[Interest] Custom QSGGeometryNode Scene blending issue

[Roberto Garrido]

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
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