[utah-glx-dev] offscreen rendering an glReadPixels.

[Lincoln M C <mc...@es...>]

Hi,
	I'm using a current version of your glx drivers with a matrox G400
in a 450MHz PII.  I'm using the texturemapper for a scientific
application. I require the result of my renderings to be placed in an
array which I can render, so I setup glx to render into a GLXPixmap.
This works really well and is very fast, but I am unable to get it to
work in direct mode.

Is direct, off screen rendering supported yet ?

My problem is with glReadPixels, it is incredibly slow. About 60ms per
call for images that are only 384x288.  I looked into the source code
and found
this was calling __glx_ReadPixels(.....).

Two of the calls inside this function seem to be taking most of the
time:-

_XReply(dpy, (xReply*)&reply, 0, xFalse); /* 53ms */
__GLX_GET_BUFFER(pixels, length, width, height, format, type);/* 19ms */

I dont suppose much can be done about this as it seems to be a X call (I 
know nothing about X)  I would assume this isn't the case in direct
mode,
but as I said my code doesn't work when direct.

I've tried XGetImage but this dosnt seem much faster, and seems total
over kill.

Can anyone think of a fast way to get my image back into my programs
address space ?

I dont mind hacking the GLX code around a bit if it would be feasible to
get a pointer to off screen memory (after making it shared). 


Any help much appreciated, 
Mike Lincoln

University of Essex.
 I