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Project Rembrandt

3,705 bytes added, 17 February
Status (07/2014): obsolete due to Compositor
{{WIP}}== Why this the name ? ==[http://en.wikipedia.org/wiki/{{Wikipedia|Rembrandt Rembrandt] }} was a dutch painter living in the 17th century, famous as one of the master of [http://en.wikipedia.org/wiki/Chiaroscuro {{Wikipedia|chiaroscuro]}}.
This project is about changing the way [[FlightGear]] renders lights, [[shadows]] and shades, and aims at making Rembrandt painting style possible in FG.
== What is it ? ==
|Deferred {{=}} rendering a geometry pass into the geometry (g-)buffer which holds positions, normals, materials, then doing all operations (texel, light, ...) in fragment passes on that buffer (or any others).<br/>
Forward {{=}} (usually) rendering part of the shading (typically ambient and diffuse light) per vertex, extrapolate all quantities which are declared as varying across triangles and hand them via rasterizer to the fragment shader which does the rest
|{{cite web |url=http://forum.flightgear.org/viewtopic.php?p=221222#p221222
|title=<nowiki>Re: Orbital Makes the Sky Black</nowiki>
|date=<nowiki>Sun Oct 19</nowiki>
The idea driving the project is to implement [http://en.wikipedia.org/wiki/Deferred_shading deferred rendering] inside FlightGear.
From the beginning FlightGear had a forward renderer that tries to render all properties of an object in one pass (shading, lighting, fog, ...), making it difficult to render more sophisticated shading
In FG, we end the rendering pipeline by displaying the [[Menubar|GUI]] and the [[HUD]].
All these stages are more precisely described if in this [http://bat710.univ-lyon1.fr/~jciehl/Public/educ/GAMA/2007/Deferred_Shading_Tutorial_SBGAMES2005.pdf tutorial] that is the basis of the current code, with some addition and modifications. C++ developers interested in learning more about how Rembrandt works, will want to look at these initial $FG_SRC commits (note that the viewer related sources are now to be found in $FG_SRC/Viewer):* 94e3ae4c675463c358071e1d9da8de583cf56e15 * 6b008126b5fd99c38778c6293c15f9721a4fa509* 64e3e98069fb2310311687b9f099b338b8b375a5* bb16463d631a8d87310a54a573966eb1cca813e6
== Caveats ==
Deferred rendering is not able to display transparency. For the moment, clouds are renderer rendered separately and should be lit and shaded by their own. Transparent surfaces are alpha-tested and not blended. They would have to be drawn in their own bin over the composited image.
It also dondoesn't fit with depth partitioning because the depth buffer should be kept to retain the view space position, so for the moment, z-fighting is quite visible. Depth partitioning with non overlapping depth range might be the solution and should be experimented at one point.
The glow pass can make certain MFD MFDs (that use emissive color) unreadable because blurredof blurring. Should be treated as transparent.
== Implementation ==
<syntaxhighlight lang="xml">
<map-size type="int">8192</map-size>
And put 4096 or 2048 instead.
You can also use a startup parameter: <code>--prop:/sim/rendering/shadows/map-size=2048</code>
=== Configurable pipeline ===
The Rembrandt renderer uses an XML file to setup its pipeline for each viewport described in the camera group. This file describes the way the intermediary buffers are setup and how the different rendering stages are sequenced. The general outline of a pipeline file is as follow :
Each effect attached to the fullscreen passes define the way blending is done between the pass and the previous accumulation of render.
=== C++ implementation ===
On the C++ side, Rembrandt is set up in those steps:
* ''buildRenderingPipeline()'' is the last common function between forward rendering and deferred rendering. That's the point of start for specific deferred stuff. In this function we call ''buildDeferredPipeline()''
** ''buildDeferredPipeline()'' is just a wrapper for ''buildCameraFromRenderingPipeline()''
*** ''buildCameraFromRenderingPipeline()'' is where we initialize all buffers and create all the stages found in ''Effects/default-pipeline.xml'' with the call to ''buildBuffers()'' and ''buildStage()''
**** ''buildBuffers()'' is where we ask to build each buffer with the call to ''buildDeferredBuffer()''
***** ''buildDeferredBuffer()'' create a 2D texture
**** ''buildStage()'' is where we ask to build each camera depending on the type of the stage (geometry, lighting, shadow, fullscreen, display) with the call to ''buildDeferred*Camera()'' (where * is the stage type)
***** ''buildDeferred*Camera()''is where we build the camera, for each camera we attach the required buffers with the call to ''buildAttachments()''.
