void mainImage( out vec4 O, vec2 u )
{   vec2  U = u / iResolution.xy;                       // normalized coordinates
    float z = 1.-.7*U.y;                                // depth along the plane
    O = 1.5*textureLod(iChannel0, 
                       vec2( U.x-.5 , 1 )/ z -.1*iTime, // perspective parameterization
                       5.+log2(abs(1./z - 1.5)) ).xxxx; // log2( circle of confusion ) 
  //O = sqrt(O);                                        // cheap sRGB convertion
}