SDWebImage主线之图片解码

SDWebImage主线设计的解码大致有两种：普通解码和渐进式解码。本文只对普通解码进行解析。

普通解码又分为正常解码和大图解码。

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普通解码

普通解码从 -[SDWebImageDownloaderOperation URLSession:task:didCompleteWithError:] 发起

更多解析请参考SDWebImage主线梳理（二）

1. 同步锁加持，发送两个通知:SDWebImageDownloadStopNotification, SDWebImageDownloadFinishNotification

2. 如果 self.callbackBlocks 有 LoaderCompletedBlock(key=kCompletedCallbackKey), 继续

3. self.imageData(NSMutableData) 在 didReceiveData: 方法中一点一点拼接的可变data

4. 异步串行队列(self.coderQueue)，调用 SDImageLoaderDecodeImageData() 解码imageData，输出UIImage

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解码函数 SDImageLoaderDecodeImageData()

SDImageLoaderDecodeImageData() 函数在 SDImageLoader.m 中

1. SDImageScaleFactorForKey(NSString * key)，返回一个屏幕倍数因子。

   1. 如果key是普通名字，判断key包不包含"@2x."或者"@3x."，包含就返回这个倍数因子。
   2. 如果key是一个URL，百分号编码的下的@ = %40，判断key包不包含"%402x."或者"%403x."。

2. 如果不是“仅解码第一帧”并且是动图，则装载动图的所有帧

3. 如果不是动图，-[SDImageCodersManager(sharedManager) decodedImageWithData:imageData options:]，参考详解一

4. 动图不解码

5. 如果应该解码，判断是否应该按比例缩小图片

6. 按比例缩小图片：-[SDImageCoderHelper decodedAndScaledDownImageWithImage:limitBytes:]，参考详解二

7. 不缩小图片：-[SDImageCoderHelper decodedImageWithImage:]，参考详解三

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详解一

+[SDImageCodersManager sharedManager] 方法

调用 -[SDImageCodersManager new] 即 -[SDImageCodersManager init]

+ (nonnull instancetype)sharedManager {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

-[SDImageCodersManager init] 方法

1. 创建可变数组，初始化赋值 SDImageIOCoder, SDImageGIFCoder, SDImageAPNGCoder; 都是调用 sharedCoder 方法获取, 待展开
2. 创建信号量线程锁，保存在 SDImageCodersManager 的 _codersLock 属性中

- (instancetype)init {
    if (self = [super init]) {
        // initialize with default coders
        _imageCoders = [NSMutableArray arrayWithArray:@[[SDImageIOCoder sharedCoder], [SDImageGIFCoder sharedCoder], [SDImageAPNGCoder sharedCoder]]];
        _codersLock = dispatch_semaphore_create(1);
    }
    return self;
}

-[SDImageCodersManager decodedImageWithData:options:] 方法

1. 取出(加锁) _imageCoders, SDImageIOCoder, SDImageGIFCoder, SDImageAPNGCoder;

2. 反转遍历 _imageCoders ，顺序为 SDImageAPNGCoder -> SDImageGIFCoder -> SDImageIOCoder

3. 每个 coder 都调用 canDecodeFromData: 方法，判断是否是可以解码的格式

   1. -[SDImageIOAnimatedCoder canDecodeFromData:]，取两个值(图片格式)进行对比
      1. +[NSData(ImageContentType) sd_imageFormatForImageData:] 获取data中包含的图片格式
         1. -[NSData getBytes:ength:] 获取的一个字节(第一个字节)可以区分图片格式
      2. (self.class.imageFormat 也就是 +[SDImageAPNGCoder imageFormat]) == SDImageFormatPNG

4. 如果返回值为YES则 coder 调用 decodedImageWithData:options: 方法，输出 image。

   1. -[SDImageIOAnimatedCoder decodedImageWithData:options:] ::: 此处并不是解码的地方，只是将压缩的图片二进制流读取到UIImage中。

