介绍：

压: 指文件体积变小，但是像素数不变，长宽尺寸不变，那么质量可能下降。
缩: 指文件的尺寸变小，也就是像素数减少，而长宽尺寸变小，文件体积同样会减小。

应用:

在实际开发中,我们经常会对图片进行处理,满足开发需求,以下介绍三种图片压缩处理:

1.压缩图片质量(图片体积减小,像素不变)

两种读取图片数据的简单方法:
(1).UIImageJPEGRepresentation函数需要两个参数:图片的引用和压缩系数,压缩体积不是随压缩系数比例变化的。
(2).UIImagePNGRepresentation只需要图片引用作为参数。
注意:
UIImagePNGRepresentation(image) 要比UIImageJPEGRepresentation(image, 1.0) 返回的图片数据量大很多
建议优先使用 UIImageJPEGRepresentation ,并可根据自己的实际使用场景,设置压缩系数,进一步降低图片数据量大小。

+(UIImage *)reduceImage:(UIImage *)image percent:(float)percent
{
   NSData *imageData = UIImageJPEGRepresentation(image, percent);
   UIImage *newImage = [UIImage imageWithData:imageData];
   return newImage;
}

2.图片压缩到指定尺寸
+ (UIImage *) scaleImage:(UIImage *) image withNewSize:(CGSize) newSize
{
UIGraphicsBeginImageContext(newSize);
[image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}

3.压缩到指定尺寸等比例压缩

 -(UIImage *) imageCompressForSize:(UIImage *)sourceImage targetSize:(CGSize)size{
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = size.width;
    CGFloat targetHeight = size.height;
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0, 0.0);
    if(CGSizeEqualToSize(imageSize, size) == NO){
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        if(widthFactor > heightFactor){
            scaleFactor = widthFactor;
        }
        else{
            scaleFactor = heightFactor;
        }
        scaledWidth = width * scaleFactor;
        scaledHeight = height * scaleFactor;
        if(widthFactor > heightFactor){
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
        }else if(widthFactor < heightFactor){
        thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }

    UIGraphicsBeginImageContext(size);
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width = scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    [sourceImage drawInRect:thumbnailRect];
    newImage = UIGraphicsGetImageFromCurrentImageContext();

    if(newImage == nil){
        NSLog(@"scale image fail");
   }
    UIGraphicsEndImageContext();
    return newImage;
}

-(UIImage *) imageCompressForWidth:(UIImage *)sourceImage targetWidth:(CGFloat)defineWidth
{
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = defineWidth;
    CGFloat targetHeight = height / (width / targetWidth);
    CGSize size = CGSizeMake(targetWidth, targetHeight);
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0, 0.0);
    if(CGSizeEqualToSize(imageSize, size) == NO){
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
    if(widthFactor > heightFactor){
        scaleFactor = widthFactor;
    }
    else{
        scaleFactor = heightFactor;
    }
    scaledWidth = width * scaleFactor;
    scaledHeight = height * scaleFactor;
    if(widthFactor > heightFactor){
        thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
     }else if(widthFactor < heightFactor){
        thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }
    UIGraphicsBeginImageContext(size);
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width = scaledWidth;
    thumbnailRect.size.height = scaledHeight;

    [sourceImage drawInRect:thumbnailRect];

    newImage = UIGraphicsGetImageFromCurrentImageContext();
    if(newImage == nil){
       NSLog(@"scale image fail");
    }
    UIGraphicsEndImageContext();
    return newImage;
}

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