[Interest] Custom QSGGeometryNode Scene blending issue

[Roberto Garrido]

Hi Gunnar,
we were creating our texture using "quickWindow->createTextureFromId",
passing our FBO texture id as parameter, but forgot to add the "
CreateTextureOption::TextureHasAlphaChannel". I think we have the desired
effect now.

Thank you so much for helping us to narrow this down.
Cheers,
Robert.

On 24 June 2016 at 10:40, Gunnar Sletta <gunnar at sletta.org> wrote:

> Since the window's background is being cleared when you draw the
> framebuffer, it sounds as the window's default framebuffer is bound at the
> time of clearing. That or that the FBO is indeed cleared to fully
> transparent and rendered without blending, thus overwriting the pixels
> window.
>
> To further debug this, I would have first checked the contents of the FBO,
> using QOpenGLFramebufferObject::toImage() or using glReadPixels if this is
> your own FBO, and then look at the blending that happens in the scene graph
> if that one is ok. What does the QSGTexture instance look like that you use
> to wrap the FBO. Does it report that there is alpha?
>
> QSG_VISUALIZE=overdraw will tell you whether the FBO is drawn in the scene
> graph using blending or not.
>
> Or simply use QQuickFramebufferObject, of course :)
>
> cheers,
> Gunnar
>
> > On 23 Jun 2016, at 17:26, Roberto Garrido <
> robertogarridomartin at gmail.com> wrote:
> >
> > Gunnar,
> > thanks for your detailed response.
> > We don't discard going for the QQuickFramebufferObject soon, but
> currently what we have is the Item/QSGGeometryNode setup.
> > I've been doing some tests, and I cannot manage to get the desired
> effect. The most silly example I can think of is to clear our buffer with a
> fully transparent color (as you suggested), then render a fullscreen quad
> with also a fully transparent color (using premultiplied alpha -> 1, 1 -
> SRC_ALPHA). That should give us fully transparent item, and the qml scene
> behind should be shown. But what we see is the desktop itself, not the QML
> items behind. Does it give you any clue on what could I be doing wrong?
> >
> > Our node (derived from QSGGeometryNode) takes a QSGTextureMaterial (with
> the default shader for this type of material), and we have also activated
> the QSGMaterial::Blending flag.
> >
> > Thanks,
> > Robert.
> >
> > On 7 June 2016 at 09:41, Gunnar Sletta <gunnar at sletta.org> wrote:
> > Hi Roberto,
> >
> > The opacity of any geometry node is decided by the opacity that comes
> out of the fragment shader of its material. Depending on how the alpha
> logic exists in your custom GL renderer, there are two ways to do this.
> >
> > Option 1: If most of the FBO is partially transparent, or if there are
> bits and pieces that are overlaid on top of the rest of the UI, like
> controls on a HUD, then you want to render into an FBO with an alpha
> buffer, clear it with fully transparent, draw your custom GL stuff and then
> alpha blend the entire FBO on top of the QML scene. You do alpha blending
> of the fbo content by setting the QSGMaterial::Blending flag and returning
> a premultiplied alpha color from the fragment shader. The
> QSGTextureMaterial has a fragment shader that does the right thing.
> >
> > Option 2: If you have an opaque FBO with certain sections, say a given
> rectangle, fully transparent, you can draw the FBO as fully opaque, but
> create a geometry that excludes the fully transparent region. This has the
> benefit you will be drawing more opaque stuff which is generally faster for
> the GPU to handle, compared to blended stuff.
> >
> > It is of course possible to do a mixture of 1 and 2, by creating two
> separate materials, both referring to the FBO. For the fully opaque
> sections of the FBO, you use an opaque material to draw it which is fast.
> For the semi-transparent sections of the FBO you use a different geometry
> node with a material that enables blending. The fully transparent regions
> are fully excluded from either geometry, so those cost nothing. (something
> like the MaskedImage I have here:
> https://github.com/qtproject/playground-scenegraph/tree/master/shapes)
> >
> > Btw, if you used QQuickFramebufferObject instead of rolling your own
> custom Item/node/material, you would get option 1 by default :)
> >
> > cheers,
> > Gunnar
> >
> > > On 06 Jun 2016, at 18:05, Roberto Garrido <
> robertogarridomartin at gmail.com> wrote:
> > >
> > > Hi all!
> > > We have a custom QQuickItem which in turn creates a custom
> QSGGeometryNode. We reimplement the QSGGeometryNode's preprocess() method
> in order to draw our scene to a FrameBufferObject with a texture attached.
> This OpenGL texture ID is shared with the QSGGeometryNode's material. This
> way, we can render our custom OpenGL scene into a QML item. So far, so good.
> > >
> > > However, we want to use alpha blending on some parts of our scene, in
> order to let the user see the QML items behind our custom item. Something
> similar to what Item::opacity value is doing, but only for a specific area
> of our Item. It's like having a hole inside our Item that allows the user
> to see what's behind. The information about the specific region that should
> be transparent is handled inside our custom OpenGL renderer.
> > >
> > > I've been trying to find out what Qt is doing in order to change the
> opacity of a QML item, with no luck.
> > >
> > > Any idea on how can we achieve this effect, with our Item/Node
> configuration?
> > > Thanks in advance,
> > > Robert.
> > >
> > > --
> > > Website: http://robertogarrido.com
> > > Twitter: http://twitter.com/che1404
> > > LinkedIn: http://es.linkedin.com/in/robertogarrido/en
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> >
> >
> >
> > --
> > Website: http://robertogarrido.com
> > Twitter: http://twitter.com/che1404
> > LinkedIn: http://es.linkedin.com/in/robertogarrido/en
>
>


-- 
Website: http://robertogarrido.com
Twitter: http://twitter.com/che1404
LinkedIn: http://es.linkedin.com/in/robertogarrido/en
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