5. 打断for循环，返回image

// -[SDImageCodersManager decodedImageWithData:options:]
- (UIImage *)decodedImageWithData:(NSData *)data options:(nullable SDImageCoderOptions *)options {
    if (!data) {
        return nil;
    }
    UIImage *image;
    NSArray> *coders = self.coders;
    for (id coder in coders.reverseObjectEnumerator) {
        if ([coder canDecodeFromData:data]) {
            image = [coder decodedImageWithData:data options:options];
            break;
        }
    }
    
    return image;
}

// -[SDImageIOAnimatedCoder canDecodeFromData:]
- (BOOL)canDecodeFromData:(nullable NSData *)data {
    return ([NSData sd_imageFormatForImageData:data] == self.class.imageFormat);
}

@implementation NSData (ImageContentType)

+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data {
    if (!data) {
        return SDImageFormatUndefined;
    }
    // File signatures table: http://www.garykessler.net/library/file_sigs.html
    uint8_t c;
    [data getBytes:&c length:1];
    switch (c) {
        case 0xFF:
            return SDImageFormatJPEG;
        case 0x89:
            return SDImageFormatPNG;
        case 0x47:
            return SDImageFormatGIF;
        case 0x49:
        case 0x4D:
            return SDImageFormatTIFF;
        case 0x52: {
            if (data.length >= 12) {
                //RIFF....WEBP
                NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
                if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
                    return SDImageFormatWebP;
                }
            }
            break;
        }
        case 0x00: {
            if (data.length >= 12) {
                //....ftypheic ....ftypheix ....ftyphevc ....ftyphevx
                NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(4, 8)] encoding:NSASCIIStringEncoding];
                if ([testString isEqualToString:@"ftypheic"]
                    || [testString isEqualToString:@"ftypheix"]
                    || [testString isEqualToString:@"ftyphevc"]
                    || [testString isEqualToString:@"ftyphevx"]) {
                    return SDImageFormatHEIC;
                }
                //....ftypmif1 ....ftypmsf1
                if ([testString isEqualToString:@"ftypmif1"] || [testString isEqualToString:@"ftypmsf1"]) {
                    return SDImageFormatHEIF;
                }
            }
            break;
        }
    }
    return SDImageFormatUndefined;
}

// -[SDImageIOAnimatedCoder decodedImageWithData:options:]
- (UIImage *)decodedImageWithData:(NSData *)data options:(nullable SDImageCoderOptions *)options {
    if (!data) {
        return nil;
    }
    CGFloat scale = 1;
    NSNumber *scaleFactor = options[SDImageCoderDecodeScaleFactor];
    if (scaleFactor != nil) {
        scale = MAX([scaleFactor doubleValue], 1);
    }
    
#if SD_MAC
    SDAnimatedImageRep *imageRep = [[SDAnimatedImageRep alloc] initWithData:data];
    NSSize size = NSMakeSize(imageRep.pixelsWide / scale, imageRep.pixelsHigh / scale);
    imageRep.size = size;
    NSImage *animatedImage = [[NSImage alloc] initWithSize:size];
    [animatedImage addRepresentation:imageRep];
    return animatedImage;
#else
    
    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
    if (!source) {
        return nil;
    }
    size_t count = CGImageSourceGetCount(source);
    UIImage *animatedImage;
    
    BOOL decodeFirstFrame = [options[SDImageCoderDecodeFirstFrameOnly] boolValue];
    if (decodeFirstFrame || count <= 1) {
        animatedImage = [[UIImage alloc] initWithData:data scale:scale];
    } else {
        NSMutableArray *frames = [NSMutableArray array];
        
        for (size_t i = 0; i < count; i++) {
            CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, i, NULL);
            if (!imageRef) {
                continue;
            }
            
            NSTimeInterval duration = [self.class frameDurationAtIndex:i source:source];
            UIImage *image = [[UIImage alloc] initWithCGImage:imageRef scale:scale orientation:UIImageOrientationUp];
            CGImageRelease(imageRef);
            
            SDImageFrame *frame = [SDImageFrame frameWithImage:image duration:duration];
            [frames addObject:frame];
        }
        
