19. Invert

Inverts colors on a portion of a region.

void btprnt_invert(btprnt_region *r,
                   int xoff, int yoff,
                   int w, int h);

Later, add bounds checking.

void btprnt_invert(btprnt_region *r,
                   int xoff, int yoff,
                   int w, int h)
{
    int x, y;
    /* TODO: add bounds checking */
    for (x = 0; x < w; x++) {
        for (y = 0; y < h; y ++) {
            int s;
            int xp, yp;
            xp = x + xoff;
            yp = y + yoff;
            s = btprnt_region_read(r, xp, yp);

            if (s) {
                btprnt_region_draw(r, xp, yp, 0);
            } else {
                btprnt_region_draw(r, xp, yp, 1);
            }
        }
    }

}

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