Strange looking sift features

[jhaggle]

I am using sift feature detection on images of chessbord with charuco patterns, and except for that a completely black background. Sift should not find any features where there is only black background. But when i choose "show features" i get the image below. What is the green and orange markers? And why are there orange markers in the bottom of the image?

[fabiencastan]

Are you sure that the image is fully back? Can you check the values with the color-picker? Maybe you have some noise or compression artefacts. The points in orange means that the points have been matched to other images, which sounds very surprising here.

[jhaggle]

Thank you for your replay. Yes, looking closer at the image, I can se that it contains compression artefacts as you suggested. See images below. The object in the image is masked out so that the contrast of the surroundings may be increased. The artefacts of course creates a LOT of extra keypoints/features. Originally I have png-images. However, I need to add metadata to the images in order for Meshroom to initiate a rig. But Meshroom refuses to read metadata from png-files and then does not initiate a rig unless I change the images to jpg-format. Any suggestion on how to solve this?

[simogasp]

This is a known issue with the latest release as the sift threshold changed and they can pick up the slightest change in the greylevel values. If you do not depend on a particular feature of the latest release you can use the previous release 2020.1.1, even only for the feature extraction part, ie instead of executing the latest binary you can try to use the previous one. Not ideal but it should you get going. And if you don't need sift at all cost you can still try to use one of the akaze.

[jhaggle]

Ok too bad that they didn't make it possible to set the sensibility threshold directly in meshroom as you can do when running sift with for example openCV. Would be a huge benefit.

[FlachyJoe]

You can convert your PNG to 100% quality JPEG so you shouldn't have compression artifacts. With ImageMagic: convert -quality 100 image.png image.jpg

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