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vec4 shader(vec2 texcoord)
{
    vec4 src = texture2D(texture, texcoord);

    // Luminance
    float lum = dot(src.rgb, vec3(0.299, 0.587, 0.114));

    // Simple scanlines
    float scan = 0.85 + 0.15 * sin(texcoord.y * 700.0);

    // Green phosphor
    vec3 green = vec3(0.0, lum, 0.0) * scan;

    return vec4(green, 1.0);
}