0.92: Downscaling with gamma correction posterizes dark areas

[Daniel710]

XnView: MP 0.92 - 64 bit
OS: Linux

Downscaling an image with gamma correction posterizes dark areas. See the attached image (50% size of the original).

To reproduce, downscale an image with "Use gamma correction" enabled.

PIA11667.jpg

[xnview]

The original have posterized area

[Daniel710]

The posterized image you see in my post is the downscaled version of the original. I don't see any artifacts on the original image:

[Daniel710]

Still reproducible in XnView: MP 1.3 (and also in XnResize, btw).

[xnview]

i can't reproduce with your sample image, which output size do you use? could you post the output?

[Daniel710]

Try downscale to e.g. 1806x961 pixels. You can see the output image in the first post: download/file.php?id=5149&mode=view

[xnview]

where do you see "posterized area"?

[Daniel710]

On the Saturn rings. I attached an image with side-by-side comparison.

[xnview]

yes a known issue on dark color

[damian101]

Internal bit depth needs to be higher, then this won't happen. Currently, it seems to be 8 bits per channel, which is a joke, awful banding everywhere in dark gradients. Linear light needs to be processed at 16 bits per channel (sufficient for this) or higher internally.

[Rotareneg]

Just ran into this issue and was wondering what on earth was going wrong until I tried turning gamma correction off.

Top image is original, bottom is resized to 1919x1079, nearest neighbor with gamma correction enabled:

RGB_gradient.png

RGB_gradient_gamma_correct_scaled.png

[xnview]