Question

I’m a trying to create a gray IplImage from a gray scaled UIImage and Im using the method below.

- (IplImage *)createGrayIplImageFromUIImage:(UIImage *)image {

    CGImageRef imageRef = [image CGImage];

    CFDataRef dat = CGDataProviderCopyData(CGImageGetDataProvider(imageRef));

    const unsigned char *buffer = CFDataGetBytePtr(dat);
    IplImage *iplimage = cvCreateImage(cvSize(image.size.width, image.size.height), IPL_DEPTH_8U, 1);
    for (int i=0; i<image.size.width*image.size.height; i++) {
        iplimage->imageData[i] = buffer[i];
        NSLog(@"in creategrayiplimage - %d --- %d",buffer[i], iplimage->imageData[i]);
    }
    return iplimage;
}

When i log the data in for loop, buffer has correct values (0-255) but iplimage->imageData has signed values even its IPL_DEPTH_8U.

e.g

in creategrayiplimage – 149 — -107
in creategrayiplimage – 247 — -9
in creategrayiplimage – 127 — 127
in creategrayiplimage – 128 — -128

This seems like a small problem but I am stuck and I wonder why i get minus values from iplimage. Thanks in advance

Answer 1

A byte is a byte is a byte. The data itself doesn’t know whether it’s signed or unsigned — it’s all in how you interpret the data.

Change your NSLog() statement to use %u (unsigned int) instead of %d (signed int) for the format specifier and your data will be displayed unsigned.