OpenCV - How to apply adaptive threshold to an image on iOS

Question

I am trying to apply adaptive thresholding to an image of an A4 paper as shown below:

I use the c

Answer 1

You can use OpenCV framework and implement below code

 +(UIImage *)blackandWhite:(UIImage *)processedImage
    {
        cv::Mat original = [MMOpenCVHelper cvMatGrayFromAdjustedUIImage:processedImage];

        cv::Mat new_image = cv::Mat::zeros( original.size(), original.type() );

        original.convertTo(new_image, -1, 1.4, -50);
        original.release();

        UIImage *blackWhiteImage=[MMOpenCVHelper UIImageFromCVMat:new_image];
        new_image.release();

        return blackWhiteImage;
    }

+ (cv::Mat)cvMatGrayFromAdjustedUIImage:(UIImage *)image
{
    cv::Mat cvMat = [self cvMatFromAdjustedUIImage:image];
    cv::Mat grayMat;
    if ( cvMat.channels() == 1 ) {
        grayMat = cvMat;
    }
    else {
        grayMat = cv :: Mat( cvMat.rows,cvMat.cols, CV_8UC1 );
        cv::cvtColor( cvMat, grayMat, cv::COLOR_BGR2GRAY );
    }
    return grayMat;
}

+ (cv::Mat)cvMatFromAdjustedUIImage:(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

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

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


+ (UIImage *)UIImageFromCVMat:(cv::Mat)cvMat
{
    NSData *data = [NSData dataWithBytes:cvMat.data length:cvMat.elemSize() * cvMat.total()];

    CGColorSpaceRef colorSpace;

    if (cvMat.elemSize() == 1) {
        colorSpace = CGColorSpaceCreateDeviceGray();
    } else {
        colorSpace = CGColorSpaceCreateDeviceRGB();
    }

    CGDataProviderRef provider = CGDataProviderCreateWithCFData((__bridge CFDataRef)data);

    CGImageRef imageRef = CGImageCreate(cvMat.cols,                                     // Width
                                        cvMat.rows,                                     // Height
                                        8,                                              // Bits per component
                                        8 * cvMat.elemSize(),                           // Bits per pixel
                                        cvMat.step[0],                                  // Bytes per row
                                        colorSpace,                                     // Colorspace
                                        kCGImageAlphaNone | kCGBitmapByteOrderDefault,  // Bitmap info flags
                                        provider,                                       // CGDataProviderRef
                                        NULL,                                           // Decode
                                        false,                                          // Should interpolate
                                        kCGRenderingIntentDefault);                     // Intent

    UIImage *image = [[UIImage alloc] initWithCGImage:imageRef];
    CGImageRelease(imageRef);
    CGDataProviderRelease(provider);
    CGColorSpaceRelease(colorSpace);

    return image;
}

its working for me check the output for your document

Answer 2

Try this:

+ (UIImage *)processImageWithOpenCV:(UIImage*)inputImage {
    cv::Mat cvImage = [inputImage CVMat];
    threshold(cvImage, cvImage, 128, 255, cv::THRESH_BINARY);
    return [UIImage imageWithCVMat:cvImage];
}

Result image: