[OpenGL ES] NV_pixel_buffer_object doesn't define GL_STREAM_READ

[emersion]

OpenGL ES 2.0 core only defines GL_STREAM_WRITE. NV_pixel_buffer_object advertises that it allows "Asynchronous glReadPixels". The code examples use GL_STREAM_READ. However including both <GLES2/gl2.h> and <GLES2/gl2ext.h> isn't enough to get access to this symbol necessary for creating a PBO that can be read from. No other GLES2 extension seems to define it.

Is this an oversight?

cc @pow2clk

[pow2clk]

@mheyer32 is the contact on this spec now. These were pretty heavily "inspired" by the desktop specs. It's entirely possible that the enum was inadvertently copied from there.

[nvmheyer]

@nvmheyer would be me with NVIDIA hat on :-)

This is certainly a mistake in the NV_pbo extension specs (which are in fact based off the desktop extension spec language) to use GL_STREAM_READ. The other example uses GL_STREAM_DRAW.

One could argue that the EXT_map_buffer_range extension should have introduced new usage hints as it allows reading back from buffers for the first time in GLES.

This should not be a huge problem, though. The buffer usage hints are not a good indicator of how a buffer is going to be used. The NVIDIA driver, for instance, will mostly ignore the hints and instead infer the optimal buffer storage from the actual usage.

[emersion]

This should not be a huge problem, though.

Ah, indeed, it seems like this is just a hint and that it doesn't constrain the actual usage. It's unfortunate that there's no "no hint" enum value.

Mesa also ignores the hint in this situation: https://gitlab.freedesktop.org/mesa/mesa/-/blob/e5339fe4a47c242693962c9f90bbab8b74935cba/src/mesa/state_tracker/st_cb_bufferobjects.c#L248

This is certainly a mistake in the NV_pbo extension specs

Should we somehow fix the examples?

One could argue that the EXT_map_buffer_range extension should have introduced new usage hints as it allows reading back from buffers for the first time in GLES.

Indeed. I'm not sure if/how we can fix that.

[pdaniell-nv]

@nvmheyer Would you mind creating a PR to fix this minor issue? Thanks.