图片压缩 浅析

原文链接：http://www.jianshu.com/p/fb6a072fba47

最近做的项目中需要上传头像，发表内容的时候也要涉及到图片上传，我直接用的原图上传，但是由于公司网络差，原图太大，老是加载好久好久，所以需要把原图裁剪或者修改分辨率之后再上传，找了好久，做了很多尝试才解决问题，下面记录一下，以便帮助自己查找，也希望能帮助其他人。

我们一般图片上传最好使用UIImageJPEGRepresentation(image, 0.1)这个方法先把图片处理一下，其实这个方法把图片压缩之后，图片大小会小很多（小多少待会再讲），但是在app上人眼的感觉差不多，我对图片的处理如下：

从相册拿到图片后，先经过方法修改图片的大小，也就是分辨率，方法如下：

- (UIImage *)scaleImage:(UIImage *)image toSize:(CGSize)size
{
    UIGraphicsBeginImageContextWithOptions(size,NO,0.0);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return scaledImage;
}

因为我是要上传头像，所以我把图片的size设置为(128,128)，这里我在模拟器中截取到了一个320*320的图片，然后经过这个方法压缩后得到的图片是20.3KB，然后我在上传之前又用UIImageJPEGRepresentation(image, 0.05)把图片品质压缩了一下，图片的大小变成了1.5KB左右，这样的小图，在app上加载起来快急了，之前那个七八十KB的图片经常都加载不出来呢，现在OK了~。

总结：1、用 - (UIImage *)scaleImage:(UIImage *)image toSize:(CGSize)size 修改图片的分辨率；

  2、用 UIImageJPEGRepresentation(image,0.1)这个方法来压缩图片品质。

我们可以在不减小图片的分辨率（质量可以适当减小）的情况下，显著减小图片的大小

float kCompressionQuality = 0.3; // 具体大小自己调
NSData *photo = UIImageJPEGRepresentation(UIImage, kCompressionQuality);

上面方法等价于下面: 压缩图片质量

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

//将图片压缩到指定比例

-(UIImage *)scaleToSize:(UIImage *)aImage size:(CGSize)size{

    //创建context,并将其设置为正在使用的context
    UIGraphicsBeginImageContext(size);
    //绘制出图片(大小已经改变)
    [aImage drawInRect:CGRectMake(0, 0, size.width, size.height)];
    //获取改变大小之后的图片
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    //context出栈
    UIGraphicsEndImageContext();
    return newImage; //返回获得的图片
}

等比例压缩

//等比例压缩
-(UIImage *)oldImage:(UIImage *)oldImage toSize:(CGSize)size{

    UIImage *newImage = nil;//新照片对象
    CGSize theSize = oldImage.size;//压缩前图片size

    CGFloat width = theSize.width; //压缩前图片width
    CGFloat height = theSize.height;//压缩前图片height

    CGFloat newWidth = size.width; //压缩后图片width
    CGFloat newHeight = size.height;//压缩后图片height

    CGFloat scaleFactor = 0.0;//初值

    CGFloat toWidth = newWidth;//压缩后图片width
    CGFloat toHeight = newHeight;//压缩后图片height

    CGPoint thumnailPoint = CGPointMake(0.0, 0.0);//给初值

    if (CGSizeEqualToSize(theSize, size) == NO) {
        //判断是不是已经满足 theSize = size 要求

        CGFloat widthFac = newWidth/width;
        CGFloat heithrFac = newHeight/height;
        if (widthFac > heithrFac) {
            scaleFactor = widthFac;
        }else {
            scaleFactor = heithrFac;
        }
        //不满足做等比例缩小处理
        toWidth = width *scaleFactor;
        toHeight = height *scaleFactor;

        if (widthFac > heithrFac) {
            thumnailPoint.y = (newHeight - toHeight)* 0.5;
        }else if (widthFac < heithrFac){
            thumnailPoint.x = (newWidth - toWidth)* 0.5;
        }
    }

