José Fonseca pisze:
> On Tue, 2009-07-28 at 08:09 -0700, Michał Król wrote:
>
>> José Fonseca pisze:
>>
>>> I found one other problem in the way we use 4 x 8bit color formats:
>>> sometimes we interpret them as arithmetically coded in an unsigned (e.g
>>> src/gallium/auxiliary/util/u_tile.c when reading/writing
>>> color/depth/stencil buffers), sometimes we interpret them (e.g.
>>> src/gallium/auxiliary/translate/translate_generic.c when reading/writing
>>> vertex buffers). And these actually mean different things on
>>> little-endian architectures.
>>>
>>>
>>>
>> Some text is missing from the first sentence. I am guessing that
>> sometimes we interpret them as an array of bytes, right?
>>
>
> Right ;)
>
>
>>> I think the only viable option is to distinguish between these two kinds
>>> in the cases where it is ambiguous, like
>>>
>>> PIPE_FORMAT_R8G8B8A8_UNORM /* a | ( b << 8) | (g << 8) | (r << 24) */
>>> PIPE_FORMAT_RGBA8_UNORM /* {r, g, b, a} */
>>>
>>> Since there are legitimate uses in for both (color buffers, and vertex
>>> buffers).
>>>
>>> Anybody has better ideas?
>>>
>>>
>> We should go with and stick to a single convention. I don't know, maybe
>> for example this:
>>
>> A16R16G16B16
>>
>> The format description above would indicate that we are dealing with a
>> 64-bit entity with bits being numbered from right to left. That would
>> mean the B component occupies first 16 bits (bytes 0:1), the G component
>> next 16 bits (bytes 2:3) and so on. Because there is no implied dword
>> and encoding using shifts, we could easily write some code that decodes
>> the format in a portable way across LE and BE architectures.
>>
>
> Are these semantics followed by GL? (D3D does't matter much since it is
> only used on x86 anyway). Because if not we need to choose different
> formats according to the endianness.
>
>
I don't think they follow them everywhere. But the point is we can just
look at GL spec, see the convention and try to match it to a gallium format.
> Note also that formats like A1R5G5B5 can only be defined in terms of
> shits. Furthermore, the channel that starts from bit 0 is B, and not A
> as one would conclude from the your rule above.
>
>
Well, yes and no. Let's break A1R5G5B5 into bits using my definition.
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
A R R R R R G G G G G B B B B B
It means B is in byte 0, G in bytes 0 and 1, R and A are in byte 1. To
extract them correctly you need to use masking and shifting wisely, but
because you do it on a byte level, you are not concerned about
endianness, as endianness is about byte ordering in a word, not bit
ordering in a byte. The concept of a word does not exist here.
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