Re: Replacing the animal photos in the ‘erase’ activities

[Karl Ove Hufthammer]

Karl Ove Hufthammer skreiv 20.02.2022 18:24:
>
>> But first thing before moving to it, is to make sure we can package it
>> and make it work fine for all our supported platforms.
>
> I hope it does. I have tested WebP on a few images, and while very 
> high compression levels (of course) did result in visible artefacts, 
> these artefacts were much less *annoying* than the corresponding JPEG 
> artefacts. So with WebP we can probably use both high resolution and 
> higher compression.

Here are a couple of example images to illustrate this. The file size of 
the animal images in GCompris range from 220 KiB to 38 KiB (the latter 
being a *very* low-quality image). If we use a target file size of 150 
KiB, we will be able to include ~35 images.

My first example image is this cheetah:
https://commons.wikimedia.org/wiki/File:Cheetah_(Acinonyx_jubatus)_female_2.jpg

After cropping, resizing to 1920 × 1080 and subtle sharpening, it looks 
like this in full quality (~2 MiB):
https://huftis.org/kritikk/gcompris/cheetah-original.jpg
(If viewing in a browser, remember to click to zoom in and see it in 
full resolution.)

The ~150 KiB JPEG shows ugly banding/posterisation in the background:
https://huftis.org/kritikk/gcompris/cheetah-jpeg-150.jpg
(I used Gimp to save it, but a JPEG of similar size converted with 
ImageMagick corresponds to a -quality setting of < 20, i.e., very low 
quality.)

The similarly-sized WebP has acceptable artefacts:
https://huftis.org/kritikk/gcompris/cheetah-webp-150.webp


Here are corresponding images for this horse image:
https://upload.wikimedia.org/wikipedia/commons/f/fa/Zaniskari_Horse_in_Ladakh.jpg

https://huftis.org/kritikk/gcompris/horse-original.jpg
https://huftis.org/kritikk/gcompris/horse-jpeg-150.jpg
https://huftis.org/kritikk/gcompris/horse-webp-150.webp

Here I actually think the JPEG looks slightly better. The WebP version 
loses some details in the darker, front part of the horse. I wonder if 
this is a general problem with WebP, as you can see the same 
over-smoothing effect in the red eye of the cheetah.

Also note that here I have used a target size of 150 KiB. But some 
images have more details, and will benefit from lower compression levels 
(higher file sizes), while some have fewer details, and can be reduced 
(perhaps much) more in file size without much loss of quality. I think 
it’s better to aim for a target *average* file size of, for example, 150 
KiB and use the ‘-quality’/‘-q’ parameter when converting the images. 
Then every image should be of ~equal quality, while the file sizes will 
vary.

-- 
Karl Ove Hufthammer
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