iOS图片压缩方案

作者: 愤怒小鸟飞呀飞 | 来源:发表于2019-07-09 18:02 被阅读0次

一、概述

“压” 是指文件体积变小，但是像素数不变，长宽尺寸不变，那么质量可能下降。（图片压完之后，占用内存的bitMap不变）
“缩” 是指文件的尺寸变小，也就是像素数减少，而长宽尺寸变小，文件体积同样会减小。

二、“压”

1、图片质量压缩原理
采用一些算法，将几个像素点用一个像素点的数据表示
2、图片压完之后，占用内存的bitMap不变论证：

UIImage *image = [UIImage imageNamed:@"WechatIMG435.jpeg"];
    NSData *imgData = [NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"WechatIMG435" ofType:@"jpeg"]];
    NSLog(@"imgData:%ld",imgData.length);
    
    ;
    CFDataRef rawData = CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage));
    NSLog(@"rawData.length:%ld",(long)CFDataGetLength(rawData));
    
    NSLog(@"----------------------我是分割线---------------------");
    
    NSData *compressData = UIImageJPEGRepresentation(image, 0.1);
    UIImage *compressImg = [UIImage imageWithData:compressData];
    
    
    NSString *newPath = [NSString stringWithFormat:@"%@/%@",[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) firstObject],@"compressImg.jpeg"];
   BOOL a =  [compressData writeToFile:newPath atomically:YES];
    NSLog(@"imgData:%ld",compressData.length);
    CFDataRef compressRawData = CGDataProviderCopyData(CGImageGetDataProvider(compressImg.CGImage));
    NSLog(@"rawData.length:%ld",(long)CFDataGetLength(compressRawData));

打印结果：

2019-07-09 18:01:38.172504+0800 Img[47267:15114761] libMobileGestalt MobileGestalt.c:890: MGIsDeviceOneOfType is not supported on this platform.
2019-07-09 18:01:38.263858+0800 Img[47267:15114761] [framework] CUIThemeStore: No theme registered with id=0
2019-07-09 18:01:38.330603+0800 Img[47267:15114761] imgData:10983
2019-07-09 18:01:38.331847+0800 Img[47267:15114761] rawData.length:480000
2019-07-09 18:01:38.331955+0800 Img[47267:15114761] ----------------------我是分割线---------------------
2019-07-09 18:01:38.334699+0800 Img[47267:15114761] imgData:5560
2019-07-09 18:01:38.335806+0800 Img[47267:15114761] rawData.length:480000

如上，图片"压"后，占用内存的bitmap没有改变

三、“缩”
1、原理
图片尺寸压缩，减小体积，图片bitmap减小
2、方案
使用ImageIO接口，避免在改变图片大小的过程中产生临时的bitmap，就能够在很大程度上减少内存的占有从而避免由此导致的app闪退问题。

    UIImage *image = [UIImage imageNamed:@"WechatIMG435.jpeg"];
    NSData *imgData = [NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"WechatIMG435" ofType:@"jpeg"]];
    NSLog(@"imgData:%ld",imgData.length);
    
    ;
    CFDataRef rawData = CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage));
    NSLog(@"rawData.length:%ld",(long)CFDataGetLength(rawData));
    
    NSLog(@"----------------------我是分割线---------------------");
    
    UIImage *sizeCompressImg = [self thumbnailForData:imgData maxPixelSize:image.size.width * 0.25];
    CFDataRef sizeCompressRawData = CGDataProviderCopyData(CGImageGetDataProvider(sizeCompressImg.CGImage));
    NSLog(@"rawData.length:%ld",(long)CFDataGetLength(sizeCompressRawData));

打印结果

2019-07-09 20:10:04.300806+0800 Img[48279:15418157] libMobileGestalt MobileGestalt.c:890: MGIsDeviceOneOfType is not supported on this platform.
2019-07-09 20:10:04.378632+0800 Img[48279:15418157] [framework] CUIThemeStore: No theme registered with id=0
2019-07-09 20:10:04.448211+0800 Img[48279:15418157] imgData:10983
2019-07-09 20:10:04.449632+0800 Img[48279:15418157] rawData.length:480000
2019-07-09 20:10:04.449805+0800 Img[48279:15418157] ----------------------我是分割线---------------------
2019-07-09 20:10:04.450898+0800 Img[48279:15418157] rawData.length:30000