        NSUInteger loopCount = [self.class imageLoopCountWithSource:source];
        
        animatedImage = [SDImageCoderHelper animatedImageWithFrames:frames];
        animatedImage.sd_imageLoopCount = loopCount;
    }
    animatedImage.sd_imageFormat = self.class.imageFormat;
    CFRelease(source);
    
    return animatedImage;
#endif
}

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详解二-解码大图

+[SDImageCoderHelper decodedAndScaledDownImageWithImage:limitBytes:] 方法

1. +[SDImageCoderHelper shouldDecodeImage:]

   • 已解码过(判断关联属性sd_isDecoded)的不再解码；
   • 图片为nil不解码；
   • 动图不解码；

2. +[SDImageCoderHelper shouldScaleDownImage:limitBytes:]，判断是否是要比例缩小，如果不需要则直接正常解码图片即可

   1. 限制图片当超过多少字节时需要缩小，可以手动设置也可以走SD的默认值
   2. SD默认当图片总像素数量超过 262144(60M所拥有的像素数量)需要缩小
   3. 只要限制的像素数量比源像素数量小即需要缩小。PS：太小了也不行，至少要超过(大于) 1M 才需要。

3. 初始化目标总像素量(destTotalPixels，60M的总像素) 以及目标图片的单片(tile)总像素量(tileTotalPixels，20M的总像素)

更多的解析看源码中添加的注释吧

+ (UIImage *)decodedAndScaledDownImageWithImage:(UIImage *)image limitBytes:(NSUInteger)bytes {
#if SD_MAC
    return image;
#else
    if (![self shouldDecodeImage:image]) {
        return image;
    }
    
    if (![self shouldScaleDownImage:image limitBytes:bytes]) {
        return [self decodedImageWithImage:image];
    }
    
    CGFloat destTotalPixels;
    CGFloat tileTotalPixels;
    if (bytes > 0) {
        destTotalPixels = bytes / kBytesPerPixel;
        tileTotalPixels = destTotalPixels / 3;
    } else {
        destTotalPixels = kDestTotalPixels;
        tileTotalPixels = kTileTotalPixels;
    }
    CGContextRef destContext;
    
    // autorelease the bitmap context and all vars to help system to free memory when there are memory warning.
    // on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
    @autoreleasepool {
        CGImageRef sourceImageRef = image.CGImage;
        
        CGSize sourceResolution = CGSizeZero;
        sourceResolution.width = CGImageGetWidth(sourceImageRef);
        sourceResolution.height = CGImageGetHeight(sourceImageRef);
        CGFloat sourceTotalPixels = sourceResolution.width * sourceResolution.height;
        // Determine the scale ratio to apply to the input image
        // that results in an output image of the defined size.
        // see kDestImageSizeMB, and how it relates to destTotalPixels.
        // 此处开方，假设destTotalPixels = 60M(拥有的像素量), sourceTotalPixels = 240M, imageScale = 1/2。
        // 因为dest的总像素量需要宽高相乘才能得到，所以需要比例值需要开方。
        CGFloat imageScale = sqrt(destTotalPixels / sourceTotalPixels);
        CGSize destResolution = CGSizeZero;
        destResolution.width = (int)(sourceResolution.width * imageScale);
        destResolution.height = (int)(sourceResolution.height * imageScale);
        
        // device color space
        CGColorSpaceRef colorspaceRef = [self colorSpaceGetDeviceRGB];
        BOOL hasAlpha = [self CGImageContainsAlpha:sourceImageRef];
        // iOS display alpha info (BGRA8888/BGRX8888)
        CGBitmapInfo bitmapInfo = kCGBitmapByteOrder32Host;
        bitmapInfo |= hasAlpha ? kCGImageAlphaPremultipliedFirst : kCGImageAlphaNoneSkipFirst;
        
        // kCGImageAlphaNone is not supported in CGBitmapContextCreate.
        // Since the original image here has no alpha info, use kCGImageAlphaNoneSkipFirst
        // to create bitmap graphics contexts without alpha info.
        destContext = CGBitmapContextCreate(NULL,
                                            destResolution.width,
                                            destResolution.height,
                                            kBitsPerComponent,
                                            0,
                                            colorspaceRef,
                                            bitmapInfo);
        
        if (destContext == NULL) {
            return image;
        }
        CGContextSetInterpolationQuality(destContext, kCGInterpolationHigh);
        
