[Mesa-dev] [PATCH 1/2] i965: Fix mipmap offsets for HiZ and separate stencil buffers.

[Paul Berry]

On 25 April 2012 11:13, Roland Scheidegger <sroland at vmware.com> wrote:

> Am 24.04.2012 21:23, schrieb Paul Berry:
> > When rendering to a miplevel other than 0 within a color, depth,
> > stencil, or HiZ buffer, we need to tell the GPU to render to an offset
> > within the buffer, so that the data is written into the correct
> > miplevel.  We do this using a coarse offset (in pages), and a fine
> > adjustment (the so-called "tile_x" and "tile_y" values, which are
> > measured in pixels).
>
> Just wondering, can't you get the gpu to do these offset calculations
> for you?


In theory yes, but my understanding is that there are enough limitations
that we've elected not to.  The limitations I'm aware of are:

1. Gen6 HiZ buffers (and possibly also separate stencil buffers?) don't
respect the miplevel setting, so if we do this we'll have to do extra work
to get HiZ and separate stencil to work properly.
2. The LOD we're rendering to must match for depth, stencil, and all color
buffers.  AFAICT, GL doesn't require this (though it's possible we might be
able to work around it by reporting GL_FRAMEBUFFER_UNSUPPORTED, I'm not
sure).

The decision was made a long time before I joined the project, so I don't
know the full history.  I would like to do some experiments to try getting
the gpu to do the offset calculations, but I'm not sure when I'll have time
to do that :)


> After all dx10 allows you to have multiple mip levels bound as
> a render target at once, where the actual mip level being rendered to is
> determined by a system value (GL 3.3 (?) can do the same with the
> gl_Layer output of geometry shader). How do you handle that if you have
> to adjust the offset manually?
>

I'm not familiar with dx10, but the GL 3.3 feature you're talking about is
for rendering to multiple layers of 3D textures and texture arrays, and
multiple faces of cubemaps.  It doesn't have anything to do with
miplevels.  We implement gl_Layer yet (since we don't support geometry
shaders) but I just did a quick scan of the docs and it looks like we'll be
able to support it without a problem, since the offsets to get into a
different layer and the offsets to get into a different miplevel are
additive.
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