ivec2 itexSize =textureSize(rbga_tex,0);
       vec2 texSize =  vec2(itexSize);
       vec2 texelSize = 1.0 / (texSize / 2.0);
       vec2 pixel = uv * texSize + 0.5;
       pixel = (floor(pixel) / texSize) - vec2(texelSize/2.0);

        //bilinear filtering:
            float sc = 4.0;
          vec3 ts = texture(rbga_tex,   pixel ).rgb * 0.25; // center
          vec3 tr = texture(rbga_tex,   pixel + vec2(texelSize.x * sc, 0.0) ).rgb  * 0.25;
          vec3 bl = texture(rbga_tex,   pixel + vec2(0.0, texelSize.y * sc) ).rgb * 0.25;
          vec3 br = texture(rbga_tex,   pixel + vec2(texelSize.x, texelSize.y) * sc).rgb * 0.25;

         vec3 tex1 = (ts + tr + bl + br) ;


        // calculate and  write U...

        // calculate and write V...