将2张图片叠加绘制成1张图片

- (void)composeImg {

    UIImage *img = [UIImage imageNamed:@"0.png"];

    CGImageRef imgRef = img.CGImage;

    CGFloat w = CGImageGetWidth(imgRef);

    CGFloat h = CGImageGetHeight(imgRef);

    //以1.png的图大小为底图    UIImage *img1 = [UIImage imageNamed:@"1.png"];

    CGImageRef imgRef1 = img1.CGImage;

    CGFloat w1 = CGImageGetWidth(imgRef1);

    CGFloat h1 = CGImageGetHeight(imgRef1);

    //以1.png的图大小为画布创建上下文    UIGraphicsBeginImageContext(CGSizeMake(w1, h1));

    [img1 drawInRect:CGRectMake(0, 0, w1, h1)];//先把1.png 画到上下文中    

[img drawInRect:CGRectMake(100, 100, w, h)];//再把小图放在上下文中  

  UIImage *resultImg = UIGraphicsGetImageFromCurrentImageContext();//从当前上下文中获得最终图片   

 UIGraphicsEndImageContext();//关闭上下文    

    NSString *path = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];

    NSString *filePath = [path stringByAppendingPathComponent:@"01.png"];

    [UIImagePNGRepresentation(resultImg) writeToFile:filePath atomically:YES];//保存图片到沙盒    

   CGImageRelease(imgRef);

    CGImageRelease(imgRef1);

}