Mike Westall wrote:
>
> We have an application (``instant'' radiosity) that needs to read
> back depth information from the graphics card. We find that this
> operation appears to work fine
>
> (1) using software only rendering (e.g. by not loading r128.o
> and
> (2) on Nvidia GeForce2's
>
> It appears to not work so fine when using DRI accelerated r128
> or mga. It may be some manner of synchronization problem as putting
> a getchar() between the draw and the readback on the r128 made
> it work on the r128. However, it never works with a sleep()
> interposed.
>
> I have attached a small sample program that illustrates the
> problem. It first renders a smooth shaded triangle with
> depth information. It then does a glFinish() and sleep(10)
>
> When it wakes up it reads back the depth info and renders it
> as a pixmap in monochromatic green. The triangle should
> appear slightly darker than the background.
>
> It does so using (1) or (2) above. Using accelerated DRI
> rendering no triangle is visible as the depth info
> returned is constant.
>
> Thanks for any insights (precision or otherwise),
I've had problems with readback synchronizaton with the mga driver.
That is, glReadPixels doesn't reliably flush the rendering hardware
so you don't get back the correct results. I don't know how to fix
this. I never had this trouble with the R128.
I'm afraid I'm too busy to look into this.
-Brian
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