[PATCH v3 2/2] wined3d: Simplify the odd line condition in gen_yv12_read().

[Anton Baskanov]

On среда, 1 сентября 2021 г. 22:53:14 +07 you wrote:
> > Am 31.08.2021 um 19:26 schrieb Anton Baskanov <baskanov at gmail.com>:
> > 
> > -    shader_addline(buffer, "    if (fract(floor(texcoord.y * size.y) *
> > 0.25 + 1.0 / 6.0) >= 0.5)\n"); +    shader_addline(buffer, "    if
> > (fract(texcoord.y * size.y * 0.25) >= 0.5)\n");
> Some archeology 2c: the code in arb_program_shader.c has this to say about
> the 1/6 offset:
> 
>         shader_addline(buffer, "ADD texcrd2.x, texcrd2.y, yv12_coef.y;\n");
> /* To avoid 0.5 == 0.5 comparisons */
> 
> I don't remember why I was scared of 0.5 == 0.5 comparisons or why exactly
> they might happen (at the exact edge of the texture probably? But even then
> I'd expect the texture coordinate to be not quite 1.0), why I was scared of
> equality here ('SGE' is set greater equal after all. Driver bugs?). It was
> 13 years ago when I wrote the ARB version of this.
> 
> The ARB code does use 0.25 though, like the change your previous patch
> introduces.

Weirdly, the WINED3D_GL_RES_TYPE_TEX_2D branch in the ARB code does not 
multiply the floor result by anything at all. This means that the condition is 
always false. I tried to set shader_backend to arb, and I saw 4-pixel tall 
jaggies around the color transitions, which confirm this.

> 
> My gut feeling says that those shaders should be safe re magnifying filters,
> but if the blit scales the YUV image down while converting to RGB it'll
> probably break in interesting ways.