Avoiding/minimizing the threshold effect

[butterw]

https://github.com/butterw/bShaders/blob/master/edge/bCartoonize.avs
original picture: https://forum.videohelp.com/images/imgfiles/MxS8q29.jpg

vid_yuv.TCannyMod(mode=1, sigma=0, sobel=true, chroma=2).Invert("Y")
darker means stronger edge detection:
https://raw.githubusercontent.com/butterw/bShaders/master/assets/mpv_invSobel_Luma_noThreshold.jpg


To achieve a realtime cartoon effect, I've used Luma Sobel edge detection inverted (no thresholding) +keep UV. This works decently to hide noisy/weak edge detection but also causes black/dark elements to become white. In the above picture I've applied a Threshold on dark elements to bring back some of their color. The issue is that the Luma threshold (between 2 different math expressions) is now clearly visible on the ground.

What should I do to solve (or at least reduce) this type of thresholding issue ? The problem can also be present with Luma bandify color posterization: https://forum.videohelp.com/threads/400218-%5BAvisynth-%5D-How-to-display-a-luminance-map
The threshold can cause ugly blocky moving artifacts at the border between two bands to be highlighted (particularly in background shadow areas).

Suggested techniques:
- selective blurring at the boundary to smooth the transition between bands
- add dithering

[butterw]

On compressed video, pre-processing the YV12 source with the mplayer spatio-temporal denoiser hqdn3d(2.) seemed to help. It smooths/slows the boundary noise making it less of an issue.

Are there any Avisynth+ implementations of adaptative thresholding ?