ShaderToy demo

shader-toy-demo.glsl

float utilityFunction(float t, float value)
{
    return t;
}

float subFunction(float t, vec2 uv)
{
        return utilityFunction(t, texture(iChannel0, uv)[0]);
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 v_uv = fragCoord/iResolution.xy;
    float a = subFunction(0.5, v_uv);
    fragColor = vec4(1.0,a,0.0,1.0);
}