300x300 像素 Png 图像到字节数组

Question

My Objective is to extract 300x300 pixel frame from a CVImageBuffer (camera stream) and convert it in to a UInt Byte Array. Technically the array size should be 90,000. However I'm getting a much more larger value. Any help would much appreciate to spot the mistake.

Method that convert the Image buffer to UIImage

 func getImageFromSampleBuffer(image_buffer : CVImageBuffer?) -> UIImage?
  {
    if let imageBuffer = image_buffer {
        // Lock the base address of the pixel buffer
        CVPixelBufferLockBaseAddress(imageBuffer, CVPixelBufferLockFlags.readOnly);


        // Get the number of bytes per row for the pixel buffer
        let baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);

        // Get the number of bytes per row for the pixel buffer
        let bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
        // Get the pixel buffer width and height
        let width = CVPixelBufferGetWidth(imageBuffer);
        let height = CVPixelBufferGetHeight(imageBuffer);

        // Create a device-dependent RGB color space
        let colorSpace = CGColorSpaceCreateDeviceRGB();

        // Create a bitmap graphics context with the sample buffer data
        var bitmapInfo: UInt32 = CGBitmapInfo.byteOrder32Little.rawValue
        bitmapInfo |= CGImageAlphaInfo.premultipliedFirst.rawValue & CGBitmapInfo.alphaInfoMask.rawValue
        //let bitmapInfo: UInt32 = CGBitmapInfo.alphaInfoMask.rawValue
        let context = CGContext.init(data: baseAddress, width: width, height: height, bitsPerComponent: 8, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo)
        // Create a Quartz image from the pixel data in the bitmap graphics context
        let quartzImage = context?.makeImage();
        // Unlock the pixel buffer
        CVPixelBufferUnlockBaseAddress(imageBuffer, CVPixelBufferLockFlags.readOnly);

        // Create an image object from the Quartz image
        let image = UIImage.init(cgImage: quartzImage!);

        return (image);
    }
   return nil
  }

}

Extension that convert the image dat in to UInt8 Array

 extension Data {
   func toByteArray() -> [UInt8]? {
    var byteData = [UInt8](repeating:0, count: self.count)
    self.copyBytes(to: &byteData, count: self.count)
    return byteData
   }
 }

Usage of the Code

    let image = getImageFromSampleBuffer(image_buffer: imageBuffer)

   //Issue*******
    if let byteArrayOfImage = image.copy(newSize: CGSize(width: 300, height: 300))?.pngData()?.toByteArray(){

       print(byteArrayOfImage.count) // Technically should print 90,000. However it prints a large value
    }

What am i missing here

Answer 1

Your expectation Technically the array size should be 90,000 may be wrong.

1. If your CVPixelBuffer 's format is BRGA or other 4-channel type, then the array size will be w*h*4=360,000 , where w*4 or, more general, w*channel_count is the row stride .

2. CVPixelBuffer is usually aligned by 16 bytes: https://developer.apple.com/documentation/corevideo/kcvpixelbufferbytesperrowalignmentkey?language=objc So the real row stride may be even more than w*channel_count .

You should check the value bytesPerRow to understand your whole problem.