[Development] Fwd: ShaderEffect on animated custom OpenGL QQuickItem is not animated.

Sletta Gunnar Gunnar.Sletta at digia.com
Wed Jul 10 16:14:09 CEST 2013

On 9. juli 2013, at 09:11, "Dr. André Bergner" <andre.bergner.0 at googlemail.com<mailto:andre.bergner.0 at googlemail.com>> wrote:

Hi Qt-devs,

I have problems applying a ShaderEffect to a custom QQuickItem drawing
raw OpenGL directives.

I have a little QML script as test case, which I copied from here:

I replaced the image of that example with my custom QQuickItem drawing
raw OpenGL. This custom item I created following this example:
I.e., I subclassed QQuickItem. Within updatePaintNode I generate a new
QSGSimpleTextureNode having a FBO as texture. On the
QQuickWindow::beforeRendering signal I draw my OpenGL stuff into that

My custom OpenGLItem has some property exposed to QML and is animated
using a Timer within the QML script.

Now, my problem is that while the Blur is inactive my custom OpenGL
code is nicely drawn into the item and animated properly. When I
switch on the shader the shader is computed only once and stays then
static. The animation -- even though still rendered into the FBO -- is
not picked up by the ShaderEffect item. The only way to trigger an
update of the shader. This will generate a new QSGSimpleTextureNode
with a fresh FBO.

When I test the Blur with a small animated Rectangle put inside
another Rectangle this animation is visible within the blur.

What do I need to add to my custom QQuickItem in order to signal
changes and trigger redraws of dependent items such as that blur? I
could not find any apportierte signal. What else do I miss?

Try calling QSGNode::markDirty(QSGNode::DirtyMaterial) on the node after changing the FBO. The example should strictly speaking also be doing this...

However, if you intend to use the texture as input to a ShaderEffect, you might want to consider implenementing the QQuickItem::textureProvider() function to save the extra FBO.


Thanks a lot in advance,

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