acessing a 1D array with row/col

imagePtr[i*cols + j]
where i and j are the row/col coordinates. this isn't right, i can't remember how else to do it. i know it's something LIKE this, just not exactly what, and google isn't cooperating. do i need to use the memory locations?

>>7
it's supposed to be the negative of the grey scale color pixel value stored in (i,j), where maxval is the maximum value of the scale.