0.84: Artefacts when high quality zooming 4-color greyscale

[helmut]

XnView: MP 0.84
OS: Windows 10 - 64 bit

When high quality zooming into a 4-color greyscale image you will see artefacts even though the original image is completely white.

Effect: Unexpected behaviour

To reproduce:
1. Download and save the attached PNG image file.
2. Open the PNG image file in XnView's viewer (View mode).
Please note that the image doesn't contain anything and is completely white. (correct)
3. Convert the image to a 4-colour greyscale image (Image » Change Colour depth). When converting, don't use any dithering ("None").
4. Select "Zoom in" for "High zoom quality" in Tools » Settings | View | View.
5. Zoom slowly into the image
Actual behaviour (bug): When zooming into the white image you see a "grid" of black pixels at 200%, 400%, 800%, and 1600%.

Expected behaviour: When zooming into a white image you should see white - nothing else.

Note: The problem might be related to 0.84: Crash when high quality zooming b/w image.

[helmut]

Could someone please try to reproduce the problem?

[XnTriq]

When zooming into the white image you see a "grid" of black pixels at 200%, 400%, 800%, and 1600%.

Confirmed

Related bug in XnView Classic: Faint grid overlayed on high quality downsampled view

[helmut]

Thank you for confirming, XnTriq.

Reproduced

[xnview]

O.k., thank you, I can also reproduce the problem. Issue 1237 is fixed in next version.

[xnview]

This problem is supposed to be fixed in XnView MP 0.87. Please check and confirm the bug fix here.

[XnTriq]

Tested in 0.87 beta 3: Fixed!