****** ''buildDeferredGeometryCamera()'' c.f [http://wiki.flightgear.org/Project_Rembrandt#What_is_it_.3F What is it ?]
****** ''buildDeferredShadowCamera()'' c.f [http://wiki.flightgear.org/Project_Rembrandt#What_is_it_.3F What is it ?]
****** ''buildDeferredLightingCamera()'' Only for the lighting camera (''buildDeferredLightingCamera()'') we have to build passes who is called with ''buildPass()'' c.f [http://wiki.flightgear.org/Project_Rembrandt#What_is_it_.3F What is it ?]
****** ''buildDeferredFullscreenCamera()'' c.f [http://wiki.flightgear.org/Project_Rembrandt#What_is_it_.3F What is it ?]
****** ''buildDeferredDisplayCamera()'' c.f [http://wiki.flightgear.org/Project_Rembrandt#What_is_it_.3F What is it ?]
== Running Flightgear with Rembrandt ==
For a list of converted aircraft, please see [http://forum.flightgear.org/forums/viewtopic.php?f=4&t=17536#p166346].
=== Registering all translucent surfaces ===
If opaque surface need to have special effect, for example to apply bump mapping, this effect should use the "RenderBin" bin, or the rendering hint set to "opaque", and the G-buffer needs to be initialized correctly in the Geometry stage.
A surface that uses the [[ALS technical notes#ALS glass effect|ALS glass effect]] does not need to be separately registered for Rembrandt, this is done automatically.
=== Making sure that all geometry will cast shadow ===
|Three element vector. <c> element is the constant factor, <l> element is the linear factor and <&lt;q> &gt; element is the quadratic factor.<br />
Attenuation of color at distance d is [[File:Spotlight_attenuation.png]]
=== Mac Issues ===
More and more Apple/Mac users are reporting issues related to running Rembrandt [http://forum.flightgear.org/forums/search.php?st=0&sk=t&sd=d&sr=posts&keywords=rembrandt+mac].
Looking at the Mac GPU specs, it isn't clear if the Mac/ATI hardware/driver is generally insufficient, it should seem possible to run a customized Rembrandt setup with acceptable frame rates at 15-35 fps (assuming everything else being disabled for starters).
GLSL compilers have varying quality and especially the Mac (ATI/AMD) GLSL compilers are known to have issues with more sophisticated/nested constructs, so that it may help to reduce complexity of GLSL statements by splitting them up, instead of using nested anonymous vectors or functions (fatal error C9999: Nested functions, aborting!) for example.
So, the specific issue on Mac is some shaders being miscompiled, so the frame-rates are particularly bad, since the driver is hitting (slow) error paths. Certainly some Rembrandt-related shades fail to compile, though whether or not these are optional or required ones, isn't clear (Fred?). Some users reported that the errors and problems after upgrading their OS from OsX, 10.6.8 to OSX 10.8.2 (Mountain Lion) [http://forum.flightgear.org/forums/viewtopic.php?f=47&t=19070&p=177106&hilit=rembrandt+mac#p177106] or OS Snow Leopard (10.6. to Mountain Lion (10.8.2), downloaded XQuartz 2.7.4 [http://forum.flightgear.org/forums/viewtopic.php?f=21&t=19000&p=177570&hilit=rembrandt+mac#p176972].
The shader errors would suggest that certain GLSL constructs are not supported by the ATI/AMD glsl compiler - this seems to be a known issue: [https://groups.google.com/forum/?fromgroups=#!topic/comp.graphics.api.opengl/H_FLQCQE1i4] To address this, one would need to port the corresponding shaders - like just was done to get rid of the constructs that caused errors on old GeForce 7x generation hardware. It seems the hardware is not the problem, but the driver being way out of date. I find the fact that it's the Cg compiler and not a native glsl compiler that returns the errors very strange.
