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Mat from UIImage (iOS) differences between emulator and device

asked 2013-11-14 03:18:59 -0500

Ricciolo gravatar image

I'm using this function to convert UIImage to Mat

- (cv::Mat)cvMatFromUIImage:(UIImage *)image
{
CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
CGFloat cols = image.size.width;
CGFloat rows = image.size.height;

cv::Mat cvMat(rows, cols, CV_8UC4); // 8 bits per component, 4 channels (color channels + alpha)

CGContextRef contextRef = CGBitmapContextCreate(cvMat.data,                 // Pointer to  data
                                         cols,                       // Width of bitmap
                                         rows,                       // Height of bitmap
                                         8,                          // Bits per component
                                         cvMat.step[0],              // Bytes per row
                                         colorSpace,                 // Colorspace
                                         kCGImageAlphaNoneSkipLast |
                                         kCGBitmapByteOrderDefault); // Bitmap info flags

CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
CGContextRelease(contextRef);

return cvMat

On emulator it works great and I get image processing as expected. When using the same code on the device with the same image (same jpeg file) I get unexpected result. Looking inside Mat I see different pixel data. Do you know why?

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Could you attach results for simulator and real device? What version of iOS do you use? What device?

AlexanderShishkov gravatar imageAlexanderShishkov ( 2013-11-14 04:06:51 -0500 )edit

Device is iPhone 4 with iOS 7.0.3 If I dump cvMat.at<uchar>(0,0) cvMat.at<uchar>(1,0) I get on simulator: c b on iphone: c c

Ricciolo gravatar imageRicciolo ( 2013-11-14 04:32:17 -0500 )edit

That's odd, but using imread the cv::Mat is correct. But the image will come from UIImagePickerController so I need to convert from UIImage

Ricciolo gravatar imageRicciolo ( 2013-11-14 08:37:09 -0500 )edit

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answered 2013-11-14 04:11:32 -0500

AlexanderShishkov gravatar image

Coud you try code from OpenCV:

void UIImageToMat(const UIImage* image,
                         cv::Mat& m, bool alphaExist) {
    CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
    CGFloat cols = image.size.width, rows = image.size.height;
    CGContextRef contextRef;
    CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast;
    if (CGColorSpaceGetModel(colorSpace) == 0)
    {
        m.create(rows, cols, CV_8UC1); // 8 bits per component, 1 channel
        bitmapInfo = kCGImageAlphaNone;
        if (!alphaExist)
            bitmapInfo = kCGImageAlphaNone;
        contextRef = CGBitmapContextCreate(m.data, m.cols, m.rows, 8,
                                           m.step[0], colorSpace,
                                           bitmapInfo);
    }
    else
    {
        m.create(rows, cols, CV_8UC4); // 8 bits per component, 4 channels
        if (!alphaExist)
            bitmapInfo = kCGImageAlphaNoneSkipLast |
                                kCGBitmapByteOrderDefault;
        contextRef = CGBitmapContextCreate(m.data, m.cols, m.rows, 8,
                                           m.step[0], colorSpace,
                                           bitmapInfo);
    }
    CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows),
                       image.CGImage);
    CGContextRelease(contextRef);
    CGColorSpaceRelease(colorSpace);
}
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Same issue :-(

Ricciolo gravatar imageRicciolo ( 2013-11-14 04:32:59 -0500 )edit
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Asked: 2013-11-14 03:18:59 -0500

Seen: 1,049 times

Last updated: Nov 14 '13