frameDrawer = [[AVFrameDrawer alloc] initWithSize:CGSizeMake(targetWidth, targetHeight)
                               contextInitailizeBlock:contextInitialization];
    
    frameDrawer.contextUpdateBlock = ^BOOL(CGContextRef context, CGSize size, CMTime time) {

        //
        // video overlay CoreGraphics code goes here
        //
        
        return YES; // return YES if context was modified
    };