+
+ if (alpha != NULL)
+ {
+ // src_alpha starts at the first pixel and increases by 1 after each step
+ // (a step here is the copy of the alpha value of one pixel)
+ const unsigned char *src_alpha = M_IMGDATA->m_alpha;
+ // dest_alpha starts just beyond the first line, decreases before each step,
+ // and after each line is finished, increases by 2 widths (skipping the line
+ // just copied and the line that will be copied next)
+ unsigned char *dest_alpha = alpha + width;
+
+ for (long jj = 0; jj < height; ++jj)
+ {
+ for (long i = 0; i < width; ++i) {
+ *(--dest_alpha) = *(src_alpha++); // copy one pixel
+ }
+ dest_alpha += 2 * width; // advance beyond the end of the next line
+ }
+ }