IOS图片压缩

iPhone拍出来的照片一般大小在1-10M,我们在上传照片时，不可能上传如此大的图片到服务器，一般我们会对照片进行压缩。

常用的做法是，使用这个函数对图片压缩 UIImageJPEGRepresentation(UIImage * __nonnull image, CGFloat compressionQuality); 但这个函数有一个临界值，不能对图片无限制压缩，一般到当压缩比传入0.1已达到临界值了。这时如果我想把5M的图片压缩到50K是行不通的。

如果图太大，我们要求压缩的比例太大就不能使用以上的方法来压缩，这时就要先对图片裁剪，裁剪后再使用这个函数压缩，这样才能达到想要的大小。为了不使图片变形，裁剪时要按原图的宽高比来裁剪。可以写成UIImage的分类方法代码如下：

- (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize
{
    UIImage *sourceImage = self;
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = targetSize.width;
    CGFloat targetHeight = targetSize.height;
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0,0.0);
    
    if (CGSizeEqualToSize(imageSize, targetSize) == NO)
    {
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        
        if (widthFactor > heightFactor)
            scaleFactor = widthFactor; // scale to fit height
        else
            scaleFactor = heightFactor; // scale to fit width
        scaledWidth= width * scaleFactor;
        scaledHeight = height * scaleFactor;
        
        // center the image
        if (widthFactor > heightFactor)
        {
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
        }
        else if (widthFactor < heightFactor)
        {
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }
    
    UIGraphicsBeginImageContext(targetSize); // this will crop
    
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width= scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    
    [sourceImage drawInRect:thumbnailRect];
    
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    if(newImage == nil)
        NSLog(@"could not scale image");
    
    //pop the context to get back to the default
    UIGraphicsEndImageContext();
    return newImage;
}

然后给系统工具类写一个压缩图片的类方法，根据不同大小的图片采用不同大小的裁剪比和压缩比，将压缩后的图片保证在50K左右的大小。

+ (NSData *)compressWithOrgImg:(UIImage *)img
{
    
    NSData *imageData = UIImageJPEGRepresentation(img, 1);
    float length = imageData.length;
    length = length/1024;
    NSLog(@"压缩前的大小：%fKB",length);
    // 裁剪比例
    CGFloat cout = 0.5;
    
    // 压缩比例
    CGFloat imgCout = 0.1;
    if(length > 25000){ // 25M以上的图片
        cout = 0.1;
        imgCout = 0;
    }else if(length > 10000){ // 10M以上的图片
        cout = 0.2;
        imgCout = 0;
    }else if (length > 5000) { // 5M以上的图片
        cout = 0.3;
        imgCout = 0;
    }else if (length > 1500) { // 如果原图大于1.5M就换一个压缩级别
        cout = 0.7;
        imgCout = 0.1;
    }else if (length > 1000) {
        cout = 0.8;
        imgCout = 0.2;
    }else if (length > 500) {
        cout = 0.8;
        imgCout = 0.3;
    }else{
        cout = 0.9;
        imgCout = 50/length;
    }
    
    
    // 按裁剪比例裁剪
    UIImage *compressImage =  [img imageByScalingAndCroppingForSize:CGSizeMake(img.size.width * cout, img.size.height *cout)];
    
    
    // 那压缩比例压缩
    imageData = UIImageJPEGRepresentation(compressImage, imgCout);
    
    length= imageData.length / 1024;
    NSLog(@"裁剪比例：%f，压缩比例：%f,压缩后的大小：%fKB",cout,imgCout,length);
    return imageData;
}