Swift 版本获取错误的像素数据，但 Object-c 版本使用方法 "CGBitmapContextCreate" 从同一图像获取正确的像素数据

Question

简直不敢相信自己的眼睛，它们基本上是相同的代码，只是将Object-c代码转换为swift代码，但是Object-c代码总是得到正确的答案，但是swift 代码有时会得到正确答案，有时会出错。

Swift 翻译：

class ImageProcessor1 {
    class func processImage(image: UIImage) {
        guard let cgImage = image.cgImage else {
            return
        }
        let width = Int(image.size.width)
        let height = Int(image.size.height)
        let bytesPerRow = width * 4
        let imageData = UnsafeMutablePointer<UInt32>.allocate(capacity: width * height)
        let colorSpace = CGColorSpaceCreateDeviceRGB()

        let bitmapInfo: UInt32 = CGBitmapInfo.byteOrder32Big.rawValue | CGImageAlphaInfo.premultipliedLast.rawValue
        guard let imageContext = CGContext(data: imageData, width: width, height: height, bitsPerComponent: 8, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {
            return
        }
        imageContext.draw(cgImage, in: CGRect(origin: .zero, size: image.size))
        print("---------data from Swift version----------")
        for i in 0..<width * height {
            print(imageData[i])
        }
    }
}

Objective-C 翻译：

- (UIImage *)processUsingPixels:(UIImage*)inputImage {

  // 1. Get the raw pixels of the image
  UInt32 * inputPixels;

  CGImageRef inputCGImage = [inputImage CGImage];
  NSUInteger inputWidth = CGImageGetWidth(inputCGImage);
  NSUInteger inputHeight = CGImageGetHeight(inputCGImage);

  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

  NSUInteger bytesPerPixel = 4;
  NSUInteger bitsPerComponent = 8;

  NSUInteger inputBytesPerRow = bytesPerPixel * inputWidth;

  inputPixels = (UInt32 *)calloc(inputHeight * inputWidth, sizeof(UInt32));

  CGContextRef context = CGBitmapContextCreate(inputPixels, inputWidth, inputHeight,
                                               bitsPerComponent, inputBytesPerRow, colorSpace,
                                               kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

  CGContextDrawImage(context, CGRectMake(0, 0, inputWidth, inputHeight), inputCGImage);

    NSLog(@"---------data from Object-c version----------");
    UInt32 * currentPixel = inputPixels;
    for (NSUInteger j = 0; j < inputHeight; j++) {
        for (NSUInteger i = 0; i < inputWidth; i++) {
            UInt32 color = *currentPixel;
            NSLog(@"%u", color);
            currentPixel++;
        }
    }
  return inputImage;
}

https://github.com/tuchangwei/Pixel

有空

如果你得到相同的答案，请运行多次。

Answer 1

你的 Objective-C 和 Swift 代码 have leaks. Also your Swift code is not initializing the allocated memory. When I initialized 内存，我没有看到任何区别:

imageData.initialize(repeating: 0, count: width * height)

FWIW，虽然 allocate 不初始化内存缓冲区，但 calloc 会：

... The allocated memory is filled with bytes of value zero.

但就个人而言，我建议您完全摆脱分配内存的工作，为 data 参数传递 nil，然后使用 bindMemory 访问该缓冲区.如果你这样做，正如 the documentation 所说：

Pass NULL if you want this function to allocate memory for the bitmap. This frees you from managing your own memory, which reduces memory leak issues.

因此，也许：

class func processImage(image: UIImage) {
    guard let cgImage = image.cgImage else {
        return
    }
    let width = cgImage.width
    let height = cgImage.height
    let bytesPerRow = width * 4

    let colorSpace = CGColorSpaceCreateDeviceRGB()

    let bitmapInfo: UInt32 = CGBitmapInfo.byteOrder32Big.rawValue | CGImageAlphaInfo.premultipliedLast.rawValue
    guard
        let imageContext = CGContext(data: nil, width: width, height: height, bitsPerComponent: 8, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo),
        let rawPointer = imageContext.data
    else {
        return
    }

    let pixelBuffer = rawPointer.bindMemory(to: UInt32.self, capacity: width * height)

    imageContext.draw(cgImage, in: CGRect(origin: .zero, size: CGSize(width: width, height: height)))
    print("---------data from Swift version----------")
    for i in 0..<width * height {
        print(pixelBuffer[i])
    }
}

Swift 版本获取错误的像素数据，但 Object-c 版本使用方法 "CGBitmapContextCreate" 从同一图像获取正确的像素数据

Swift version get wrong pixels data but the Object-c version get the right pixels data from the same image using method "CGBitmapContextCreate"

swift

cgbitmapcontextcreate