iOS图片叠加

在做QQ、微信分享时，需要做图片叠加。因服务端返回的图片一般是jpg(1x)，和本地图片的scale(2x或3x)不一致；大小也可能不一致，做相应等比的放大缩小。

//jpg的scale是1x，一般以本地的scale为准，本地和网络图宽高比一致
- (UIImage *)componseImage:(UIImage *)netImg localImage:(UIImage *)localImg {
    UIGraphicsBeginImageContextWithOptions(localImg.size, NO, localImg.scale);
    netImg = [self scaleImage:netImg widthScale:localImg.size.width/netImg.size.width heightScale:localImg.size.height/netImg.size.height imgScale:localImg.scale];
    [netImg drawInRect:CGRectMake(0, 0, netImg.size.width, netImg.size.height)];
    [localImg drawInRect:CGRectMake(0, 0, localImg.size.width, localImg.size.height)];
    UIImage *resultImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return resultImage;
}

- (UIImage *)scaleImage:(UIImage *)image
             widthScale:(float)widthScale
            heightScale:(float)heightScale
               imgScale:(CGFloat)imgScale
{
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(image.size.width * widthScale, image.size.height * heightScale), NO, imgScale);
    [image drawInRect:CGRectMake(0, 0, image.size.width * widthScale, image.size.height * heightScale)];
    UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return scaledImage;
}