分析：图片bitmap占用从 480000 减小到 30000减小了16倍

image.png

四、图片质量压缩方案探索
1、分次循环压缩

    ///  对图片进行质量压缩
    ///
    /// - Parameter targetByte: 压缩目标字节数
    /// - Returns: 压缩之后的data 对象 
    func compressByQulity(toTargetByte targetByte:NSInteger)->NSData?{
        let bigImage = self
        
        let imgData = bigImage.dataFromImage()
        guard imgData != nil else{
            return nil
        }
        var targetImageData:Data = imgData!
        print("lalala compress before:%d",targetImageData.count)
        //分次循环压缩
        //当前imgData 大小
        var targetImageDataLength:Int = targetImageData.count
        //上次压缩imgData 大小
        var lastTargetImageDataLength : Int = 0
        //png jpeg应机器学习同学要求，同时采用jpeg压缩格式
        while(targetImageDataLength > targetByte ){
            //上次压缩图片大小赋值
            lastTargetImageDataLength = targetImageDataLength;
            
            //压缩
            let img =  UIImage.init(data: targetImageData)
            guard img != nil else{
                break
            }
            let compressImgData = img!.jpegData(compressionQuality: CGFloat(UIImage.qualityPerCompressRate))
            guard compressImgData != nil else{
                break
            }
            targetImageData = compressImgData!
            targetImageDataLength = targetImageData.count
            guard targetImageDataLength < lastTargetImageDataLength else {
                break
            }
        }
        print("lalala compress after:%d",targetImageData.count)
        return targetImageData as NSData
    }

lalala =-------我是分割线-----------------
lalala compress before:%d 1811323
lalala compress after:%d 543473
lalala compress after:%d 544901
lalala =-------我是分割线-----------------
lalala compress before:%d 1559291
lalala compress after:%d 526630
lalala compress after:%d 527984
lalala =-------我是分割线-----------------
lalala compress before:%d 2522415
lalala compress after:%d 766597
lalala compress after:%d 767281
lalala =-------我是分割线-----------------
lalala compress before:%d 2287580
lalala compress after:%d 753524
lalala compress after:%d 754047

以上算法存在以下问题
（1）、第二次一定比第一次大，所以只能跑一遍
（2）、UIImage 与 data互转来回互转 data数据会改变即 UIImageJPEGRepresentation 1.0 从本地读取到data数据也会改变，图片的质量也会受到影响

  UIImage *image = [UIImage imageNamed:@"WechatIMG435.jpeg"];
    NSData *imgData = [NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"WechatIMG435" ofType:@"jpeg"]];
    NSLog(@"imgData:%ld",imgData.length);
    
    NSData *jpegData = UIImageJPEGRepresentation(image, 1.0);
    NSLog(@"jpegData:%ld",jpegData.length);
    
    UIImage *newImage = [UIImage imageWithData:jpegData];
    NSData *newData = UIImageJPEGRepresentation(newImage, 1.0);
    NSLog(@"newData:%ld",newData.length);

打印结果：

2019-07-09 20:56:55.355619+0800 Img[48690:15554037] imgData:10983
2019-07-09 20:56:55.359325+0800 Img[48690:15554037] jpegData:47681
2019-07-09 20:56:55.363122+0800 Img[48690:15554037] newData:51046

2、二分法寻找最佳压缩率
(1)UIImageJPEGRepresentation(image, scale) API调研
a、图片压缩测试原图大小：7801942

scale	实际	正常比例计算
0.1	114823	780192
0.2	127503	1560384
0.3	156226	2340577
0.4	204067
0.5	261299
0.6	331804
0.7	438994
0.8	508924
0.9	640913