        // Now define the size of the rectangle to be used for the
        // incremental blits from the input image to the output image.
        // we use a source tile width equal to the width of the source
        // image due to the way that iOS retrieves image data from disk.
        // iOS must decode an image from disk in full width 'bands', even
        // if current graphics context is clipped to a subrect within that
        // band. Therefore we fully utilize all of the pixel data that results
        // from a decoding opertion by achnoring our tile size to the full
        // width of the input image.
        CGRect sourceTile = CGRectZero;
        sourceTile.size.width = sourceResolution.width;
        // The source tile height is dynamic. Since we specified the size
        // of the source tile in MB, see how many rows of pixels high it
        // can be given the input image width.
        sourceTile.size.height = (int)(tileTotalPixels / sourceTile.size.width );
        sourceTile.origin.x = 0.0f;
        // The output tile is the same proportions as the input tile, but
        // scaled to image scale.
        CGRect destTile;
        destTile.size.width = destResolution.width;
        destTile.size.height = sourceTile.size.height * imageScale;
        destTile.origin.x = 0.0f;
        // The source seem overlap is proportionate to the destination seem overlap.
        // this is the amount of pixels to overlap each tile as we assemble the ouput image.
        float sourceSeemOverlap = (int)((kDestSeemOverlap/destResolution.height)*sourceResolution.height);
        CGImageRef sourceTileImageRef;
        // calculate the number of read/write operations required to assemble the
        // output image.
        int iterations = (int)( sourceResolution.height / sourceTile.size.height );
        // If tile height doesn't divide the image height evenly, add another iteration
        // to account for the remaining pixels.
        int remainder = (int)sourceResolution.height % (int)sourceTile.size.height;
        if(remainder) {
            iterations++;
        }
        // Add seem overlaps to the tiles, but save the original tile height for y coordinate calculations.
        float sourceTileHeightMinusOverlap = sourceTile.size.height;
        sourceTile.size.height += sourceSeemOverlap;
        destTile.size.height += kDestSeemOverlap;
        
        for( int y = 0; y < iterations; ++y ) {
            @autoreleasepool {
                sourceTile.origin.y = y * sourceTileHeightMinusOverlap + sourceSeemOverlap;
                destTile.origin.y = destResolution.height - (( y + 1 ) * sourceTileHeightMinusOverlap * imageScale + kDestSeemOverlap);// 用户空间和设备空间的坐标系转换
                sourceTileImageRef = CGImageCreateWithImageInRect( sourceImageRef, sourceTile );
                if( y == iterations - 1 && remainder ) {// 最后一次并且高不能被整除
                    float dify = destTile.size.height;
                    // 获取不能被整除的最后一小块真实高度
                    destTile.size.height = CGImageGetHeight( sourceTileImageRef ) * imageScale;
                    // 修正y值
                    dify -= destTile.size.height;
                    destTile.origin.y += dify;
                }
                CGContextDrawImage( destContext, destTile, sourceTileImageRef );
                CGImageRelease( sourceTileImageRef );
            }
        }
        
        CGImageRef destImageRef = CGBitmapContextCreateImage(destContext);
        CGContextRelease(destContext);
        if (destImageRef == NULL) {
            return image;
        }
        UIImage *destImage = [[UIImage alloc] initWithCGImage:destImageRef scale:image.scale orientation:image.imageOrientation];
        CGImageRelease(destImageRef);
        if (destImage == nil) {
            return image;
        }
        destImage.sd_isDecoded = YES;
        destImage.sd_imageFormat = image.sd_imageFormat;
        return destImage;
    }
#endif
}

这个解码大图不是SD的原创，是苹果官方给出的解决方案，提供demo可下载。想要了解更多，就去《Large Image Downsizing》探索伟大航路吧！
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详解三-正常解码

