Phototonic vs feh
When comparing Phototonic vs feh, the Slant community recommends feh for most people. In the question“What are the best FOSS image viewers for UNIX-like systems?” feh is ranked 3rd while Phototonic is ranked 15th.
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Pros
Pro Clean and functional UI
Beside being pretty clean and easy-to-use, the UI is also pretty fast.
Pro Can be easily keyboard driven
Lots of pre-defined keyboard shortcuts.
Pro Highly customizable
Lots of options to personalize your experience. You can set Photonic to fit images by width and height, or width or height; rotate according to Exif orientation; show/hide image name in viewer; adjust space between thumbnails; adjust delay between slides.
Pro Loads up images extremely fast
Pro Minimalistic
Goes well with tiling window managers.
Cons
Con No Dark mode
And not a lot of customization available.
Con No WebP image format support
Con Junky rescaling by default in WMs
Fixable by running these commands:
echo "alias feh='feh --scale-down --image-bg black & disown" >> ~/.bashrc
source ~/.bash_profile
But after entering those two commands for all users, feh works flawlessly, and is extremely responsive. Would definitely recommend.