How to avoid aliasing for image on an epub?
 

Hello mobileread community!
As you can see on the following image, the gray and black strokes in this PNG are aliased. I can achieve a perfect quality with SVGs but other issues force me to switch to PNGs (some stroke just doesn’t appear for unknown reasons).

https://i.stack.imgur.com/In2hk.png

Original PNG image


Does anyone have similar experience? Is that related to my device (Kobo Aura)?

Your problem is probably similar: Resize the image with a decent image editing software to a reasonable size, and avoid the device having to do the resizing (which creates the aliasing).

Thank you for your answer. How can I know what is the best size for my device? How can I make sure this size will fit only to my device?

A matter of taste, but I personally find that the quality of the display starts to deteriorate visibly after more than 2x magnification.
An online Swiss Knife in the field of good image editing is https://www10.lunapic.com/editor/. If you choose the scale option, the width / height ratio remains the same, see attachment.

The image is 24 bits with an alpha channel (transparency).
That's expecting a fairly sophisticated reader. And it's unnecessary

Just make it greyscale, black and grey on white.
Reduced the palette to 4 colours, the filesize is 1/6th, and should work better.
https://imgur.com/dWQwkeR
I use Irfanview to do simple transformations like this.

if you want to scale it, it can do that too: here's 800 px wide. Even smaller file.

That’s a good advice, thank you. I will try to lower the general quality and remove the alpha channel. That can explain the aliasing. Thanks!

But do not forget to test the images with a 6" 300DPI Reader to make sure the image sizes are large enough.