[Bug 31499] [r300g] KWin Lanczos filter problems - Black Windows using effects

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=31499

--- Comment #6 from Tom Stellard <tstellar at gmail.com> 2010-11-20 00:07:55 PST ---
As the shader is currently written, it should use 50 constants, your hardware
only supports 32.  So unless the KDE devs can modify the shader to use less
constants, it won't work with your hardware.  It might be possible to rewrite
it to use 32 or fewer constants.  It looks like there are only two
possibilities for values in the offset array (although I could be wrong about
this), so rewriting it like this might work:

uniform sampler2D texUnit;
uniform vec4 offsetValues[6];
uniform vec4 kernel[25];
unifrom vec4 offsetsAandB;

void main(void)
{
    vec2 offsetValueA = offsetsAandB.xy;
    vec2 offsetValueB = offsetsAandB.zw;
    vec4 sum = texture2D(texUnit, gl_TexCoord[0].st) * kernel[0];
    for (int i = 1; i < 25; i++) {
        vec2 offset;
        int index = (i - 1) / 4;
        vec4 offsetIndexValues = offsetValues[index];
        switch((i - 1) % 4) {
        case 0:
            offset = (offsetIndexValues.x == 1.0 ? offsetValueA :
offsetValueB);
            break;
        case 1:
            offset = (offsetIndexValues.y == 1.0 ? offsetValueA :
offsetValueB);
            break;
        case 2:
            offset = (offsetIndexValues.z == 1.0 ? offsetValueA :
offsetValueB);
            break;
        case 3:
            offset = (offsetIndexValues.w == 1.0 ? offsetValueA :
offsetValueB);
            break;
        }
        sum += texture2D(texUnit, gl_TexCoord[0].st - offset) * kernel[i];
        sum += texture2D(texUnit, gl_TexCoord[0].st + offset) * kernel[i];
    }
    gl_FragColor = sum;
}


The basic idea is to store an array of boolean values (1.0 and  0.0) that tell
you what offset value (offsetValueA or offsetValueB) to choose for each value
of i.  So for i = 1, check offsetValues[0].x and for i = 2, check
offsetValues[0].y, i = 5, check offsetValues[1].x, etc.  If the compiler is
smart enough, the above shader will only have 32 constants.

The KDE developers might be able to spot some simpler ways to reduce the number
of constants, since they have a better idea of what the code is doing.  At the
very least, the vec2 offset array could be packed into a vec4 array, that would
reduce the number of constants by 12.

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