<![CDATA[how to handle millions of items ?]]>i am using QGraphicsView and QGraphicsScene to display millions of items, but it's very slow.
is there any way to improve the GUI speed with this framework ?
I can see many commercial tools can handle even billion of items, how to achieve this ?

]]>https://forum.qt.io/topic/101540/how-to-handle-millions-of-itemsRSS for NodeWed, 29 Apr 2026 11:23:43 GMTMon, 08 Apr 2019 09:24:35 GMT60<![CDATA[Reply to how to handle millions of items ? on Wed, 17 Apr 2019 22:11:22 GMT]]>As suggested you should research LOD algorithms. For example this may be of some use or maybe a clustering algorithm. You may also benefit from a space partitioning scheme that's not the one implemented by the graphics scene, but something custom. In any case such problems are not trivial.

]]>https://forum.qt.io/post/523697https://forum.qt.io/post/523697Wed, 17 Apr 2019 22:11:22 GMT<![CDATA[Reply to how to handle millions of items ? on Wed, 17 Apr 2019 04:33:42 GMT]]>@kenchan Thanks. they are single polygons, some of them may overlapping each other.
yes, I agree I need to minimum the piexl coverage, that means filter out the ones need to render.
the question is how to filter them out according to view port with so many polygons quickly.
when zoom to an area, it should respond quickly, when fit the view, also quickly.
I don't think it's a special case just for me. this should be a general case in many applications.
So i want to see if there is existing framework which can handle such case.

]]>https://forum.qt.io/post/523455https://forum.qt.io/post/523455Wed, 17 Apr 2019 04:33:42 GMT<![CDATA[Reply to how to handle millions of items ? on Mon, 15 Apr 2019 14:21:47 GMT]]>drawing millions of objects in qt

]]>https://forum.qt.io/post/523120https://forum.qt.io/post/523120Mon, 15 Apr 2019 14:21:47 GMT<![CDATA[Reply to how to handle millions of items ? on Mon, 15 Apr 2019 11:55:15 GMT]]>@student
are these polygon shapes single polygons or meshes of polygons?
Are they overlapping i.e. can be hidden behind or hide others?
Do they have some measure of coherence you can exploit?

These are the kind of questions you should be asking yourself so you can determine the best strategies for LOD.
The obvious strategies are minimum pixel coverage, removing items completely hidden items, removing items outside the viewport.

If you really want to do it brute force use the GPU directly via OpenGL calls to do the drawing and don't rely on the Qt graphics items at all. Even then you might need to use some clever LOD strategies with that many polygons.

]]>https://forum.qt.io/post/523100https://forum.qt.io/post/523100Mon, 15 Apr 2019 11:55:15 GMT<![CDATA[Reply to how to handle millions of items ? on Mon, 15 Apr 2019 05:56:12 GMT]]>any new idea?

]]>https://forum.qt.io/post/523016https://forum.qt.io/post/523016Mon, 15 Apr 2019 05:56:12 GMT<![CDATA[Reply to how to handle millions of items ? on Thu, 11 Apr 2019 02:46:02 GMT]]>@SGaist no, just 2D polygons.

]]>https://forum.qt.io/post/522431https://forum.qt.io/post/522431Thu, 11 Apr 2019 02:46:02 GMT<![CDATA[Reply to how to handle millions of items ? on Wed, 10 Apr 2019 21:53:16 GMT]]>Is it a 3D scene ?

]]>https://forum.qt.io/post/522427https://forum.qt.io/post/522427Wed, 10 Apr 2019 21:53:16 GMT<![CDATA[Reply to how to handle millions of items ? on Wed, 10 Apr 2019 02:14:14 GMT]]>@SGaist I have many polygon shapes, and need to view / edit them, for view/edit, i think just when zoom in, without zoom in, a rough picture is enough.
as you said: """ There's no use trying to render each feather of a Kiwi bird if you are looking at it from the top of a building.

The same goes the other way around, there's no use rendering the full building since you're on top of it trying to look at a Kiwi bird."""

that's it.

]]>https://forum.qt.io/post/522208https://forum.qt.io/post/522208Wed, 10 Apr 2019 02:14:14 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 21:44:22 GMT]]>Before going further, what kind of rendering are you looking at ? Just to ensure you are choosing the right tool for your task.

]]>https://forum.qt.io/post/522194https://forum.qt.io/post/522194Tue, 09 Apr 2019 21:44:22 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 07:23:57 GMT]]>@SGaist thanks for your reply, i am a newer to Qt.
yes, I don't want to show them at full pic view, so I just show them when zoom in to a specific value, in the paint function for each item, I use :

option->levelOfDetailFromTransform(painter->worldTransform()) < SpecificValue: return

but it doesn't help much.

Could you help to describe more about how to filter them ? or is there any example ?
thanks very much !

]]>https://forum.qt.io/post/522003https://forum.qt.io/post/522003Tue, 09 Apr 2019 07:23:57 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 08:01:31 GMT]]>@jsulm yes, agree with you. i believe they must have some mechanisms to process this.
you know, we just want to see detail when zoom in to a specific level, otherwise, just show a rough picture. that's why I asked this question here to see if anyone have experience on such case. in the other world, how to process large amount of polygon shapes by using Qt.
i am a new to Qt :-)

]]>https://forum.qt.io/post/521997https://forum.qt.io/post/521997Tue, 09 Apr 2019 08:01:31 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 07:07:19 GMT]]>@student said in how to handle millions of items ?:

they can show billions of polygon shapes

I doubt that. It is simply not possible to see so many objects on a display (a display does not even has so many pixels...). I'm quite sure they reduce the amount of objects to what actually makes sense.

]]>https://forum.qt.io/post/521994https://forum.qt.io/post/521994Tue, 09 Apr 2019 07:07:19 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 06:58:25 GMT]]>@aha_1980 yes, actually i have more than 4 millions of objects (polygon shapes) that could be 100000000. i want to show a blurred picture unless zoom in.
i see many EDA tools, like synopsys, cadence, they can show billions of polygon shapes.

]]>https://forum.qt.io/post/521993https://forum.qt.io/post/521993Tue, 09 Apr 2019 06:58:25 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 06:50:51 GMT]]>@student

Have you thought that each element needs memory too? Holding 4 million objects in memory may be challenging.

If you really need so many items, you will need to invest some more energy.

]]>https://forum.qt.io/post/521992https://forum.qt.io/post/521992Tue, 09 Apr 2019 06:50:51 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 06:46:22 GMT]]>@aha_1980 thanks, I tried this example before, when I increase the number to 4 millions, it's also very slow . it doesn't help too much.

]]>https://forum.qt.io/post/521990https://forum.qt.io/post/521990Tue, 09 Apr 2019 06:46:22 GMT<![CDATA[Reply to how to handle millions of items ? on Tue, 09 Apr 2019 06:38:28 GMT]]>@student said in how to handle millions of items ?:

or is there any example ?

https://doc.qt.io/qt-5/qtwidgets-graphicsview-chip-example.html

]]>https://forum.qt.io/post/521987https://forum.qt.io/post/521987Tue, 09 Apr 2019 06:38:28 GMT<![CDATA[Reply to how to handle millions of items ? on Mon, 08 Apr 2019 10:24:28 GMT]]>Hi and welcome to devnet,

The usual technique is: render only what make sense to show.

There's no use trying to render each feather of a Kiwi bird if you are looking at it from the top of a building.

The same goes the other way around, there's no use rendering the full building since you're on top of it trying to look at a Kiwi bird.

]]>https://forum.qt.io/post/521849https://forum.qt.io/post/521849Mon, 08 Apr 2019 10:24:28 GMT