+ (CGImageRef)screenShot:(NSScreen *)screen
{
    CFArrayRef windowsRef = CGWindowListCreate(kCGWindowListOptionOnScreenOnly, kCGNullWindowID);

    NSRect rect = [screen frame];
    NSRect mainRect = [NSScreen mainScreen].frame;
    for (NSScreen *subScreen in [NSScreen screens]) {
        if ((int) subScreen.frame.origin.x == 0 && (int) subScreen.frame.origin.y == 0) {
            mainRect = subScreen.frame;
        }
    }
    rect = NSMakeRect(rect.origin.x, (mainRect.size.height) - (rect.origin.y + rect.size.height), rect.size.width, rect.size.height);

    NSLog(@"screenShot: %@", NSStringFromRect(rect));
    CGImageRef imgRef = CGWindowListCreateImageFromArray(rect, windowsRef, kCGWindowImageDefault);
    CFRelease(windowsRef);

    return imgRef;
}