iOS校验图片每个像素透明度

- (void)canAsNightClubBodyImage:(void(^)(BOOL can))finish {
    CGImageRef imgref = self.CGImage;

    //   获取图片宽高（总像素数）
    size_t width = CGImageGetWidth(imgref);
    size_t height = CGImageGetHeight(imgref);
    // 每行像素的总字节数
    size_t bytesPerRow = CGImageGetBytesPerRow(imgref);
    // 每个像素多少位(RGBA每个8位，所以这里是32位)         ps:（一个字节8位）
    size_t bitsPerPixel = CGImageGetBitsPerPixel(imgref);
    CGDataProviderRef dataProvider = CGImageGetDataProvider(imgref);
    CFDataRef data = CGDataProviderCopyData(dataProvider);

    UInt8 *buffer = (UInt8*)CFDataGetBytePtr(data);// 图片数据的首地址
    
    int area = 24;
    int total = 0;
    int alphaZero = 0;
    // left top
    for (int x = 0; x < area; x ++ ) {
        for (int y = 0; y < area; y ++) {
            total ++;
            //每个像素的首地址
            UInt8 *pt = buffer + y * bytesPerRow + x * (bitsPerPixel/8);
//            UInt8  red = *pt;
//            UInt8  green = *(pt+1);   //指针向后移动一个字节
//            UInt8  blue = *(pt+2);
            UInt8  alpha = *(pt+3);
            if (alpha < 0.1) {
                alphaZero ++;
            }
        }
    }
    // left bottom
    for (int x = 0; x < area; x ++ ) {
        for (long y = height - area; y < height; y ++) {
            total ++;
            UInt8 *pt = buffer + y * bytesPerRow + x * (bitsPerPixel/8);
            UInt8  alpha = *(pt+3);
            if (alpha < 0.1) {
                alphaZero ++;
            }
        }
    }
    // right top
    for (long x = width - area; x < width; x ++ ) {
        for (int y = 0; y < area; y ++) {
            total ++;
            UInt8 *pt = buffer + y * bytesPerRow + x * (bitsPerPixel/8);
            UInt8  alpha = *(pt+3);
            if (alpha < 0.1) {
                alphaZero ++;
            }
        }
    }
    // right bottom
    for (long x = width - area; x < width; x ++ ) {
        for (long y = height - area; y < height; y ++) {
            total ++;
            UInt8 *pt = buffer + y * bytesPerRow + x * (bitsPerPixel/8);
            UInt8  alpha = *(pt+3);
            if (alpha < 0.1) {
                alphaZero ++;
            }
        }
    }
    
    // 有这行会崩溃 CGDataProviderRelease(dataProvider);
    
    float percent = alphaZero*1.0/total;
    finish(percent > 0.2);
}