[AppStream] Rethink how we handle the icon cache in AppStream

[Richard Hughes]

On 6 October 2016 at 20:53, Matthias Klumpp <matthias at tenstral.net> wrote:
> 1) Graphics people told me that downscaling 64x64px icons to e.g.
> 48x48px yields worse results than downscaling 128x128px to 64x64px -

Not unexpected, 64->48 is non power of two.

> so, should we ship bigger icons by default and drop 64x64?

I think 64x64 would be sorely missed.

> 2) Many software centers want huge icons (KDE Discover, Elementary
> AppCenter), and if only the 64x64px icons are available, the
> experience isn't great. Again, should we waste more bandwith and
> diskspace and ship bigger icons by default?

How many upstream apps ship an icon larger than 64px -- same question
again for 256 and 512? I can't believe 48x48 is even legible on a
hidpi screen. It's not like every upstream app ships 512px versions of
it's icons.

> 3) Many people want SVG icons in AppStream too, which would solve the
> file-size problem, but SVGs render very slowly (Richard says up to 50x
> slower than PNGs). We need to render possibly hundreds of icons in
> AppStream GUI frontends, so if it really is that slow we should not do
> it.

Also, there are quite a few files that render fine with inkscape but
fail to render correctly with rsvg.

> I would like to be convinced that SVGs are a good idea and render
> quickly enough before cosnidering to allow them.

And the memory footprint of doing so, especially for threaded apps
like g-s that are doing lots of things all in parallel.

> 4) Issue https://github.com/ximion/appstream/issues/21 exists to
> properly provide different DPI for icons, instead of just declaring
> 128x128px to be "HiDPI"

Why not declare that 512x512 or 256x256 is the new HiDPI? Anything
powerful enough to have a HiDPI screen is going to be capable of
scaling an image to an integer division in super quick time. We just
then ship 64x64 always and then the biggest of the available 128, 256
or 512 icons.

> especially since there are only few apps shipping low-res icons

I think this is a good point; unless _everything_ ships HC icons it
looks basically awful. The same effect can be seen when you see a
toolbar with 90% symbolics and 10% full color icons; it just looks
really bad.

Richard.