determine padding for scanlines in copy.c

Question

Line 65 in copy.c

int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

Question is why do we need last modulo operator in bold? Even without it, it gives the right answer.

Answer 1

It's not needed for a padding of 1, 2, or 3 bytes.

But imagine what happened if bi.biWidth were a multiple of 4.

bi.biWidth * sizeof(RGBTRIPLE) would be a multiple of 4.

(bi.biWidth * sizeof(RGBTRIPLE)) % 4 would be 0.

4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4 would be 4, but should be 0. You'd fix that by applying another %4.