[Mesa-dev] [PATCH 2/4] gallium: extend resource_resolve to accommodate BlitFramebuffer

Christoph Bumiller e0425955 at student.tuwien.ac.at
Tue Jul 26 11:18:08 PDT 2011

On 07/26/2011 01:49 AM, Roland Scheidegger wrote:
> (Strange thought sent that before - mail client going crazy...)
>> Resolving color buffers is pretty well defined (for standard msaa at
>> least). I have no idea how that's supposed to be defined for depth
>> and stencil values. Frankly I'm not sure what glBlitFramebuffer is
>> supposed to do here, maybe it's defined somewhere but even applying
>> the term "resolve" to a depth buffer seems very iffy. At the very
>> least it needs to be documented in the gallium docs what "resolving"
>> a depth/stencil buffer really means.
> Hmm actually it must be like ReadPixels. So it is "recommended" that implementations just use the centermost sample, but this is not required. In particular "any function using depth/stencil values" is valid.
> Taking this to the extreme, this means just returning 0 is valid (f = 0*sample0 + 0*sample1...) though probably not recommended...
> Averaging would be allowed as would be any other crazy stuff. In any case calling this step, whatever it does, "resolve" seems abusive with results possibly quite implementation dependent. I have no idea what nv50 does here though I guess given the loose definition it should certainly fit the requirements.
But that's great, it means drivers can't do anything wrong here.

And either way, GL demands that you can "resolve" it so you better
support it. nv50 gallium would follow the behaviour of the binary
driver, which is take the value from a single sample.
Should hw for some reason not be able to do this (I can't imagine how),
well, they can return black. Or whatever the st would do instead.

Now again about the rest of the new arguments:

pipe_resource *aux (from my first reply):
The are multisampling implementations that store extra sample data
required for resolve in a separate buffer, nv's coverage sampling for
I think the pipe driver can store a link to such a buffer in the
associated colour resource, so, forget about that one.

Now the important ones:

mask: If depth resolve is supported, you are not allowed to clobber
stencil. You have to allow the pipe driver to not do double the work.

If the user resolves to the (n != 0)-th level of a texture, you
certainly want this; doing an extra copy of several MiB only because you
don't like the looks of an extra parameter in the function declaration
is insane from any angle.

dstx, dsty, box:
BlitFramebuffer honours scissor state. What if the user is scissoring
away half of the destination ? Do you really want to push through the
other half, all that to temporary storage, and then again blit several
MiB of data over, just because you don't like to clutter the interface ?

When the user is able to resolve to the window system buffer (if mesa is
kind enough to choose the same component ordering, or if the "same
format" requirement is interpreted loosely - this is certainly possible
with the binary drivers), then you have to do this flip.
And you don't want to ALWAYS do double the work. You *really* do not.
Not !


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