opencl - Need function that mirrors image coordinates outside of boundaries -

opencl - Need function that mirrors image coordinates outside of boundaries -

i performing nxn image convolution on image of dimensions w , h, n much smaller w or h. given coordinate alpha, calculate new coordinate safe_alpha guaranteed lie in bounds. coordinates outside of boundary should mirrored.

i.e., if

alpha = (w-1) +2;

then

safe_alpha = (w-1) -2;

this running on gpu, prefer avoid conditionals in calculation.

mirroring on outer boundary simple:

safe_alpha = -| (w-1) - alpha| + alpha;

but need mirror on both.

edit:

here seems work, not sure if fastest:

safe_alpha = |alpha| + (alpha/(w-1)) * (alpha - (w-1));

i don't know opencl details, can avoid border info mirroring using appropriate convolution window @ edges: convolution window values double image border values. need n/2 array of convolution windows (once derived original n*n window) every 4 border , bit more corners, no conditionals , no devisions @ all.

image-processing opencl