Re: [Mesa3d-dev] Bug in z-depth

[Brian P. <br...@va...>]

Klaus Niederkrueger wrote:
> 
> bash$ LD_PRELOAD=~/Mesa/src/.libs/libGL.so.1.2.030500 ./walktest funky -video=opengl
> Using alternative 3D driver: crystalspace.graphics3d.opengl
> Crystal Space version 0.17 r001 [Linux-Intel-GCC] (Wed, 1-Sep-2000).
> Created by Jorrit Tyberghein and others...
> 
> Video driver GL/X version (direct renderer)
> Visual ID: 20, 16bit TrueColor
> Frame buffer: 16bit R5:G6:B5:A0, level 0, double buffered,
> Depth buffer: 8bit                                   <<<------
> OpenGL renderer: Mesa X11 version 1.2 Mesa 3.5 beta
> Using windowed mode at resolution 640x480.
> Loading world '/lev/funky'...
> 
> It's 8bit, while with Mesa-3.2 it is 16bit.

I grabbed the Crystal Space sources and grep'd for glXChooseVisual.
In plugins/video/canvas/openglx/glx2d.cpp in the csGraphics2DGLX::Open()
function I see this:

  int desired_attributes[] =
  {
    GLX_RGBA, 
    GLX_DEPTH_SIZE, 8, 
    GLX_RED_SIZE, 4,
    GLX_BLUE_SIZE, 4,
    GLX_GREEN_SIZE, 4,
    None,
    None
  };

The GLX spec says that this should choose a visual with the deepest depth
buffer of at least 8 bits.  While this is perfectly legal, Mesa has a
problem with this.  Mesa's GLX, as we all know, is not real a GLX but an
emulation and it can't do everything right.

When Mesa sees GLX_DEPTH_SIZE=0, it doesn't allocate a depth buffer.
If GLX_DEPTH_SIZE=1, it allocates a 16-bit depth buffer (the default
depth buffer size).
Otherwise, if GLX_DEPTH_SIZE=N and N>1, then Mesa allocates an N-bit
depth buffer.  This allows one to get a deeper depth buffer (such as
24-bits) if needed.  I didn't think anyone would ever request a depth
of 1 to 15 bits so the code is kind of dumb when that happens.

There's two possible solutions:

1. Modify Crystal Space to specify GLX_DEPTH_SIZE=1 or GLX_DEPTH_SIZE=16.

2. Modify Mesa so that a 16-bit (the default) depth buffer is chosen if
GLX_DEPTH_SIZE is between 1 and 16.

Any comments?

Is anyone on this list also involved in the Crystal Space project?

-Brian