    //创建context,并将其设置为正在使用的context
    UIGraphicsBeginImageContext(size);
    CGRect thumbnailRect  = CGRectZero;
    thumbnailRect.origin = thumnailPoint;
    thumbnailRect.size.width = toWidth;
    thumbnailRect.size.height = toHeight;

    //绘制出图片(大小已经改变)
    [oldImage drawInRect:thumbnailRect];

    newImage = UIGraphicsGetImageFromCurrentImageContext();

    //结果判断
    if (newImage == nil) {
        [NSException exceptionWithName:@"提示" reason:@"Error:image scale fail" userInfo:nil];
    }
    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;
}

图片的压缩其实是俩概念，
1、是 “压” 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，
2、是 “缩” 文件的尺寸变小，也就是像素数减少。长宽尺寸变小，文件体积同样会减小。
这个 UIImageJPEGRepresentation(image, 0.0)，是1的功能。
这个 [sourceImage drawInRect:CGRectMake(0,0,targetWidth, targetHeight)] 是2的功能。
所以，这俩你得结合使用来满足需求，不然你一味的用1，导致，图片模糊的不行，但是尺寸还是很大。

我们还可以对图片进行部分截取

-(UIImage*)getSubImage:(CGRect)rect
{
    CGImageRefsubImageRef =CGImageCreateWithImageInRect(self.CGImage,rect);
    CGRectsmallBounds =CGRectMake(0,0, CGImageGetWidth(subImageRef),CGImageGetHeight(subImageRef));

    UIGraphicsBeginImageContext(smallBounds.size);
    CGContextRefcontext =UIGraphicsGetCurrentContext();
    CGContextDrawImage(context,smallBounds,subImageRef);
    UIImage*smallImage =[UIImage imageWithCGImage:subImageRef];
    UIGraphicsEndImageContext();

    returnsmallImage;
}

//--------------截取部分图片到指定位置-------------------------

图片(UIImage*)img
要截取的起始坐标sx:(int)sx1 sy:(int)sy1
要截取的长度和宽度sw:(int)sw1 sh:(int)sh1
最终要显示的坐标desx:(int)desx1 desy:(int)desy1

-(UIImage*)objectiveDrawRegion:(UIImage*)img sx:(int)sx1 sy:(int)sy1sw:(int)sw1 sh:(int)sh1 desx:(int)desx1 desy:(int)desy1{
[selfsaveImage:imgname:@"objectiveDrawRegion1.png"];

//创建图片缓冲
void*imageDataRegion=malloc(screenWidth*screenHeight*32);
CGColorSpaceRefiColorSpaceRegion=CGColorSpaceCreateDeviceRGB();
    CGContextRefiDeviceRegion=CGBitmapContextCreate(imageDataRegion,screenWidth,screenHeight,8,4*screenWidth,iColorSpaceRegion,kCGImageAlphaPremultipliedLast);

//剪切区域
    CGRectclipRegion=CGRectMake(sx1,sy1,sw1,sh1);
    CGContextClipToRect(iDeviceRegion,clipRegion);

    CGFloatwidthf=img.size.width;
    CGFloatheightf=img.size.height;

CGRect  cg=CGRectMake(0.0,0.0, widthf, heightf);
//画底图
    CGContextDrawImage(iDeviceRegion,cg,img.CGImage);

//将缓冲形成图片
    CGImageRefioffRegion=CGBitmapContextCreateImage(iDeviceRegion);

CGRect  cg1=CGRectMake(desx1,desy1, sw1, sh1);
UIImage *ui=[UIImageimageWithCGImage:ioffRegion];

CGContextDrawImage(当前context,cg1,ui.CGImage);

//清除缓冲
   CGColorSpaceRelease(iColorSpaceRegion);
  CGContextRelease(iDeviceRegion);
   CGImageRelease(ioffRegion);
  free(imageDataRegion);
//    iDeviceRegion=NULL;
//    imageDataRegion=0;

returnui;
}

参考：http://www.henishuo.com/ios-image-compressed/