kernel void toGrey(read_only image2d_t srcImg, write_only image2d_t dstImg)
{
  const sampler_t smp =
    CLK_NORMALIZED_COORDS_FALSE | //Natural coordinates
    CLK_ADDRESS_CLAMP_TO_EDGE | //Clamp to zeros
    CLK_FILTER_LINEAR;

  int2 coord = (int2)(get_global_id(0), get_global_id(1));

  uint4 rgba = read_imageui(srcImg, smp, coord); //The byte order is RGBA

  float4 rgbafloat = convert_float4(rgba) / 255.0f; //Convert to normalized [0..1] float

  //Convert RGB to luminance (make the image grayscale).
  float luminance =  sqrt(0.241f * rgbafloat.z * rgbafloat.z + 0.691f *
                      rgbafloat.y * rgbafloat.y + 0.068f * rgbafloat.x * rgbafloat.x);

  rgba.x = rgba.y = rgba.z = (uint) (luminance * 255.0f);
  rgba.w = 255;

  write_imageui(dstImg, coord, rgba);
}