__global__ void
d_calculateShadowness(float * shadowness,
                    bool * imMASK,
                    float * imBLURRED,
                    float * shadowM,
                    float * shadowMsum,
                    unsigned int imH,
                    unsigned int imW
                    ){
    //calculate shadowness, obviously
    unsigned int x = threadIdx.x; //0...imW
    float s = 0;
    unsigned int idx;
    for(unsigned int i=imH-1; i>0; i--){
        idx = i*imW+x;
        shadowness[idx] = imMASK[idx]?s/shadowMsum[i]:INVALID_VALUE;
        s += imBLURRED[idx]*shadowM[i];
    }
}