QML - load multiple Images simultaneously

[huilui]

Hi guys,

I'm currently trying to move an existing application I have that is based on QWidgets over to QML. I was hesitant at first, but QML does work rather well for this app. However, I do have one slight "problem" or drawback at the moment:

In my application I load a bunch of images as thumbnails in a ListView (a bunch of Image elements aligned in a Row). I really like the ImageProvider feature of QML, it's really handy! However, it seems to be a little slower than what I had before in pure C++/QWidgets, even though the ImageProvider I'm using makes use of almost exactly the same code I used before to load the thumbnails (it's basically a simple scaling, database caching, returning).

So, my question is this: Does anybody know if it's possible to load multiple images simultaneously? At the moment, it loads one image at a time. However, the slow part of the ImageProvider are (I guess) the scaling and caching. Since pretty much all computers nowadays have several CPU Cores available, I'm thinking, that loading 2/3/4 images simultaneously would be significantly faster than one at a time...

Thanks guys!

huilui

[wysota]

If you are doing scaling at C++ level then it is not worth it. If you scale the image in QtQuick, you will get it almost for free as the GPU will scale the texture much faster than your CPU will scale the bitmap.

[huilui]

Well, what I did just now, is, timing every line of the ImageProvider to see exactly where time is lost along the way, and as it turns out the scaling actually isn't responsible for the slow down, but instead the storing of the thumbnail in a database was. So, now I'm using a transaction and a timer to commit all the thumbnails to the database once they're all finished loading. Thus, now I'm down at ~50-80ms per thumbnail created, which is about as fast as it is in the QWidget version.
(I'm going to stick with doing all that in C++, as I'm also doing some painting on the thumbnails using QPainter...)

However:

If you are doing scaling at C++ level then it is not worth it.

I'm still confused why you think loading 2-4 images simultaneously on different CPU cores wouldn't give any benefits speedwise?

[wysota]

I'm still confused why you think loading 2-4 images simultaneously on different CPU cores wouldn't give any benefits speedwise?

Because your GPU will do it about 100 (or more) times faster. I'm not saying you shouldn't do it anyway if you intend to stick with scaling in C++. But if you ever decide to do some animations or alike in QtQuick where you will want to scale the thumbnail up (e.g. to highlight it), you will lose quality.

As far as painting on the image is concerned, you can still do it in QML using a custom item (derived from QQuickPaintedItem) keeping all the benefits of QML and QtQuick.