When comparing XnView vs nomacs, the Slant community recommends nomacs for most people. In the question“What are the best FOSS image viewers for Linux?” nomacs is ranked 2nd while XnView is ranked 4th.
Ranked in these QuestionsQuestion Ranking
Pro Supports over 500 image formats
Pro Includes simple editing tools
Typical image editing tools are included. For instance, there are tools for color and size manipulation, several filters and effects.
Pro Available for macOS, Linux and Windows
Pro Easy redate and view configuration
Pro Compare images side-by-side
To delete blurry phone pics, etc.
XnView speaks over 30 languages.
Pro Portable version for Windows available
Pro Highly customizable
Pro Detailed view of image metadata
Pro Lots of image adjustment tools
Pro Simple and pleasant look
No cramped toolbars or large icons; the UI is cohesive, minimal and keeps out of your way.
Starts very quickly and has a small memory footprint.
Pro You can work with hot keys that can be reassigned as you want them
Pro Frameless view
Pro Allows to cut (frame) images
It is important if you have to work with photos of documents and want to cut off the background on which you made photos.
Pro Flexible thumbnail views
Can display thumbnails in a strip or a zoomable grid as well, also has hotkey for both of these. Opening a folder is one of the main actions on the toolbar!
Has builds for Windows, Mac and Linux.
Pro Remembers last used folders
Often used folders (e.g., screenshots) can be quickly accessed.
Pro Batch image processing
Has tools for applying conversions etc. to many files at once.
Pro Can print to printer
Unlike most other image viewers.
Pro Supports new image formats out of the box
Such as WebP, AVIF, etc.
Pro Comparing two images
This feature is called Synchronization. With the synchronization it is possible that multiple viewers perform the same action (like panning, zooming, etc.). This feature is useful when comparing two images. See more here.
Con No Dark mode
Con Very poor quality and very invazive on windows 10
Convert all images to 8bit rgb
[not suport 16bit or 32bit, cmyk, Lab, multichannel] so you never be able to see real original image, you only see the result of conversion...on windows 10 it enforced self promotion to default app for every type/format of image...
Con No custom installation - if you need i.e. a language package, you need to install the full version
Con Free only for private use
For use in a company 1 license starts at 26 EUR.
Con Doesn't show movies on phone in correct position
Some of mp4 files shows in upside down position.
Con Doesn't pan/zoom fluidly with touchscreen
Con No support for JPEG-XL
Con Only partial support for HEIC
Apple's HEIC format is only partially supported. Nomacs can open individual images, but can't navigate among them.
Con Heavy compared to other Linux image viewers
Uses more memory and CPU than other lightweight image viewers. High CPU usage when starting.
Con Returns to default settings after update
Con Recent version has slowed down
The latest version (3.8) suddenly became very slow in Fedora 26. Slow to step from image to image; slow to open a directory; slow slow slow. Downgrading to the previous version (3.6) restored speed.