Write both files to disk

gistfile1.m

NSRect offscreenRect = NSMakeRect(0.0, 0.0, 20.0, 12.0);
NSBitmapImageRep* bitmap = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:nil
                                                                   pixelsWide:offscreenRect.size.width
                                                                   pixelsHigh:offscreenRect.size.height
                                                                bitsPerSample:8
                                                              samplesPerPixel:4
                                                                     hasAlpha:YES
                                                                     isPlanar:NO
                                                               colorSpaceName:NSCalibratedRGBColorSpace
                                                                 bitmapFormat:0
                                                                  bytesPerRow:(4 * offscreenRect.size.width)
                                                                 bitsPerPixel:32];

NSGraphicsContext* originalContext = [NSGraphicsContext currentContext];
[NSGraphicsContext setCurrentContext:[NSGraphicsContext graphicsContextWithBitmapImageRep:bitmap]];

[NSGraphicsContext saveGraphicsState];

//clear the image rep. This is faster than filling with [NSColor clearColor].
unsigned char *bitmapData = [bitmap bitmapData];
if (bitmapData)
    bzero(bitmapData, [bitmap bytesPerRow] * [bitmap pixelsHigh]);

NSRect shadowRect = centeredRect;
shadowRect.origin.y -= 1.0f;
[shadowImage drawInRect:NSMakeRect(0, 0, 20, 11) fromRect:NSRectFromCGRect([shadowImage extent]) operation:NSCompositeSourceOver fraction:0.75f];
[drawnImage drawInRect:NSMakeRect(0, 1, 20, 11) fromRect:NSZeroRect operation:NSCompositeSourceOver fraction:1.0];

//restore the original graphics context
[NSGraphicsContext restoreGraphicsState];
[NSGraphicsContext setCurrentContext:originalContext];

//get PNG data from the image rep
NSData* pngData = [bitmap representationUsingType:NSPNGFileType properties:nil];
NSError* error;
if(![pngData writeToURL:[NSURL fileURLWithPath:[NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:@"%ld.png", frameNumber+1]]] options:NSDataWritingAtomic error:&error])
{
    NSLog(@"%@",error);
}