[Interest] QOpenGLFramebufferObject, render to FBO using texture from another Image item?

[Ola Røer Thorsen]

I recently discovered the new way of doing render-to-FBO in Qt 5.2.0. It's
a major improvement compared with what had to be done previously, I think!
:-)

However I have run into something tricky as I want to do something more
advanced:

I need to render some textured geometry into an FBO. The use-case is
something similar to correcting for lens artifacts in a photograph, and the
geometry is too advanced for using the Shadereffect items (can't use a
regular mesh grid).

The texture is loaded from a .jpg file on disk. The image is rather large,
so I want to use similar mechanisms as used by the Quick2 Image item
(cache, asynchronous loading, etc).

It's not a good idea to use the internals from qquickimage, so I thought of
doing something similar to what the Shadereffectsource item does, by using
the texture created by an Image item. This way, I can re-use all the
advanced things the Image item does internally for loading an image.

I have now got a class inheriting QQuickFramebufferObject. It has a
sourceItem property, which holds a QQuickItem*. In my QML code, i set this
to be a Image item.

Image {
   id: rawImage
   source: "picture.jpg"
   visible: false
   asynchronous: true

   onStatusChanged: {
      (status === Image.Ready) {
         fboRender.updateTexture(); // this calls "update();" on the item
in c++
      }
   }
}

MyFboRenderer {
   id: fboRender
   sourceItem: rawImage
}

In c++, I now need to safely pass the texture eventually found in the
sourceItem to my QQuickFramebufferObject::Renderer. This has to be
thread-safe in every way. As the Image and FBO render items in QML are to
be used in model view delegates, they are created and deleted runtime.

Any hints on how to accomplish this would be very welcome!

Things I've found so far are:
- calling "sourceItem()->textureProvider()->texture()" from the renderer's
"synchronize" function returns "0" even if synchronize was triggered as
result of an update called when the image has completed loading.
- calling the same from the render-function gives a texture valid texture,
and this sortof works, but there is no obvious safe way of accessing
"sourceItem" at this point. I've tried to store a pointer to either the
source item or the QQuickFramebufferObject, but both of these might be
deleted/invalid when render() is called and crashes do happen.

Cheers
Ola
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