'alpha to coverage' and multiple render targets

[DrGoatcabin]

I can't see anything answering this in the ARB_multisample spec (correct me if I'm wrong), so does anyone know if 'alpha to coverage' should work when outputing to multiple colour buffers (COLOR0/1/2/3). If so, does anyone know the official line on whether the coverage mask calculation is based on the COLOR0 alpha of the fragment output or something else?

[dorbie]

Hmm... I wouldn't rely on this. Do you have multiple multisample buffers?

[Humus]

I'm not 100% sure if that's mentioned anywhere in the spec, but I would expect it to use glFragData[0].a

[arekkusu]

This is specified in ARB_draw_buffers:

If there are multiple output colors
being written to multiple buffers, the alpha used in alpha to
coverage and alpha test is the alpha of output color 0.

[DrGoatcabin]

That's exactly what I wanted to know.
Cheers, arekkusu.

(sorry for delay in acknowledgement. I hadn't subscribed to email updates).