[Interest] Questions regarding Scene Graph (QSGClipNode, QSGTexture, QSGOpacityNode)

[Gunnar Sletta]

1. Both the transform node and the clip node for the child hierarchy is managed internally in the QQuickItem and you don't have access to those. The paint node is just the visual part, hence the name "paint node" :)

As you don't have access to the item's clip node, you can't do clipping using this API. Your options would be to either use a ShaderEffect + ShaderEffectSource to make an opacity mask (like OpactiyMask from QtGraphicalEffect) or to abuse the knowledge that there will be a clip node as one of your paint node's direct or close to direct parents. The opacity mask approach comes at a perf cost and abusing the clip node is subject to future changes in the structure, so neither options are ideal, but those are the ones available :p

2. Items beneath a clip node are rendered. We can easily reject an entire sub tree when opacity is 0, but we would have to check the bounds of every single leaf node in the case of clipping, and we've decided to not do that.

3. Nodes will have the OwnedByParent flag set by default, so they will be gathered along with everything else when the scene graph is nuked upon window closing. If you take a node out of the scene graph (by receiving non-null and returning null in updatePaintNode, for instance), you are yourself responsible for deleting that node as it will no longer be managed by its parent.

cheers,
Gunnar


> On 09 May 2016, at 22:54, Artem Fedoskin <afedoskin3 at gmail.com> wrote:
> 
> Thank you for your reply Gunnar.
> 
> I found a great class for drawing polygons in QML here http://gitlab.unique-conception.org/qt-libraries/lib-qt-qml-tricks (QQuickPolygon) and adopted triangulation methods of this class to my needs.
> But as always, a bunch of questions arose :)
> 	• Initially, I thought that Scene Graph reparents all visual children of the QQuickItem to the node that is returned from call UpdatePaintNode. However, when I tried set to return QSGClippingNode from the parent  item's updatePaintNode, nodes of its children QQuickItems weren't reparented to the clipping node. I also noticed from debugger output that when I set QQuickItem's property "clip" to true, new QSGClippingNode is created implicitly, to which all nodes of child QQuickItems are reparented. How can I do something like native QML clipping but with my own clipping geometry?
> 	• Is the node that is hided under the area outside clipping polygon rendered? I noticed that nodes with 0 opacity in debugger has "BLOCKED" state whereas hided by clipping nodes has the same state as visible.
> 	• In which cases are nodes deleted? If I don't delete a main node of QQuickItem in cases when boundingRect is empty as I saw in examples from Qt can I be sure that throughout the whole run of my program the nodes won't be implicitly deleted by Scene Graph. Is it very important to delete nodes when boundingRect is empty?
> Thank you very much for your help. You keep my project going :)
> 
> Regards, Artem
> 
> 
> 2016-05-09 11:31 GMT+02:00 Gunnar Sletta <gunnar at sletta.org>:
> 
> > On 08 May 2016, at 01:22, Artem Fedoskin <afedoskin3 at gmail.com> wrote:
> >
> > 1. Could you tell me please how one can set custom geometry for QSGClipNode (not the rectangular one)? Should I just create QSGeometry with QSGGeometry::defaultAttributes_Point2D() attribute and add vertices? Will Scene Graph recognize that polygon is closed (I need clipping to have a form of circle)?
> 
> You need to add triangles though, not a polygon outline. And when you add triangles, closing the polygon is no longer an issue :)
> 
> > 2. If I want to maintain a cache for a bunch of QSGTextures and reuse them for changing texture of QSGSimpleTextureNode how should I delete them? Should I just explicitly delete them in destructor of QSGNode derived object in which I hold them?
> 
> Yes, you need to manage them yourself and deleting them from a QSGNode is the most sensible place as QSGNodes will be cleaned up automatically.
> 
> > 3. If I set opacity to 0 does this mean that node is not rendered? How is it different from just taking a node out of a node tree?
> 
> The approaches should be pretty much the same thing internally.
> 
> cheers,
> Gunnar
> 
> >
> > Regards, Artem Fedoskin
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