Another FG 2.10 user on MacPro 3.2 GHz Quad-Core Xeon, 8GB RAM, MacOS X 10.6.8. ATI Radeon HD 5870 (gl-vendor:ATI Technologies Inc., gl-version:2.1 ATI-1.6.36 gl-renderer:ATI Radeon HD 5870 OpenGL Engine, gl-shading-language-version:1.20) report said "Rembrandt is still unusable on my Mac. One frame every 4-5 seconds and it looks weird."[http://forum.flightgear.org/forums/viewtopic.php?f=68&t=18839&p=175737&hilit=rembrandt+mac#p175684]
It's also worth noting that a number of FG 2.8 users reported that Rembrandt would still work for them using the 2.8 binary, unlike the 2.10 binary on Mac OSX version 10.7.5 on an iMac (AMD Radeon HD Graphics with 512MB) [http://forum.flightgear.org/forums/viewtopic.php?f=21&t=19000&hilit=rembrandt+mac#p176460]. So the issue seems to occur largely in combination with older Mac OS versions and newer FG versions (>=2.8+) [http://forum.flightgear.org/forums/viewtopic.php?f=4&t=19396&hilit=rembrandt+mac#p179456].
It appears it is the Rembrandt lighting causing issues while in non rembrandt mode with older Mac OS + FG 2.8 and better. On other OS setups, the rembrandt lighting gets ignored when rembrandt is turned off, but not in this case. Both lighting modes are present and creating the weird light cone effects [http://forum.flightgear.org/forums/viewtopic.php?f=4&t=19396&hilit=rembrandt+mac&start=15#p179655].
If there are any console messages (like warnings or errors) shown, that would be helpful to know. A number of rembrandt related changes got fixed by Fred like this. Obviously, it is difficult for shader developers to troubleshoot shader related issues that they cannot reproduce with their own hardware.
Some Mac folks have recently reported some success, it may be a good idea to search the forum for details, see for example:
* http://forum.flightgear.org/forums/viewtopic.php?f=21&t=19000&p=177570&hilit=rembrandt+mac#p176972
{{cquote|I'm seeing regular crashes here from ALS and Rembrandt, but that's nothing new.<ref>{{cite web |url=http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg40024.html|title=<nowiki>Re: [Flightgear-devel] 2.10.1</nowiki>|author=Vivian Meazza|date=Fri, 03 May 2013 10:16:13 -0700}}</ref>|Vivian Meazza}}
{{cquote|concerning the larger issue of different rendering pipelines / approaches, my opinion is, and remains, that the long-term solution is separate viewer codebases - while a plethora would be bad, we would already benefit from a 'fixed-function, no shaders' renderer codebase distinct from a Rembrandt renderer and modern, forward-rendering OpenGL 3.x pipeline. This needs the viewer to be cleanly split out from the simulation backend, via HLA, which is exactly what Mathias (and soon, myself) are working towards, but slowly.<ref>{{cite web |url=http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg39922.html|title=<nowiki>Re: [Flightgear-devel] Atmospheric Light Scattering</nowiki>|author=James Turner|date=Thu, 25 Apr 2013 08:09:08 -0700}}</ref>|James Turner}}
{{cquote|Like you and Thorsten, I see a very significant drop (50%) in frame-rate with Rembrandt, even without most the nice features such as shadows which has put me off using it. I had thought it was just because my box is now underpowered (the march of technology....) but it sounds like other are
seeing similar issues and it would be worth some further investigation.<ref>{{cite web |url=http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg40304.html|title=<nowiki>Re: [Flightgear-devel] reminder: entering feature freeze now</nowiki>|author=Stuart Buchanan|date=Fri, 21 Jun 2013 01:40:15 -0700}}</ref>|Stuart Buchanan}}
=== Completed tasks ===
* {{cite web |url=http://advances.realtimerendering.com/s2012/Epic/The%20Technology%20Behind%20the%20Elemental%20Demo%2016x9.pptx |author=Martin Mittring |title=The Technology behind the Unreal Engine 4 Elemental Demo |accessdate=15 September 2012 }}
[[Category:Shader development]]
[[Category:Core development projects]]
[[fr:Projet Rembrandt]]

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