how to handle millions of items ?
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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.
@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 ! -
Before going further, what kind of rendering are you looking at ? Just to ensure you are choosing the right tool for your task.
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Before going further, what kind of rendering are you looking at ? Just to ensure you are choosing the right tool for your task.
@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.
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Is it a 3D scene ?
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@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.
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@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.
@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. -
@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.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.
