Dumb 5-point gradient OpenGL shader.

gradientShader.fsh

void main() {
    // Set the output color to red.
    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);

    vec2 texCoord = cc_FragTexCoord1;

    float l0 = 0.2, l1 = 0.1, l2 = 0.0, l3 = 0.1, l4 = 0.2;
    float y0 = 1.0, y1 = 0.85, y2 = 0.5, y3 = 0.15, y4 = 0.0;

    float level = 0.2;
    if (texCoord.y > y1) {
        level = (texCoord.y - y1) * (l0 - l1)/(y0 - y1) + l1;
    } else if (texCoord.y > y2) {
        level = (texCoord.y - y2) * (l1 - l2)/(y1 - y2) + l2;
    } else if (texCoord.y > y3) {
        level = (texCoord.y - y3) * (l2 - l3)/(y2 - y3) + l3;
    } else if (texCoord.y > y4) {
        level = (texCoord.y - y4) * (l3 - l4)/(y3 - y4) + l4;
    }

    gl_FragColor = vec4(level, level, level, 1.0);
}