The Harris corner detector is invariant to rotation, illumination variation and image noise. It operates on a grayscale BW8 image and delivers a vector of points of interest.
Harris corner detector example
The EasyImage Harris corner detector requires three parameters:
- The integration scale σi: the standard deviation of the Gaussian Filter used for scale analysis.
σd = 0,7 x σi, where σd is the differentiation scale: the standard deviation of the Gaussian Filter used for noise reduction during computation of the gradient. - A corner threshold: a fraction ranging from 0 to 1 of the maximum value of the cornerness of the source image.
- A Boolean that toggles sub-pixel detection.
The following characteristics are available for every point of interest:
- Corner position (pixel coordinates with sub-pixel accuracy if enabled).
- Cornerness measurement.
- Gradient magnitude with regards to the differentiation scale σd.
- Gradient value along the X-axis with regards to the differentiation scale σd.
- Gradient value along the Y-axis with regards to the differentiation scale σd
The API of the Harris corner detector is a single class named EHarrisCornerDetector and these methods:
- Apply: applies the Harris corner detector on an image/ROI.
- EHarrisCornerDetector: constructs a EHarrisCornerDetector object initialized to its default values.
- GetDerivationScale: returns the current derivation scale.
- GetScale: returns the integration scale.
- GetThreshold: returns the current threshold.
- GetThresholdingMode: returns the current thresholding mode for the cornerness measure.
- IsGradientNormalizationEnabled: returns whether the gradient is normalized before the computation of the cornerness measure.
- IsSubpixelPrecisionEnabled: returns whether the sub-pixel interpolation is enabled.
- SetDerivationScale: sets the derivation scale.
- SetGradientNormalizationEnabled: sets whether the gradient is normalized before the computation of the cornerness measure.
- SetScale: sets the integration scale.
- SetSubpixelPrecisionEnabled: sets whether the sub-pixel interpolation is enabled.
- SetThreshold: sets the threshold on the cornerness measure for a pixel to be considered as a corner.
- SetThresholdingMode: sets the thresholding mode for the cornerness measure.
Basic usage of Harris Corner Detector
An object of the EHarrisCornerDetector class can be reused across Harris detector applications, in order to reduce the setup time.
- Create an instance of the detector and set the appropriate method, for instance, the integration scale, SetScale, with the structures of interest that could have a spatial extent of 2 pixels.
- Apply the detector with two arguments to the new image : the input image and the interest points in the input image EHarrisInterestPoints.
- Access the individual elements of the output vector.