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Harris Corner Detection Behavior #47

Open fmascarich opened 5 years ago

fmascarich commented 5 years ago

I'm running the Harris corner detector example, and I've gotten some fairly strange results. This is my input image: corner Here is the output_hls.png: output_hls Here is the output_ocv.png: output_ocv

bgouthamb commented 5 years ago

@fmascarich There will be a minimal difference in the output images of xfOpenCV function (output_hls.png) and the reference function(output_ocv.png). Since xfOpenCV works on fixed point format and the reference function uses floating point, there will be a little variation in the computation values, that causes the results to differ.

fmascarich commented 5 years ago

@bgouthamb

I would think that floating point vs fixed point differences might explain some discretization error such that there might be a pixel difference between the two implementations, however I intended to point out the missing corners and the differences when compared to the results of the official opencv implementation. Here's the result using the official opencv function:

image

output_hls finds corners on the border of the image and misses a couple in the middle.

output_ocv is missing a significant number of corners in a cross pattern on the central axis.

I'm just wondering if there's something I'm not configuring correctly or if there's buggy behavior in the xfopencv implementation.

bgouthamb commented 5 years ago

@fmascarich OpenCV's cornerHarris implementation returns Harris detector responses as output. To get the corners out of these scores, few post-processing functions has to be applied. The functions to be used for post-processing is totally dependent on the target application/image.

In xfOpenCV, threshold and max-suppression functions are the post-processing functions, whereas OpenCV's example applies a different set of functions and later a threshold function with a threshold value suitable for that image, to get corners from Harris Scores.

If we apply the same post-processing functions as OpenCV, instead of max-suppression, to xfOpenCV cornerHarris, their output's both match.

Also, xfOpenCV's implementation only supports BORDER_CONSTANT. Hence, corners are being detected on the image borders.

Currently, in xfOpenCV, the post-processing functions (thresholding and max-suppression) are integrated into the cornerHarris function itself. These functions will be separated from cornerHarris in a future release.