+[SDImageCoderHelper decodedImageWithImage:] 方法

1. +[SDImageCoderHelper shouldDecodeImage:]：已解码过(判断关联属性sd_isDecoded)的不再解码；图片为nil不解码；动图不解码；

2. +[SDImageCoderHelper CGImageCreateDecoded:]

3. +[SDImageCoderHelper CGImageCreateDecoded:orientation:]：输入image.CGImage，输出解码的CGImageRef; 核心解码函数：CGContextDrawImage()

   1. 获取图片的宽高，如果方向是左、右旋转则交换宽高数据

   2. 判断是否含有alpha信息

   3. 获取32位字节顺序(kCGBitmapByteOrder32Host这个宏避免考虑大小端问题)，保存到位图信息bitmapInfo

   4. bitmapInfo 按位或添加像素格式(alpha信息)。有alpha选择kCGImageAlphaPremultipliedFirst(BGRA8888), 无alpha选择kCGImageAlphaNoneSkipFirst

   5. 调用 +[SDImageCoderHelper colorSpaceGetDeviceRGB] 获取颜色空间，一个单例

   6. 调用 CGBitmapContextCreate() 函数创建位图上下文

   7. 调用 SDCGContextTransformFromOrientation() 获取方向旋转的 CGAffineTransform，调用 CGContextConcatCTM 关联到位图上下文，坐标系转换

   8. ❗️解码：CGContextDrawImage()❗️

      1. 传入的 bytesPerRow 参数是 0，正常情况下应传入 width * bytesPerPixel，但是这里传入 0 系统会帮助计算，而且系统还加了点小优化。
      2. 可以明显的看到Xcode检测到的内存情况和CPU使用情况的变化

   9. 获取位图：CGBitmapContextCreateImage()，输入context，输出解码后的CGImageRef。

   10. 释放上下文context

4. -[UIImage initWithCGImage:scale:orientation:]，CGImage 转为 UIImage

5. UIImage 关联属性赋值：decodedImage.sd_isDecoded = YES; decodedImage.sd_imageFormat = image.sd_imageFormat;

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另外再提一句

函数 SDImageCacheDecodeImageData() 也是普通的正常解码，与我们前面说的解码函数 SDImageLoaderDecodeImageData() 逻辑基本一致。

函数实现在 SDImageCacheDefine.m 文件中，此文件仅仅只写了这一个函数，其它什么都没有。

UIImage * _Nullable SDImageCacheDecodeImageData(NSData * _Nonnull imageData, NSString * _Nonnull cacheKey, SDWebImageOptions options, SDWebImageContext * _Nullable context) {
    UIImage *image;
    BOOL decodeFirstFrame = SD_OPTIONS_CONTAINS(options, SDWebImageDecodeFirstFrameOnly);
    NSNumber *scaleValue = context[SDWebImageContextImageScaleFactor];
    CGFloat scale = scaleValue.doubleValue >= 1 ? scaleValue.doubleValue : SDImageScaleFactorForKey(cacheKey);
    SDImageCoderOptions *coderOptions = @{SDImageCoderDecodeFirstFrameOnly : @(decodeFirstFrame), SDImageCoderDecodeScaleFactor : @(scale)};
    if (context) {
        SDImageCoderMutableOptions *mutableCoderOptions = [coderOptions mutableCopy];
        [mutableCoderOptions setValue:context forKey:SDImageCoderWebImageContext];
        coderOptions = [mutableCoderOptions copy];
    }
    
