Median live filter does not show correctly in viewport

[Vozka]

When editing a night sky stack I discovered that when applying a live median filter layer to a photo, the result looks completely different in the application compared to the exported result. To be more specific, the exported picture seemed to use a larger radius size than the live filter in application.

My first thought was that maybe it's an aliasing/blurring issue related to viewport zoom, but I verified that this happens when picture is 100% zoomed in and regardless whether I use nearest neighbor or bilinear filtering. It also happens both when using GPU renderer and WARP renderer.

When changing the radius size, the live filter seems to only run at certain sizes, as if there was some radius size quantification. What usually happens is that  from 0 to 1 px it does nothing, then at 111 px it starts to work and keeps producing the same result up to 2 px, changing again at 2.1 px. If I export the picture with radius set to 1.1 px or 2.1 px, it looks the same in the application and exported.

This behavior does not manifest immediately after opening the file. The filter seems to work correctly at first, or at least the radius "quantification points" are not at _.1 pixels. It breaks for me when I start to change the radius size using my mouse - the slider jumps around, not following my cursor precisely. Even after that though the filter does not always change at 1.1, 2.1, 3.1 etc. pixels as usual, I wasn't able to find out why.

Added to this post are example pictures and the project file, which is a simple jpeg with one live median filter.

I tested it both on my desktop and my old laptop, both running Windows 10 without any unusual hardware. I use the current version 1.8.5.703 and it seems to hapen to the latest beta (downloaded today) as well.

stack.afphoto

Edited December 14, 2020 by Vozka

[Callum]

Hi Vozka,

If you zoom in or out of your document you should it flickers showing different results from the filter Your exported result is actually correct as you can see from my screenshot this is actually how the file looks when opened on my computer and viewed at 100% zoom. The following forum post should explain why this is happening. Essentially you need to make sure you are at 100% when editing or viewing the filter to ensure it will match your export.

Thanks

C

[Vozka]

Thanks for the reply @Callum, but as I wrote above:

Quote

My first thought was that maybe it's an aliasing/blurring issue related to viewport zoom, but I verified that this happens when picture is 100% zoomed in and regardless whether I use nearest neighbor or bilinear filtering. 

The screenshot is taken with 100% zoom.

I realize that the exported image is correct - the fact that exported image changes with every adjust of the live filter (while the viewport doesn't) is a good clue as well. 

When I first open the project, the image shows correctly, as in your screenshot. It is when I start adjusting the filter that it breaks, after a couple adjustments.

Edited December 16, 2020 by Vozka

[Vozka]

Pinging @Callum again since this has not been resolved.

Like I said in both my previous posts, this behavior happens with 100% zoom.

Adding a screenshot to more clearly show what happens when I export the image with 3 px radius median zoom.

This usually does not happen immediately after opening the project file, but consistently starts happening after I change up the median radius a couple times.