"Why is the compression efficiency of avifenc inferior to that of the third-party conversion software iYo Image Converter?

[sojuo]

In order to obtain an AVIF image where every pixel displayed on the screen matches the original image, I used the command avifenc --lossless -j all -y 444 --depth 12 --qcolor 100 --qalpha 100 --min 0 --max 0 --minalpha 0 --maxalpha 0 --speed 0 * *.avif and iYo Image Converter to compress PNG images. Surprisingly, avifenc was less efficient than iYo Image Converter, with avifenc's compressed images occupying more storage space than those compressed by iYo Image Converter.
Original Image:

Images generated by the avifenc command(33.9 KB):

Images converted by iYo Image Converter(23.7 KB):

iYo Image Converter Settings:

My goal is to obtain a lossless AVIF image where every pixel displayed on the screen matches the original image. I would like to know if the issue lies with the parameters I entered for avifenc, or if there is another reason why the images generated by avifenc are not the smallest. Is there a way to make avifenc generate the smallest possible AVIF image?

Additionally, you can find all the files I generated here:files.zip

[y-guyon]

Thank you for your interest in libavif.

It seems the AVIF files in your files.zip archive do not decode to exactly the same pixels as original.png when converted with avifdec. I used the imagemagick compare tool to check for PNG equivalence.

Please try the following command line:

avifenc --lossless input.png output.avif

[sojuo]

Thank you for your interest in libavif.

It seems the AVIF files in your files.zip archive do not decode to exactly the same pixels as original.png when converted with avifdec. I used the imagemagick compare tool to check for PNG equivalence.

Please try the following command line:

avifenc --lossless input.png output.avif

Thank you for the command. It generated a 32KB file, but it is still larger than the 24KB file produced by iYo Image Converter.

[y-guyon]

But is the 24KB file produced by iYo Image Converter mathematically lossless? It was not the case in my local test based on your files.zip.

If you are only interested in visually lossless and not mathematically lossless, I would suggest some high quality instead of --lossless:

avifenc -q 90 --speed 0 input.png output.avif

[sojuo]

But is the 24KB file produced by iYo Image Converter mathematically lossless? It was not the case in my local test based on your files.zip.

If you are only interested in visually lossless and not mathematically lossless, I would suggest some high quality instead of --lossless:

avifenc -q 90 --speed 0 input.png output.avif

How can I determine if this image has been converted from the original losslessly?

[y-guyon]

As I mentioned in #2533 (comment), I used avifdec to get back to PNG format, then an ImageMagick tool to highlight the differences:

avifdec iYO_Image_Converter.avif iYO_Image_Converter.avif.png
compare original.png iYO_Image_Converter.avif.png diff.png

diff.png

To doublecheck with GIMP, the bottom status bar background color goes from #073642 in original.png to #073542 in iYO_Image_Converter.avif for example.