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

Question

I am trying to apply adaptive thresholding to an image of an A4 paper as shown below:
I use the code below to apply the image manipulation:
+ (UIImage *)processImageWithOpenCV:(UIImage*)inputImage {
cv::Mat cvImage = [inputImage CVMat];
cv::Mat res;
cv::cvtColor(cvImage, cvImage, CV_RGB2GRAY);
cvImage.convertTo(cvImage,CV_32FC1,1.0/255.0);
CalcBlockMeanVariance(cvImage,res);
res=1.0-res;
res=cvImage+res;
cv::threshold(res,res, 0.85, 1, cv::THRESH_BINARY);
cv::resize(res, res, cv::Size(res.cols/2,res.rows/2));
return [UIImage imageWithCVMat:cvImage];
}
void CalcBlockMeanVariance(cv::Mat Img,cv::Mat Res,float blockSide=13) // blockSide - the parameter (set greater for larger font on image)
{
cv::Mat I;
Img.convertTo(I,CV_32FC1);
Res=cv::Mat::zeros(Img.rows/blockSide,Img.cols/blockSide,CV_32FC1);
cv::Mat inpaintmask;
cv::Mat patch;
cv::Mat smallImg;
cv::Scalar m,s;
for(int i=0;i<Img.rows-blockSide;i+=blockSide)
{
for (int j=0;j<Img.cols-blockSide;j+=blockSide)
{
patch=I(cv::Rect(j,i,blockSide,blockSide));
cv::meanStdDev(patch,m,s);
if(s[0]>0.01) // Thresholding parameter (set smaller for lower contrast image)
{
Res.at<float>(i/blockSide,j/blockSide)=m[0];
}else
{
Res.at<float>(i/blockSide,j/blockSide)=0;
}
}
}
cv::resize(I,smallImg,Res.size());
cv::threshold(Res,inpaintmask,0.02,1.0,cv::THRESH_BINARY);
cv::Mat inpainted;
smallImg.convertTo(smallImg,CV_8UC1,255);
inpaintmask.convertTo(inpaintmask,CV_8UC1);
inpaint(smallImg, inpaintmask, inpainted, 5, cv::INPAINT_TELEA);
cv::resize(inpainted,Res,Img.size());
Res.convertTo(Res,CV_8UC3);
}
Although the inputted image is greyscaled, it outputs an yellowish image as shown below:
My hypothesis is that whilst conversion between the cv::Mat and UIImage, something happened leading to the color image, however I can not figure out how to fix this issue.
**please ignore the status bar as these images are screenshots of the iOS app.
Update:
I have tried using CV_8UC1 instead of CV_8UC3 for Res.convertTo() and added cvtColor(Res, Res, CV_GRAY2BGR); but am still getting very similar results.
Could it be the conversion between cv::mat and UIImage which is causing this problem??
I want my image to be like this shown below.

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: