1.9.8: A paletted 4-bit PNG image is displayed incorrectly when zoomed in

[Lord_D]

XnView: MP 1.9.8 - 64 bit
OS: Windows 10 - 64bit

When a paletted 4-bit PNG is zoomed in, it turns into a series of stripes. Apparently, one column stretches across the entire width of the image.

Example image

gradient_dither.png (1.69 KiB) Viewed 59 times

Screenshot demonstrating the problem

screenshot.png (64.75 KiB) Viewed 59 times

ImageMagick command to create such images:

Code: Select all

magick.exe input.png -remap eink_cmap.gif output.png

Effect: A paletted 4-bit PNG display incorrectly when zoomed in.

To reproduce:
1. Open attached gradient_dither.png in XnView MP.
2. Zoom in more than 100%.
Actual behaviour (bug): The image turns into a series of stripes

Expected behaviour: The image is displayed correctly.

[user0]

I can reproduce, caused by antialiasing zoom-in filters (all except bilinear)
however, no issues with same filters during zoom-out

[xnview]

Thanks to your detailed description I can reproduce the problem.