[Mesa-dev] gallium and GL interpolation intersection issue

[Brian Paul]

On 01/09/2012 06:35 AM, Dave Airlie wrote:
> Okay so we have a gallium/GL/GLSL problem with interpolation, lets see
> if I can describe the issue:
>
> In the beginning there was GL interpolation rules, glShadeModel was
> the only information you had.
>
> Then came GLSL which allowed overriding the GL defaults using specifiers.
>
> So the gl_FrontColor variant, if left unspecified would take the value
> of the glShadeModel, if specified
> it would over-rule the current ShadeModel setting.
>
>
> So we end up something like this
>
> if (glFrontColor attrib>  0) result = glFrontColor_attrib; else result
> = shademodel.
>
> Now when we convert to gallium we lose the unspecified interpolation,
> and we generate a default interpolation
> attribute, which should usually be SMOOTH. Now we've no idea then
> inside the gallium drivers whether this smooth
> is due to an explicit smooth at the GLSL level which we can't
> override, or whether this smooth is something that we really
> wanted to be overruled by the ShadeModel bit.
>
> Now as discussed previously most hardware has support for the
> ShadeModel bit so generating multiple shaders
> for the unspecified case would be suboptimal for it.
>
> So the options I see are:
>
> (a) do it anyways, apply the patch I had earlier that derives the
> setting at the glsl->tgsi translation stage, and pretty
> much ignore it.
>
> (b) add unspecified interpolation support for Color inputs, only
> colors are affected by this, and since TGSI supports
> Colors we should probably support all the features of the GL Color
> output/inputs.
>
> Anyone any other ideas?

I'd probably go with (b), add a new TGSI_INTERPOLATE_COLOR mode that 
means "look at pipe_rasterizer_state::flatshade."

IMO, it's still attractive to get rid of 
pipe_rasterizer_state::flatshade someday.  glShadeModel no longer 
exists in OpenGL 3.2 core profile and later.

-Brian