xcode CVpixelBuffer显示负值

Question

我正在使用xcode，目前正在尝试使用以下代码从像素缓冲区中提取像素值。但是，当我打印出像素值时，它由负值组成。有人曾经遇到过这样的问题吗？

代码的一部分如下所示

- (void)captureOutput:(AVCaptureOutput*)captureOutput didOutputSampleBuffer:
(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection*)connection
{

CVImageBufferRef Buffer = CMSampleBufferGetImageBuffer(sampleBuffer);

CVPixelBufferLockBaseAddress(Buffer, 0);
uint8_t* BaseAddress = (uint8_t*)CVPixelBufferGetBaseAddressOfPlane(Buffer, 0);
size_t Width = CVPixelBufferGetWidth(Buffer);
size_t Height = CVPixelBufferGetHeight(Buffer);
if (BaseAddress)
{
    IplImage* Temporary = cvCreateImage(cvSize(Width, Height), IPL_DEPTH_8U, 4);

    Temporary->imageData = (char*)BaseAddress;
    for (int i = 0; i < Temporary->width * Temporary->height; ++i) {
        NSLog(@"Pixel value: %d",Temporary->imageData[i]);
        //where i try to print the pixels
    }
}

Answer 1

问题是，imageData of IplImage是一个签名的char。因此，任何大于127的东西都会出现为负数。

您可以简单地将其赋值给一个unsigned char，然后打印它，然后您将看到介于0到255之间的值，就像您可能预期的那样：

for (int i = 0; i < Temporary->width * Temporary->height; ++i) {
    unsigned char c = Temporary->imageData[i];
    NSLog(@"Pixel value: %u", c);
}

或者你可以用十六进制打印出来：

NSLog(@"Pixel value: %02x", c);