[Mesa-dev] Performance glxSwapBuffers 32 bit vs. 64 bit

[Michel Dänzer]

On Fre, 2011-11-11 at 21:25 +0100, Theiss, Ingo wrote: 
> Am Freitag, 11. November 2011 14:53 CET, Brian Paul <brian.e.paul at gmail.com> schrieb: 
>  
> > Ingo, if you could find out what the format/type parameters to
> > glReadPixels are, we could look into some optimization in the state
> > tracker.  I wouldn't be surprised if there's some channel swizzling or
> > format conversion going on.
>  
> Hi Brian,
> 
> I have digged around in the VirtualGL source code and hope I have
> found what you requested. Don´t blame me if I am wrong as I have very
> limited knowledge in C programming :-(

I suspect Brian might have been thinking of running your app in gdb,
setting a breakpoint in _mesa_ReadPixels and reporting the actual values
that are passed in there from glxSwapBuffers.


> Here is the (overwritten?) glReadPixels function from VirtualGL:
> 
> void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height,
>         GLenum format, GLenum type, GLvoid *pixels)
> {
>         TRY();
>         if(format==GL_COLOR_INDEX && !ctxh.overlaycurrent() && type!=GL_BITMAP)
>         {
>                 format=GL_RED;
>                 if(type==GL_BYTE || type==GL_UNSIGNED_BYTE) type=GL_UNSIGNED_BYTE;
>                 else
>                 {
>                         int rowlen=-1, align=-1;  GLubyte *buf=NULL;
>                         _glGetIntegerv(GL_PACK_ALIGNMENT, &align);
>                         _glGetIntegerv(GL_PACK_ROW_LENGTH, &rowlen);
>                         newcheck(buf=new unsigned char[width*height])
>                         if(type==GL_SHORT) type=GL_UNSIGNED_SHORT;
>                         if(type==GL_INT) type=GL_UNSIGNED_INT;
>                         glPushClientAttrib(GL_CLIENT_PIXEL_STORE_BIT);
>                         glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
>                         glPixelStorei(GL_UNPACK_ROW_LENGTH, 1);
>                         _glReadPixels(x, y, width, height, format, GL_UNSIGNED_BYTE, buf);
>                         glPopClientAttrib();
>                         _rpixelconvert(unsigned short, GL_UNSIGNED_SHORT, 2)
>                         _rpixelconvert(unsigned int, GL_UNSIGNED_INT, 4)
>                         _rpixelconvert(float, GL_FLOAT, 4)
>                         delete [] buf;
>                         return;
>                 }
>         }
>         _glReadPixels(x, y, width, height, format, type, pixels);
>         CATCH();
> }
> 
> One more noob question regarding this code: Is the glReadPixels
> function with the underscore the call to the "parent" glReadPixels
> function of libGL.so? I have not found the _glReadPixels function
> inside the VirtualGL code so this seems the only explanation to me.

The above procedure should answer this question as well. Otherwise, it
would rather be a question for VirtualGL developers, though it looks
like it is as you say.


-- 
Earthling Michel Dänzer           |                   http://www.amd.com
Libre software enthusiast         |          Debian, X and DRI developer