[cairo] [RFC] Cairo GLES2 and GL_CLAMP_TO_BORDER

[Martin Krastev]

Hi Alexandros,

I have reasons to believe the skeletal fragment shader below is more-or-less
what you've been looking for. Please note it has been tuned for emulating
GL_CLAMP_TO_BORDER with bilinear only, and as such it has certain
requirements for the TMU setup:

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);


Also, I've left some things, which would naturally enter as uniforms, as
constants here, to underline how those expressions can be done host-side.
And oh, the border color is implied black : )

                varying vec2 tc_i; // texcoord interpolant, pardon my
archaic naming convention

                uniform sampler2D albedo;

                const vec2 dim = vec2(4.0);
                const vec2 edge = vec2(0.5) - vec2(0.5) / dim;
                const vec2 bord = vec2(0.5) + vec2(0.5) / dim;

                void main()
                {
                   vec2 fade = vec2(1.0);
                   vec2 tc_d = abs(tc_i - 0.5);

                   if (tc_d.x > edge.x)
                       fade.x = (bord.x - tc_d.x) * dim.x;

                   if (tc_d.y > edge.y)
                       fade.y = (bord.y - tc_d.y) * dim.y;

                   fade = clamp(fade, 0.0, 1.0);
                   gl_FragColor = texture2D(albedo, tc_i) * fade.x * fade.y;
                }


Best regards,
martin

ps: Judging by your postings on the subject, I think you were getting to
that solution yourself.
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