[Interest] QPixmapCache considerations on ios and android

[Elvis Stansvik]

2015-10-25 15:53 GMT+01:00 maitai <maitai at virtual-winds.org>:
> Hello,
>
> I have several relatively big pixmap I'd like to manage through cache.
> Big meaning 1024x1024 at a maximum (one edge is 1024 while the other is
> less or equal). I need to keep something like 10 of those in a cache
> system. Each image being quite heavy to paint, that would help a lot.
>
> I believe each pixmap uses a maximum of 1024 * 1024 * (32 / 8 byte) /
> 1024 / 1024 = 4 Mb, right?
>
> QPixmapCache is eating way too much memory for that (40MB +
> "whatever-is-needed-for-qt" on regular memory), on android and
> especially iOS it's a no go. Plus it's kind of shared with qt own needs
> so it's hard to decide which limit to give.
>
> What would you advise? Saving the pixmap on disk, with a "kind of cache"
> system to limit it to a given size, last used is dropped etc? Using
> opengl textures to store/restore img with a similar cache system? Using
> QPixmapCache anyway? Something else?
>
> I should add that in our case it's way faster to read the pixmap from a
> pre-saved png than to re-generate it, and that quality is ok for our
> needs. So a disk-cache might be ok. But 40mb on a mobile device is
> something expensive....

If you used a disk cache with PNG files, the images would be
compressed, so probably a lot less than the 4 MB per image you
calculated above.

I guess you could also try to make a compressed in-memory cache.

Cheers,
Elvis

>
> Thanks
> Philippe Lelong
> _______________________________________________
> Interest mailing list
> Interest at qt-project.org
> http://lists.qt-project.org/mailman/listinfo/interest