[Jan]

Hey, I ran into an issue on a certain Android device, where clipping of items do not work under certain conditions. I’ve got an android tablet, where the following code only clips on Rotation angles 0 degrees, 90 degrees, 180 degrees & 270 degrees. Other platforms work fine and also another android device works fine. Seems to be a Qt bug, as it also happens in a standard Qt app. Here’s the code:

		Rectangle {
			id: clipTurn
			anchors.centerIn: parent
			width: 20
			height: 100
			color:"red"
			clip: true
			property real angle: 0
			Timer{
				running:true
				repeat: true
				interval: 400
				onTriggered:{
					clipTurn.angle+=10;
				}
			}
			Rectangle {
				anchors.horizontalCenter: parent.horizontalCenter
				width: 10
				height: 130
				color: "yellow"
			}
			transform: Rotation { origin.x: clipTurn.width/2; origin.y: clipTurn.width/2; angle: clipTurn.angle}
		}

Does anyone at least have a workaround for that?

[Jan]

An educated guess would be, that some conversion from polar- to carthesian coordinate system goes wrong…

Btw: The affected device is a Motorola Xoom with Android 4.0.4 installed

[Jan]

Ok, seems like I’ve found the reason for my issue:

http://qt-project.org/doc/qt-5/qtquick-visualcanvas-scenegraph-renderer.html

Let me cite the description for clipping: “If the item is rotated by a non-90-degree angle, the OpenGL’s stencil buffer is used.”

In my ouptut from the Android device I can see the following:

W/Qt ( 1125): scenegraph/qsgcontext.cpp:434 (virtual void QSGRenderContext::initialize(QOpenGLContext*)): QSGContext::initialize: stencil buffer support missing, expect rendering errors

So thats the root cause; but does anyone know what to do? I really need that feature 🙁

 

 

[ChristianFelgo Team]

Hi,

a workaround would be to use an Image for clipping and use an OpacityMask component, like we did for clipping the contents of a Facebook Profile image. You can see a source codes example in this related forum thread. You could also ask in the Qt Project support forums if others have another workaround for the stencil buffer issue.

Cheers, Chris

[Jan]

Good point, thanks.

I got it to work in a different way. I was able to separate my sprite into a stretchable and a non stretchable part. That way I don’t need clipping, which seems to be beneficial in regards of UI performance.

[ChristianFelgo Team]

Yes absolutely, clipping has a performance penalty so any approach that prevents it at all is a good one. 🙂

Can you maybe share the piece of code you used for splitting the sprite, as it may be interesting for other developers too?

Cheers, Chris