从上图得出实际压缩率不是传入的值，实际压缩率一定大于压缩结果
b、压缩极限测试
原图大小：193737
压缩比率
0.01 113710
对上面压缩之后的值再次压缩
0.01 113710
压缩到极限值之后，压缩之后的数据，将不在压缩
（2）、压缩目标
压缩目标物理大小 500k,容错范围 5000.8 ~~5001.2
（3）、UIImageJPEGRepresentation能力之外数据处理方案
a、图片压缩0.01之后还是不能到500k,直接返回压缩最小值
b、循环到第6次数，压缩率为0.984，已经压缩为最大值，还没有进入理想数据范围，将不在进行压缩，直接返回UIImageJPEGRepresentation 1.0压缩数值
c、本来就是有损压缩，无论png还是jpeg 统一使用jpeg压缩方案与UIImage->data转化方法

 func compressByQulity(toTargetByte targetByte:NSInteger)->NSData?{
        let bigImage = self
        
        //本来就是有损压缩，无论png还是jpeg 统一使用jpeg 注意：如果为png格式，转化之后filesize会比源文件小很多，这里将进行一次有损压缩
        let imgData = bigImage.jpegData(compressionQuality: 1.0)
        guard imgData != nil else{
            return nil
        }
        
        var targetImageData:Data = imgData!
        print("lalala =-------我是分割线-----------------")
        print("lalala compress before---",targetImageData.count)
        guard targetImageData.count > targetByte else {
            print("lalala compress after:%d",targetImageData.count)
            return targetImageData as NSData
        }
        
        //分次循环寻找compressRate大小
        //png jpeg应机器学习同学要求，同时采用jpeg压缩格式
        var compressionRate:CGFloat = 1.0
        var max:CGFloat = 1
        var min:CGFloat = 0
        //指数二分处理，s首先计算最小值
        compressionRate = CGFloat(pow(2.0, -6.0))
        let smallestImgData = bigImage.jpegData(compressionQuality: compressionRate)
        guard smallestImgData != nil else {
            return nil
        }
        
        if smallestImgData!.count < targetByte{
            for _ in 0..<6 {
                compressionRate = (max + min)/2.0
                let compressData = bigImage.jpegData(compressionQuality: compressionRate)
                guard compressData != nil else {
                    break
                }
                
                //容错区间范围0.8～1.2 409600 614400
                if compressData!.count < Int(Double(targetByte) * 0.8){
                    min = compressionRate
                }else if compressData!.count > Int(Double(targetByte) * 1.2){
                    max = compressionRate
                }else{
                    //找到压缩最佳值
                    targetImageData = compressData!
                    print("lalala ---0.9:%d~~~1.1:%d",Int(Double(targetByte) * 0.8),Int(Double(targetByte) * 1.2))
                    break
                }
                print("lalala ---\(compressionRate)----compressDatalenth:%d",compressData?.count)
            }
        }
        print("lalala compress after:%d",targetImageData.count)
        return targetImageData as NSData
    }

原因：image data 相互转换 data忽大忽小
结果：可能是300k,但是已经是jpeg能提供的最佳压缩率了

lalala compress before--- 1006180
lalala ---0.5----compressDatalenth:%d Optional(156856)
lalala ---0.75----compressDatalenth:%d Optional(266978)
lalala ---0.875----compressDatalenth:%d Optional(321495)
lalala ---0.9375----compressDatalenth:%d lalala ---0.96875----compressDatalenth:%d Optional(352076)
lalala ---0.984375----compressDatalenth:%d Optional(359699)

此方案基于 UIImageJPEGRepresentation 传入参数值由小到大，压缩值也由小变大规律
规律论证如下：

参考链接：iOS图片压缩处理
 为什么实际大小和占用空间不一样

iOS优秀的图片压缩处理方案（系统UIimagejpeg和uikit以及imageI/处理)
图像压缩原理
 图片有损压缩原理

iOS图片压缩方案

一、概述

二、“压”

相关文章

网友评论

延伸阅读

深度阅读

栏目导航

热点阅读

iOS架构