OpenCV the best method to access pixel value in a binary image (Mat)

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I want to ask a question: I'm using OpenCV 2.4 and I have to check the value (time by time) of a pixel addressed (X,Y) in a binary thresholded OpenCV Mat image.

I have to check if the pixel at the X,Y that I want to verify (and pass to a fuction) if is black or white (0 or 255)...what is the best method to do this thing?

I have searched and read about direct method (Get2D) or with pointers..but it isn't so clear for me...Image are binary, thresholded and eroded/dilated before...then, 1 channel not 3 channels..

can someone post me an example code of fuction that I've to use to do this thing?

Please, help me.

Posted 26-Nov-13 22:22pm

Domus1919

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2 solutions

Solution 1

direct memory access is lightning faster than calling some API. If you dont believe it write some test code.

I use for image manipulation and pixel access always cached off-screen bitmaps which I blitted into memory.

Posted 27-Nov-13 4:42am

KarstenK

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Domus1919 27-Nov-13 11:26am

can you explain me a part of code? for Mat image?

KarstenK 28-Nov-13 4:47am

The normal way is the "Solution "", but if you have performance problems you can use that way.

At first you need to draw your OpenCV picture on screen or a memory dc. Than you the GetDiBits API to get the pixel in binary format.

Here is a more complexexample which does that:
http://msdn.microsoft.com/en-us/library/windows/desktop/dd183402(v=vs.85).aspx

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User 59241 · Accepted Answer · 2013-11-27T13:45:00

Solution 2

It is always easier if you post your own code. This is straight out of the documentation:

C++

cv::Mat img = cv::imread("lenna.png", 0);

if(img.at<uchar>(y,x) == 255)

etc

http://docs.opencv.org/doc/user_guide/ug_mat.html[^]

Posted 27-Nov-13 13:45pm

User 59241

Updated 27-Nov-13 13:55pm

v3

Comments

Domus1919 5-Dec-13 2:12am

I'm sorry but this solution works with grey image (or color)...I have a binary thresholded image...how can I access pixel value (white or black)?