Weird artifacts when encoding PNG

[chescos]

Weird artifacts when encoding PNG

[lovell]

https://sharp.pixelplumbing.com/api-output#png

[options.quality] use the lowest number of colours needed to achieve given quality, sets palette to true

[options.palette] quantise to a palette-based image with alpha transparency support

When quality is set, palette is also selected and therefore you have chosen to use a lossy, quantised output.

[chescos]

Ah, I see. I understand that it's a lossy output, but the output from my example is still very unexpected for me. The background color is a single color, and it just adds a completely different color but just in some areas. Is this the desired/expected functionality?

[chescos]

Is there any way to prevent this from happening while still setting a quality?

[lovell]

The prebuilt binaries provided by sharp include the BSD-licenced libimagequant v2.4.1 for generating and mapping image palettes.

You may have some luck building your own libvips from source compiled with support for an alternative quantisation library such as a more recent GPL-licenced libimagequant or quantizr, but this is left as an exercise for the reader.

[chescos]

Thanks! I ended up setting dither to 0.0, which seems to fix the issue in all instances I tested. To your knowledge, should this reliably fix the issue?

[lovell]

Ah yes, this does look like dithering gone a bit wrong, so altering the dither level until you find a value that works for the type of input you'll be processing is definitely a good idea.

[lovell]

I hope this information helped. Please feel free to re-open with more details if further assistance is required.