How to avoid compreesion artifacts on segmentation?

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The compression artifacts have a low amplitude. Therefore you can use a less severe threshold to get a correct mask. Here's the result of a segmentation of the attached image with a threshold of 210 (after transforming it to grayscale):

This is valid also for segmentation: by using another value, you can obtain a correctly segmented image; no need for thresholding. Note that for compressed/noisy images, if you want to segment the hue channel, you should use the saturation channel, too (as the hue can vary a lot in low-saturation zones).