[Mesa-dev] ARB_texture_buffer_range offsets

Roland Scheidegger sroland at vmware.com
Wed Nov 5 12:08:19 PST 2014

Trying to fix some bug due to alignment issues in llvmpipe's vertex
fetch, I came across some issue with ARB_texture_buffer_range.
Namely, it looks like the offsets specified there are always in bytes,
regardless the actual format (hence, as long as the
TEXTURE_BUFFER_OFFSET_ALIGNMENT is 1, it would be allowed to have an
offset of 15 bytes for a rgba32f format for instance making all fetches
quite unaligned).
However in gallium we actually have first_elem and last_elem parameters
in the sampler views which are specified in number of elements (so takes
the format into account), which is what d3d10 does and the state tracker
translates to that apparently. IMHO d3d10 makes way more sense there
because that way the necessary alignment scales automatically depending
on the format (so, if the format is 2x16bit for instance you'd need 4
byte alignment for the offset, and only need 16 bytes alignment for
4x32bit, ensuring all lookups are always aligned). This means that 15
byte offset in the example above is completely untranslatable.
But if I see that right, OpenGL doesn't work like that, meaning
effectively gallium drivers (and I doubt most other drivers neither)
cannot actually claim to support TEXTURE_BUFFER_OFFSET_ALIGNMENT lower
than 16, even if they'd only need that for 4x32bit formats. Though most
gallium drivers indeed claim 1 right now.
Looks quite messy...


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