[Bug 97489] High latency using GL_FEEDBACK mode

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97489

--- Comment #4 from Kenneth Graunke <kenneth at whitecape.org> ---
Yes, glRenderMode has been deprecated for a long long time.

If you want to disable rendering, you can use glEnable(GL_RASTERIZER_DISCARD).
Transform feedback (GL 3.0) can record a subset of your vertex shader's outputs
into a buffer.  Another alternative would be to have your vertex shader write
data into a shader storage buffer object (SSBO) (GL 4.3).

Unfortunately, both of those options give you data pre-clipping, while
glRenderMode(GL_FEEDBACK) gives you data post-clipping.  It's a bit awkward
because there isn't an exact replacement for the deprecated functionality.

The thing is, GL_FEEDBACK and GL_SELECT don't really exist in hardware.  We
have to emulate them somehow in the driver.

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