iOS图片压缩

项目中常会遇到，上传图片的操作，由于iPhone手机直接拍照的图片往往比较大，一般3-4M,如果直接上传不做处理会浪费用户很多流量，再者有很多场景并不需要高清图片，所以在上传图片前对图片进行压缩，是很有必要的。

1.OC中的UIKit中提供了现成的压缩函数  UIImageJPEGRepresentation(UIImage * __nonnull image, CGFloat compressionQuality) ，但压缩比率只能是0.1到0.9，如果图片过大，还是无法达到我们想要的效果。

2.对于大图片（10M以上），我们可以先对图片进行裁剪，然后再压缩。这个方法能大大压缩图片的大小。以我的项目中需求为例，需求是：不管多大图片，都要压缩至50kb左右才可以上传到服务器，而且像素不能过度失真。

+ (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 if (length >100){ // 小于500k的不用裁剪
        
        imageData = UIImageJPEGRepresentation(img, 50 / imageData.length);
        float length = imageData.length;
        length = length/1024;
        NSLog(@"压缩后的大小：%fKB",length);
        return imageData;
    }else{
        
        imageData = UIImageJPEGRepresentation(img, 0.5);
        float length = imageData.length;
        length = length/1024;
        NSLog(@"压缩后的大小：%fKB",length);
        return imageData;
    }
    
    
    // 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;
}

// 裁剪
- (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;
}

代码中针对不同大小图片，给出了不同的压缩比率，以保证压缩后的图片大小都在50k左右。此处的“裁剪”是将图片的宽高等比例缩小到指定的比率