[Interest] QPixmapCache considerations on ios and android

[maitai]

Thanks Elvis for your reply,

A compressed in-memory cache would be just perfect in my case.

So I can save a QPixmap in a QByteArray in let's say png format and use 
a regular QCache instead of QPixmapCache...

I'll give it a shot for sure

Thanks again, I think you made my day :)
Philippe

Le 25-10-2015 16:20, Elvis Stansvik a écrit :
> 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
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