EL_AT( 1, 0);
            vec4 p31 = PIXEL_AT(-1, 1); vec4 p32 = PIXEL_AT( 0, 1); vec4 p33 = PIXEL_AT( 1, 1);

            if( u_step == 0 )
            {
                // https://en.wikipedia.org/wiki/Sobel_operator
                //        |+1  0 -1|        |+1 +2 +1|
                //  hor = |+2  0 -2|   ver =| 0  0  0|
                //        |+1  0 -1|        |-1 -2 -1|

                float hor = ( p11.r * 1.0 + p12.r * 2.0 + p13.r * 1.0 ) + ( p31.r * -1.0 + p32.r * -2.0 + p33.r * -1.0 );
     