[Mesa-dev] Choosing texture internal format in GLES

[Marek Olšák]

No, I said it would be better to use st_choose_matching_format in
st_ChooseTextureFormat, because st_choose_matching_format does exactly
what you're trying to do.

I have gone ahead and implemented what I had in mind. See the attached patch.

Marek


On Sat, Sep 28, 2013 at 8:45 AM, Wladimir <laanwj at gmail.com> wrote:
> Hello,
>
> Mareko and me had a small discussion about this on IRC.
>
> Basically with GLES (1.0, 2.0 at least) the application has no way to
> control the exact internal format [1]. This is intentional give the hardware
> more leeway.
>
> See http://www.khronos.org/opengles/sdk/docs/man/xhtml/glTexImage2D.xml :
> "Internalformat specifies the internal format of the texture. Must be one of
> the following symbolic constants: GL_ALPHA, GL_LUMINANCE,GL_LUMINANCE_ALPHA,
> GL_RGB, GL_RGBA."
>
> The driver is free to choose the internal format based on these general
> categories. However, with the current implementation mesa doesn't do any
> special choosing for GLES. For one, GL_RGB is always converted to RGB8. This
> can result in needless format conversion. I was helping to optimize an
> application that makes a texture:
>
>     glTexImage2D(target = GL_TEXTURE_2D, level = 0, internalformat = GL_RGB,
> width = 256, height = 256, border = 0, format = GL_RGB, type =
> GL_UNSIGNED_SHORT_5_6_5, pixels = [binary data, size = 128 kb])
>
> One of these textures is updated every frame. Every time, the image data is
> blown up to RGB8 because that's what the internal format became. This seems
> wasteful both of CPU/GPU time and memory.
>
> I proposed a crude patch that refines the internal format depending on the
> input data type (this happens after GLES input checking that
> format==internalFormat)
> https://github.com/laanwj/mesa/commit/0c71bcf1c5cc06e26bfed7a72507555fe7e7ad97
> however it doesn't take into account that the hardware may not support the
> specific formats.
>
> Mareko said it would be better to move the code to st_choose_matching_format
> inside the state tracker. This makes sense because the state tracker knows
> what formats are supported.
>
> Any other opinions on this?
>
> [1] there is an extension OES_required_internalformat which does give the
> application more control, but I'm describing behavior without this extension
>
> Wladimir
>
>
>
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