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Editorial Team
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Asked: 2026-06-05T11:06:10+00:00

I try to visualize the gradiants and angles of an image which computed by

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I try to visualize the gradiants and angles of an image which computed by the HOGDescriptor of the OpenCV Lib for Android. At the begin i have an 3 channel image Mat() with 8 bit unsigned int (CV_8UC3). The result of the computation is a MAT() (CV_32FC2) of the gradiants and a Mat() (CV_8UC2) of the angles. How can i visualize this results? What represent the values? Why have the angle Mat() 2 channels? Are the 2 channels of the gradiant Mat() the x and y component of the gradiant? I cant find documentation of the computeGradiant-Method.

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  1. Editorial Team

    HOG descriptor is an histogram of oriented gradient: it is an histogram where each bin reprezent the vote for gradient in corresponding orientation.
    In order to compute this descriptor, you should first convert you 3 channels color image into a grayscale image 

    cv::cvtColor(CV_BGR2GRAY);

    The result of “ComputeGradient” method is for exemple two images (same size as the original): x-component and y-component.
    You should then be able to compute for each pixel the gradient magnitude and orientation.

    mag=sqrt(x*x+y*y)
alpha=atan(y/x)

    Then you can fill you histogram. Note that HOG descritpor is computed by blocks and cells. See this for more detail.

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