    if (!decodeFirstFrame) {
        Class animatedImageClass = context[SDWebImageContextAnimatedImageClass];
        // check whether we should use `SDAnimatedImage`
        if ([animatedImageClass isSubclassOfClass:[UIImage class]] && [animatedImageClass conformsToProtocol:@protocol(SDAnimatedImage)]) {
            image = [[animatedImageClass alloc] initWithData:imageData scale:scale options:coderOptions];
            if (image) {
                // Preload frames if supported
                if (options & SDWebImagePreloadAllFrames && [image respondsToSelector:@selector(preloadAllFrames)]) {
                    [((id)image) preloadAllFrames];
                }
            } else {
                // Check image class matching
                if (options & SDWebImageMatchAnimatedImageClass) {
                    return nil;
                }
            }
        }
    }
    if (!image) {
        image = [[SDImageCodersManager sharedManager] decodedImageWithData:imageData options:coderOptions];
    }
    if (image) {
        BOOL shouldDecode = !SD_OPTIONS_CONTAINS(options, SDWebImageAvoidDecodeImage);
        if ([image.class conformsToProtocol:@protocol(SDAnimatedImage)]) {
            // `SDAnimatedImage` do not decode
            shouldDecode = NO;
        } else if (image.sd_isAnimated) {
            // animated image do not decode
            shouldDecode = NO;
        }
        if (shouldDecode) {
            BOOL shouldScaleDown = SD_OPTIONS_CONTAINS(options, SDWebImageScaleDownLargeImages);
            if (shouldScaleDown) {
                image = [SDImageCoderHelper decodedAndScaledDownImageWithImage:image limitBytes:0];
            } else {
                image = [SDImageCoderHelper decodedImageWithImage:image];
            }
        }
    }
    
    return image;
}

---

FAQ

-[SDWebImageDownloaderDecryptor decryptedDataWithData:response:] 内部调用私有block处理imageData，并返回NSData赋值给imageData。

• Q1: 寻找 decryptor 初始化的位置，以及 block 的实现在哪
• A1: 没有实现block的地方，默认情况下 decryptor 是nil，此为开发者来指定的属性

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

• Q2: SDImageLoader.h 和 SDImageLoader.m 文件的构成
• A2: SDImageLoader.h 只有两个函数的EXPORT声明，以及一个协议的声明；SDImageLoader.m 仅仅是两个函数的实现。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

• Q3: SDImageCacheDefine 文件和 SDImageLoader 文件
• A3: 解码代码基本一致，声明的协议不同。
  解码：
  SDImageLoader 的解码实现是 SDImageLoaderDecodeImageData()
  SDImageCacheDefine 的解码实现是 SDImageCacheDecodeImageData()
  协议：
  SDImageLoader 声明的协议是 @protocol SDImageLoader；
  SDImageCacheDefine 声明的协议是 @protocol SDImageCache；

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

• Q4: 有多少种Coder?
• A4: Coder大致分两类：苹果内建的和SD自己实现的；一共5种；

1. SDImageIOCoder 苹果内建，支持PNG, JPEG, TIFF解码，以及 GIF 第一帧的静态图。

   • 使用 UIImage *image = [[UIImage alloc] initWithData:data scale:scale]; 进行解码

2. SDImageIOAnimatedCoder 下面三种的基类

3. SDImageGIFCoder 继承自 SDImageIOAnimatedCoder

4. SDImageAPNGCoder 继承自 SDImageIOAnimatedCoder

5. SDImageHEICCoder 继承自 SDImageIOAnimatedCoder

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• Q5: CGContextSetInterpolationQuality()是在设置什么？
• A5: 在设置“插值质量”。对图片的尺寸进行缩放时，由于尺寸不同，所以在生成新图的过程中像素不可能是一一对应，因此会有插值操作。所谓插值即根据原图和目标图大小比例，结合原图像素信息生成的新的像素的过程。常见的插值算法有线性插值，双线性插值，立方卷积插值等。

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一点点图片相关小知识

以下内容均来自苹果官方文档和官方示例，地址：Quartz 2D Programming Guide.

首先普及一下图片的关键小知识：

颜色空间（ColorSpace, CS）

颜色空间最直白的说就是平时所见到的例如 RGB、CMYK 这些个东西
颜色空间文档地址

像素格式（Pixel format）

由三个数值组成: Bits per pixel (bpp)、Bits per component (bpc)、Bytes per row
像素格式文档地址

颜色分量(Color Component)；

颜色分量就是颜色空间的各个小组件。比如 RGB 颜色空间，R 是一个颜色分量，同理G和B都是颜色分量。

颜色分量与颜色空间

颜色空间对应的像素格式

每个颜色空间对应的像素格式是规定好的（定死的），即bits per pixel (bpp) 和 bits per component (bpc)；下表就是各种颜色空间对应的像素格式：

