[Mesa-dev] Gallium pixel formats on big-endian

[Jose Fonseca]

----- Original Message -----
> On Mon, 2013-02-04 at 20:17 +0100, Michel Dänzer wrote:
> > 
> > [...] how about something like this:
> > 
> > Define the packing to be in the host byte order. However, do not define
> > array formats as packed values (which makes little sense e.g. for
> > *32*32*32*32 anyway) but really just as arrays. 16- or 32-bit components
> > of arrays are again packed in host byte order. Array components of 4, 2
> > or 1 bits occupy bits [0,n-1] of byte 0, then bits [n,2n-1] of byte
> > 0, ..., bits [0,n-1] of byte 1, and so on.
> > 
> > To make the distinction between packed and array formats clear, they
> > should probably use different naming schemes. The current scheme makes
> > more sense for packed formats I think, so I'd propose separating array
> > components e.g. by an additional underscore, and ordering components
> > from lower memory address left to higher address right. As an example,
> > PIPE_FORMAT_R8G8_UNORM would become PIPE_FORMAT_R8_G8_UNORM. It would
> > probably make sense to define packed naming aliases for array formats
> > where possible, e.g. PIPE_FORMAT_R8_R8_UNORM would alias to
> > PIPE_FORMAT_R8G8_UNORM (which would be identical to MESA_FORMAT_GR88) on
> > little endian and PIPE_FORMAT_G8R8_UNORM (== MESA_FORMAT_RG88) on big
> > endian hosts. (That's using the current component ordering for packed
> > formats; IMHO the reverse order would actually make more sense for
> > those, but that might require more invasive changes...)
> 
> Actually, on second thought this means it's probably better to keep the
> current naming scheme for array formats. Maybe the component names and
> sizes could be packed together to indicate packed formats, e.g.
> PIPE_FORMAT_B5G6R5_UNORM would become PIPE_FORMAT_RGB565_UNORM and would
> be defined identically to MESA_FORMAT_RGB565.

> For my example above, PIPE_FORMAT_R8G8_UNORM would alias to
> PIPE_FORMAT_GR88_UNORM (== MESA_FORMAT_GR88) on little endian and to
> PIPE_FORMAT_RG88_UNORM (== MESA_FORMAT_RG88) on big endian.

I like this naming better, as I'd prefer the array formats to match D3D10, and also to prevent confusion with subsampled formats (like R8G8_R8B8).

Either way the renaming is just syntactic sugar and is neither urgent nor a requirement for the semantic changes. But I believe we are forming some consensus on how to move forward/

Ajax, does the above sound good to you too? I think it's not very different from your initial suggestion. 

Jose