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 <gun...@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 < > robertogarridomar...@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 <gun...@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 < > robertogarridomar...@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 > > > _______________________________________________ > > > Interest mailing list > > > Interest@qt-project.org > > > http://lists.qt-project.org/mailman/listinfo/interest > > > > > > > > > > -- > > 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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