ColorSpace	Pixel format	bitmap information constant	Availability
Null	8 bpp, 8 bpc	kCGImageAlphaOnly	Mac OS X, iOS
Gray	8 bpp, 8 bpc	kCGImageAlphaNone	Mac OS X, iOS
Gray	8 bpp, 8 bpc	kCGImageAlphaOnly	Mac OS X, iOS
RGB	16 bpp, 5 bpc	kCGImageAlphaNoneSkipFirst	Mac OS X, iOS
RGB	32 bpp, 8 bpc	kCGImageAlphaNoneSkipFirst	Mac OS X, iOS
RGB	32 bpp, 8 bpc	kCGImageAlphaNoneSkipLast	Mac OS X, iOS
RGB	32 bpp, 8 bpc	kCGImageAlphaPremultipliedFirst	Mac OS X, iOS
RGB	32 bpp, 8 bpc	kCGImageAlphaPremultipliedLast	Mac OS X, iOS

解释一下为什么上表中像素格式为啥缺了 Bytes per row 这个指标，是因为 Bytes per row 其实是通过计算确定的，Bytes per row = width * bytes per pixel。 所以有了 bpp 也就同时有了 Bytes per row

参考解码大图的官方示例

解码大图的官方示例中定义了许多宏，可以让我们很快的熟悉图片的相关知识。哪些是可以定死的，哪些是可以通过计算获得的。

#define bytesPerMB 1048576.0f 
#define bytesPerPixel 4.0f
#define pixelsPerMB ( bytesPerMB / bytesPerPixel ) // 262144 pixels, for 4 bytes per pixel.
#define destTotalPixels kDestImageSizeMB * pixelsPerMB
#define tileTotalPixels kSourceImageTileSizeMB * pixelsPerMB
#define destSeemOverlap 2.0f // the numbers of pixels to overlap the seems where tiles meet.
#define kDestImageSizeMB 60.0f // The resulting image will be (x)MB of uncompressed image data. 
#define kSourceImageTileSizeMB 20.0f // The tile size will be (x)MB of uncompressed image data. 

解码的核心函数 CGBitmapContextCreate()

翻译自 CGBitmapContextCreate() 函数的头文件注释，俺自己翻的，水平有限，请见谅

原文：The number of components for each pixel is specified by space, which may also specify a destination color profile.
译文：每个像素的颜色分量数量由“space”指定，它也可以指定目标颜色配置文件。

原文：Note that the only legal case when space can be NULL is when alpha is specified as kCGImageAlphaOnly.
译文：注意，space为NULL只有一种情况是合法的，就是当alpha被指定为kCGImageAlphaOnly。

原文：The number of bits for each component of a pixel is specified by bitsPerComponent.
译文：一个颜色分量的比特数由“bitsPerComponent”指定。

原文：The number of bytes per pixel is equal to (bitsPerComponent * number of components + 7)/8.
译文：一像素拥有的字节数 = (bitsPerComponent * number of components + 7)/8

原文：Each row of the bitmap consists of bytesPerRow bytes, which must be at least width * bytes per pixel bytes; in addition, bytesPerRow must be an integer multiple of the number of bytes per pixel.
译文：位图每一行由bytesPerRow 字节构成，bytesPerRow 必须至少是width * bytes per pixel字节。另外，bytesPerRow必须是整型数字乘以一像素拥有的字节数。

原文：data, if non-NULL, points to a block of memory at least bytesPerRow * height bytes.
译文：data如果非空，要指向一个至少是bytesPerRow * height 字节的内存块。

原文：If data is NULL, the data for context is allocated automatically and freed when the context is deallocated.
译文：如果data为空，context的data会被自动分配内存并且在context被销毁的时候释放。

原文：bitmapInfo specifies whether the bitmap should contain an alpha channel and how it's to be generated, along with whether the components are floating-point or integer
译文：bitmapInfo 指定位图是否应该包含一个alpha通道，以及如何生成它，以及颜色分量是浮点数还是整数。

CGBitmapContextCreate()