https://bugs.freedesktop.org/show_bug.cgi?id=84566
--- Comment #46 from Jason Ekstrand <ja...@jlekstrand.net> ---
(In reply to Iago Toral from comment #43)
> Jason, we are running into some issues when attempting to use
> _mesa_format_convert for glReadPixels and glGetTexImage.
>
> Generally, one thing that is different in this case is that the current
> implementation never attempts direct conversions from src to dst (except for
> a couple of specific cases that had been written ad-hoc).
>
> These functions do the conversion in 3 steps:
> 1) unpack to RGBA (float or uint).
> 2) Rebase (to consider the base format of the source pixel data).
> 3) Pack to dst (this not only packs, also handles type conversion,
> transferops, semantics specific to things like luminance formats, type
> conversions, etc).
>
> So we are hitting various issues when attempting to replace this logic with
> _mesa_format_convert:
>
> 1) You mentioned that things like transferops should be handled by the
> client. To achieve this we have _mesa_apply_rgba_transfer_ops, which
> requires an RGBA format, so converting (in the client) to RGBA first is
> required in these cases so they can use this function. I suppose this is
> okay since this is what the current implementation is doing in all cases
> right now, with or without transferops.
>
> 2) So far we have been focusing on pixel uploads. I mentioned then that we
> needed to add a new parameter to _mesa_format_convert to consider the base
> internal format we are converting to. Well, now we have the same situation
> but with the format we are converting from, and the semantics are different
> (in the sense that we need to know if we have a base format for the source
> or the destination in order to know what we need to do, like computing the
> right swizzle transform). I suppose that we could add another parameter to
> _mesa_format_convert and all the logic necessary to do the right thing in
> that case too. This, will complicate the implementation a bit I think, but I
> guess is the most consistent option. That said, the alternative would be to
> always transform to RGBA first, then call _mesa_rebase_rgba_* as the current
> code does... since the current code always convert to RGBA first we would
> not be losing performance and we would not have to add all the logic to
> _mesa_format_convert to account for a source base format. It would be less
> consistent though since _mesa_format_convert would support base formats for
> the dst but not for the src.
>
> 3) Luminance formats have special requirements. A conversion to Luminance
> from RGBA requires to do L=R+G+B for example. This is something that
> _mesa_format_convert cannot achieve at the moment, because neither
> pack/unpack functions nor _mesa_swizzle_and_convert do this kind of
> operation, so I wonder what is the right thing to do here. We could run
> another pass that computes these values after the conversion. We could do
> this inside _mesa_format_convert or in the client, after calling
> _mesa_format_convert I suppose there are no other options. The current code
> does another pass specifically for this, right before packing to the dst.
>
> 4) Step 3 does a lot of clamping work even after handling transfer ops,
> (right after packing to the dst). I am not sure that pack/unpack functions
> and _mesa_swizzle_and_convert will do the thing we need in all cases, but
> right now I don't have a specific example, so we can burn that bridge once
> we get there.
My though there was to do the following (in pseudocode)
if (!has_transfer_ops && format != LUMINANCE_ALPHA) {
mesa_format_convert(dst, src);
} else {
}
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