有时候从网络请求的图片分辨率高，缩小显示反而变模糊，主要是因为手机显示scale，导致图片锐化，可以

- (UIImage *)imageCompressWithSimple:(UIImage*)image scaledToSize:(CGSize)size {
    
    //Determine whether the screen is retina
    if([[UIScreen mainScreen] scale] == 2.0){
        UIGraphicsBeginImageContextWithOptions(size, NO, 2.0);
    }else if([[UIScreen mainScreen] scale] == 3.0) {
        UIGraphicsBeginImageContextWithOptions(size, NO, 3.0);
    }else{
        UIGraphicsBeginImageContext(size);
        
    }
    [image drawInRect:CGRectMake(0,0,size.width,size.height)];
    UIImage*newImage=UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}