格式类型：

RBGA - 包含Alpha通道
RGB
BGR
Gray - 灰度图

首先理解一张图片的组成，每张图片都是由无数个有序排列的带有颜色的像素点（Pixel）组成的，这些Pixel的排列都可以理解为一个二维数组，每个Pixel都是一个颜色点，即包含RGBA的Pixel。

什么是RGBA呢

R - Red
G - Green
B - Blue
A - Alpha 透明度

1、将UIImage转为RGBA格式

- (unsigned char *)getRGBAWithImage:(UIImage *)image
{
    int RGBA = 4;
    
    CGImageRef imageRef = [image CGImage];
    
    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char *) malloc(width * height * sizeof(unsigned char) * RGBA);
    NSUInteger bytesPerPixel = RGBA;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    
    CGColorSpaceRelease(colorSpace);
    CGContextRelease(context);
    
    return rawData;
}

继续上面的工作，获取这张图片中某个Pixel的RGBA值

- (void)getRGBAFromImage:(UIImage *)image atX:(int)xx andY:(int)yy 
{
    
    int RGBA = 4;
    
    CGImageRef imageRef = [image CGImage];
    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);
    
    // 从image的data buffer中取得影像，放入格式化后的rawData中
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char *)malloc(height * width * RGBA);
    NSUInteger bytesPerPixel = RGBA;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    
    CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);
    
    // 清空CGContextRef再绘制
    CGContextClearRect(context, CGRectMake(0.0, 0.0, width, height));
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);
    
    // 将XY坐标转成一维数组
    unsigned long byteIndex = (bytesPerRow * yy) + (bytesPerPixel * xx);
    
    // 取得RGBA位的数据
    CGFloat red   = rawData[byteIndex];
    CGFloat green = rawData[byteIndex + 1];
    CGFloat blue  = rawData[byteIndex + 2];
    CGFloat alpha = rawData[byteIndex + 3];
    
    // 利用RGB计算灰阶的亮度值
    CGFloat gray = (red + green + blue)/3 ;
    
    // 输出
    NSLog(@"%@",[NSString stringWithFormat:@"%.2f", red]);
    NSLog(@"%@",[NSString stringWithFormat:@"%.2f", green]);
    NSLog(@"%@",[NSString stringWithFormat:@"%.2f", blue]);
    NSLog(@"%@",[NSString stringWithFormat:@"%.2f", alpha]);
    NSLog(@"%@",[NSString stringWithFormat:@"%.2f", gray]);
    
    free(rawData);
}

2、将图片转为BGR格式

转换为BGR格式，顾名思义，BGR与RGBA，我们只需要把RGBA数据取到，去掉Alpha，再调换一下RGB的顺序就可以了，下面是具体调换代码：

- (unsigned char *)getBGRWithImage:(UIImage *)image
{
    int RGBA = 4;
    int RGB  = 3;
    
    CGImageRef imageRef = [image CGImage];
    
    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char *) malloc(width * height * sizeof(unsigned char) * RGBA);
    NSUInteger bytesPerPixel = RGBA;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    
    CGColorSpaceRelease(colorSpace);
    CGContextRelease(context);
    
    unsigned char * tempRawData = (unsigned char *)malloc(width * height * 3 * sizeof(unsigned char));
    
    for (int i = 0; i < width * height; i ++) {
        
        NSUInteger byteIndex = i * RGBA;
        NSUInteger newByteIndex = i * RGB;
        
        // Get RGB
        CGFloat red    = rawData[byteIndex + 0];
        CGFloat green  = rawData[byteIndex + 1];
        CGFloat blue   = rawData[byteIndex + 2];
        //CGFloat alpha  = rawData[byteIndex + 3];// 这里Alpha值是没有用的
        
        // Set RGB To New RawData
        tempRawData[newByteIndex + 0] = blue;   // B
        tempRawData[newByteIndex + 1] = green;  // G
        tempRawData[newByteIndex + 2] = red;    // R
    }
    
    return tempRawData;
}

什么是Gray灰度?

灰度（Gray scale）数字图像是每个像素只有一个采样颜色的图像。这类图像通常显示为从最暗黑色到最亮的白色的灰度

1、将一张图片转为灰度图

- (unsigned char *)getGrayWithImage:(UIImage *)image
{
    int GRAY = 1;
    
    // 获取灰度图
    CGImageRef imageRef = [image CGImage];
    
    int width = image.size.width;
    int height = image.size.height;
    unsigned char *rawData = (unsigned char *) malloc(width * height * sizeof(unsigned char));
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    NSUInteger bytesPerPixel = GRAY;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, 0);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    
    CGColorSpaceRelease(colorSpace);
    CGContextRelease(context);
    
    return rawData;
}

参考资料

彩笔的笔记本