How do I binarize a CGImage using OpenCV on iOS?

Question

In my iOS project, I have a CGImage in RGB that I'd like to binarize (convert to black and white). I would like to use OpenCV to do this, but I'm new to OpenCV. I found a book on OpenCV, but it was not for iPhone.

How can I binarize such an image using OpenCV on iOS?

By binary you mean pure black and white? And why would iPhone make any difference? — Mark Ransom, Jun 07 '12 at 17:49
Yes I mean pure black and white image. for iPhone actually i am new to openCV and I am little bit confused in cvNamedWindow and cvDestroyWindow. — Dani, Jun 07 '12 at 19:02
`cvNamedWindow()` creates a window, `cvShowImage()` displays an image inside that window, `cvWaitKey(0)` makes the window stay open until the user presses a key, and at the end of your program there should be a `cvDestroyWindow()` to release the resources previously allocated to the window. There are tons of examples around here, use the search box. — karlphillip, Jun 07 '12 at 19:47
Actually i don't wanna use cvNamedWindow and cvDestroyWindow in my iOS project code. because i think there is no concept of window in iOS. Everything is displayed on views. — Dani, Jun 08 '12 at 12:47

score 7 · Accepted Answer · answered Jun 07 '12 at 20:11

If you don't want to set up OpenCV in your iOS project, my open source GPUImage framework has two threshold filters within it for binarization of images, a simple threshold and an adaptive one based on local luminance near a pixel.

You can apply a simple threshold to an image and then extract a resulting binarized UIImage using code like the following:

UIImage *inputImage = [UIImage imageNamed:@"inputimage.png"];    
GPUImageLuminanceThresholdFilter *thresholdFilter = [[GPUImageLuminanceThresholdFilter alloc] init];
thresholdFilter.threshold = 0.5;
UIImage *thresholdFilter = [thresholdFilter imageByFilteringImage:inputImage];

(release the above filter if not using ARC in your application)

If you wish to display this image to the screen instead, you can send the thresholded output to a GPUImageView. You can also process live video with these filters, if you wish, because they are run entirely on the GPU.

score 2 · Answer 2 · edited May 23 '17 at 10:30

2

Take a look at cv::threshold() adn pass thresholdType as cv::THRESH_BINARY:

double cv::threshold(const cv::Mat& src, 
                     cv::Mat& dst, 
                     double thresh, 
                     double maxVal, 
                     int thresholdType)

This example uses the C interface of OpenCV to convert an image to black & white.

edited May 23 '17 at 10:30

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answered Jun 07 '12 at 19:40

karlphillip

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score 0 · Answer 3 · answered Sep 06 '13 at 11:12

What you want to do is remove the low rate of changes and leave the high rate of changes, this is a high pass filter. I only have experience with audio signal processing so I don't really know what options are available to you but that is the direction I would be looking.

How do I binarize a CGImage using OpenCV on iOS?

3 Answers3

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