bresenham.c

static void
render_line(struct v2 from, struct v2 to, u32 *pixels, int width, int height, u32 color)
{
    if (from.x >= width) { 
        from.x = width - 1;
    }
    
    if (to.x >= width) {
        to.x = width - 1;
    }
    
    if (from.y >= height) {
        from.y = height - 1;
    }
    
    if (to.y >= height) {
        to.y = height - 1;
    }
    
    if (from.y == to.y) {
        if (to.x < from.x) {
            int tmp = to.x;
            to.x = from.x;
            from.x = tmp;
        }
        
        u32 *out = pixels + width * from.y + from.x;
        // printf("%ld %ld -> %ld %ld\n", from.x, from.y, to.x, to.y);
        
        for (int x = from.x; x <= to.x; ++x) {
            // printf("%d\n", x); 
            *out = color;
            ++out;
        }
    } else if (from.x == to.x) {
        if (to.y < from.y) {
            int tmp = to.y;
            to.y = from.y;
            from.y = tmp;
        }
        
        u32 *out = pixels + width * from.y + from.x;
        
        //printf("%ld %ld -> %ld %ld\n", from.x, from.y, to.x, to.y);
        
        for (int y = from.y; y <= to.y; ++y) {
            out = pixels + width * y + from.x;
            *out = color;
        }
    } else {
        int x0 = from.x;
        int y0 = from.y;
        int x1 = to.x;
        int y1 = to.y;
        
        int dx = abs(x1 - x0);
        int sx = (x0 < x1 ? 1 : -1);
        int dy = -abs(y1 - y0);
        int sy = (y0 < y1 ? 1 : -1);
        int err = dx+dy;
        
        while (0 <= x0 && x0 < width && 0 <= y0 && y0 < height) {
            u32 *out = pixels + width * y0 + x0;
            *out = color;
            
            if (x0 == x1 && y0 == y1) {
                break;
            }
            
            int e2 = 2 * err;
            
            if (e2 >= dy) {
                err += dy;
                x0 += sx;
            }
            
            if (e2 <= dx) {
                err += dx;
                y0 += sy;
            }
        }
    }
}