for FlightGear has been added
  • The JSBSim flight dynamics model now has support for ground effects like bumpiness, solid-ground detection and adjusting of friction factors. Additionally, bogey type contact points sink in non-solid surfaces, making it no longer possible to ride on water.
  • YASim now has versioning support. The YASim FDM now checks a version tag in it’s configuration file to allow improvements to the YASim FDM without risking breaking older aircraft.
  • Additional aircraft have started adopting the Canvas navigation display that was introduced with FlightGear 3.0.

Graphics

  • Improved rendering of VASI, PAPI, runway and taxiway lights.
  • Orbital rendering using textures from the NASA Visible Earth project
  • Additions to the Atmospheric Light Scattering (ALS) rendering include:
    • an experimental framework to render cloud shadows on the ground (requires Advanced Weather)
    • a substantial extension of cloud layer visibility using impostor techniques to 150 km
    • a new agriculture effect allowing to render fields without tiling artifacts
    • a new forest effect to simulate managed forest, varying tree size by patch
    • sparkle and fog effect on runway lights
    • much improved visual appearance of rock faces

Usability

  • An experimental aircraft package manager allows the download of new aircraft, and changing the current aircraft in-sim.
  • A new internal web server (aka httpd) based on mongoose httpd has been added. It supports various AJAX requests, a screenshot server, a property tree browser, and a web-based moving map and much more.
  • The integrated Map dialog now uses an azimuthal equidistant projection, for better representation in polar regions and across the International Date Line.
  • Windows users are now able to use the scroll wheel in dialog lists

Canvas System

Improvements to Canvas, FlightGear’s scriptable 2D rendering system include

  • Better performance
  • MapStructure-based layers can now be customized and styled
  • Support for mouse button and modifiers
  • CanvasImage now supports the http:// protocol for dynamically retrieving raster images.
  • An experimental Map dialog using Canvas is available under the Equipment menu.
  • Canvas Layout Engine

Nasal Scripting

  • A new fully-interactive Nasal GUI console based on Canvas has been added: Interactive Nasal Console
  • the hard-coded flight path history subsystem which samples aircraft position is now exposed via Nasal.

Documentation

  • Nasal documentation is now included,

Highlighted new and improved aircraft

Other

  • A text-to-speech system based on flite+hts_engine has been implemented, which is used for ATIS and other messages.
  • Improved loading behaviour for AI/MP aircraft

Bug fixes

  • A serious bug was found late in the release causing large numbers of crashes. Fixing this delayed he release, but had a nice side-effect of improving performance on some systems that were previously CPU-limited.
  • See our bugtracker for an extensive, yet incomplete, list of the bugs fixed in this release.

Feedback on dds textures required

Should Flightgear switch to dds texture format?

What is this about?

The FG development team is considering to switch the format for terrain textures from png to dds. This would offer a number of significant advantages:

– dds is a compressed format, hence the download size of the FG base package may be decreased
– compressed dds can be directly used by many graphics cards, reducing also GPU memory consumption
– dds stores all texture resolution levels, i.e. no lower resolution levels have to be generated when the texture is used, hence it loads much faster into memory
– the resolution levels (‘mipmaps’) can be customized, allowing for some interesting effects at no performance cost

Practically all commercial simulations use dds for these reasons.

However, the dds compression algorithm is patented, which means that it is not readily available for OpenSource graphics drivers used by Linux distributions. Dependent on the specific hardware, this may or may not be a problem (modern graphics cards typically do not need the driver to process dds, for older graphics cards there are non-patented workarounds available which decompress the dds on the software level). The development team is concerned about making the Flightgear experience pleasant for all users, hence we would like to gather feedback how many users would be affected by a change in practice.

If there are no problems reported, FG will change defaults to txtures in dds format with the 3.4 release, and then phase out the use of png textures.

What would we need?

Flightgear already provides the simple option to test a dds texture set. If you are running on Linux and especially if you use an OpenSource graphics driver, please take 5 minutes to help during your next FG session:

– Open the dialog under View -> Rendering

– Under ‘Terrain texture scheme’, change the default ‘Region-specific’ to ‘Global alternative (DDS format)’ (see red circle)

– Press ‘Okay’ – FG will reload the terrain

– Do you see proper textures on the terrain (they may look different and may also not fit the location perfectly)? If yes, you’re fine. If you see monochromatic colors or other rendering artifacts, your system may have problems with dds.

– Change back to the texture scheme you like best

– Go to the wiki page and report your experiences, ideally including the graphics card you have and the driver you’re using.

Thanks for your time!

Some context for those interested

The visuals you get to see of the terrain in Flightgear depend on texture scheme and rendering scheme being used.

Simply put, the texture scheme selects a set of texture sheets which are mapped to the various landclasses in the terrain, such that a forest is rendered as forest rather than as grass. The old ‘Global’ texture scheme uses one such set everywhere in the world, the ‘Global alternative’ scheme uses a different set, but the format the textures are stored in is dds rather than png, and the ‘Regional’ scheme selects different textures based on what part of the world you are in. So the texture scheme selection governs things like the basic appearance of the terrain, the format the textures are internally stored in and the definitions where in the world certain textures should be used.

However, modern graphics cards allow to modify textures dynamically, or even create them on the fly by Procedural Texturing using shader effects. Dependent on shader quality level, these effects may have quite a pronounced impact on the visuals. If you are not running Rembrandt, you can switch the main rendering schemes runtime using the ‘Atmospheric Light Scattering’ (ALS) checkbox in the rendering dialog (blue circle in the image above) and explore what it does. So in summary, the rendering scheme selection governs just what is done in detail with the basic texture layers selected above (but, confusingly enough, shader effects may even replace textures).

Some examples exploring the different texture and rendering schemes below:

This is the South Rim of Grand Canyon using regional texture definitions and ALS procedural texturing:

Regional texture definitions allow to adjust the rock color to what is prevalent in the US Southwest, whereas the banded rock structure is not part of the texture file but generated procedurally.

Same scene using global texture definitions and ALS:

Using global textures, the rock and grass color is no longer adapted to the region, and also the shader effect no longer replaces the steepest forest patches by rock.

Same scene using global alternative (DDS) textures and ALS:

Switching to global DDS textures does not alter the visuals significantly in this case, the main difference is the texture format and detail resolution.

Same scene using regional textures and default rendering scheme:

The default rendering scheme at high quality contains some texture replacements which are coded globally into the effect framework and do not mesh too well with the regional texture colors seen elsewhere in the scene.

Same scene using global texture scheme and default rendering scheme:

Such global texture replacements in the shader however work better with a global texture scheme.

Same scene using global alternative DDS texture scheme and default rendering scheme:

Here, the dds texture scheme leads to somewhat different colors.

FG supports this wide variety of textures and rendering schemes so that users can customize the visuals to the performance offered by their computer and select the best compromise between good framerate and compelling visuals.

We need different schemes for this, since in trying to render a scene faithfully, we need to decide questions whether an average level of dust should already be included into textures (as done in the global scheme) or added dynamically according to weather (as done in the regional scheme in procedural texturing). The first alternative is preferable on low-end hardware where procedural texturing is too slow, but the second alternative works much better on high-end systems. Similarly, having different texture schemes allows us to provide a quick fallback for users who might experience problems with a dds-based scheme.