[Interest] Qt3D Multiple color targets not working

[Gil H]

Scratch that, forgot to set the item size to fill the window.  Changing that fixes it in the standalone example.

 

 

From: Interest <interest-bounces at qt-project.org> on behalf of Gil H <qt at tastytech.ca>
Date: Thursday, August 8, 2019 at 11:18 AM
To: Qt Interest <interest at qt-project.org>
Subject: Re: [Interest] Qt3D Multiple color targets not working

 

Hi guys, I got it working using Qt3DQuickWindow, but when I tried to integrate the same framegraph in my actual implementation (offscreen rendering to FBO similar to Scene3D’s code) the texture handles come back as 0.  

 

Attached is my standalone example, modified to use Scene3D to illustrate the problem.  It fails to render properly and prints errors about incomplete framebuffers.  If you uncomment the other code block in main.cpp, with a Qt3DQuickWindow it all works fine.  My use case involves taking each rendered texture and drawing it as part of an external custom GL engine.  So, I need Qt3D to render multiple textures of different views, and then I need access to those GL texture handles for manually drawing them onto a different frame buffer.

 

I know Scene3D creates its own FBO, but after digging through Qt source, I still don’t understand how that interacts with QRenderTarget nodes in the frame graph.  It is “illegal” to have this kind of graph with Scene3D? 

 

Thanks!

 

Gil

 

 

 

From: Interest <interest-bounces at qt-project.org> on behalf of Gil H <qt at tastytech.ca>
Date: Wednesday, August 7, 2019 at 9:46 AM
To: Qt Interest <interest at qt-project.org>
Subject: Re: [Interest] Qt3D Multiple color targets not working

 

Thanks Sean.  I had that framegraph set up, but I was trying to use a single RenderTarget with multiple textures attached for all the rendering, instead of what ended up working more easily, which was to simply create a separate RenderTarget for each sub-graph and just attach the textures to color0.  

 

 

From: Interest <interest-bounces at qt-project.org> on behalf of Sean Harmer <sh at theharmers.co.uk>
Date: Wednesday, August 7, 2019 at 4:40 AM
To: Qt Interest <interest at qt-project.org>
Subject: Re: [Interest] Qt3D Multiple color targets not working

 

Hi Gil,

With an FBO with multiple textures attached each texture will still have the same pixels rasterised. You can't draw different cameras and scenes into each texture at the same time. Well something like that can be done with some advanced use of the geometry shader but I don't think that's the case here.

If you want to draw 3 different scenes into 3 different textures you will need 3 different sub-trees in your framegraph. Each one will set the camera to use, the filters to select the subset of the scene graph to draw and the FBO with the target texture to render into.

Cheers,

Sean

On 01/08/2019 18:42, Gil H wrote:

Hello everyone, I’m sure I’m doing something wrong here, but basically, I’m trying to render multiple frame graph subtrees into multiple texture targets, and then draw those as part of the final scene (kind of like seeing security camera feeds on a few textured quads).  Attached is my minimal test app which shows that only textures attached to Color0 seem to contain anything.  Is it because the surface is a QWindow?  I’ve tried a more complicated example with an offscreen render surface and an FBO with multiple color attachments added to it, but the textures I got back from that were still empty.

 

Tested this on Mac with Qt 5.13.0.

 

If anyone is curious about why you would want to do this, I need to render up to 3 different Qt3D scenes for integration into an external GL engine.  So different camera views and scene sub-trees need to be drawn onto separate textures each frame.

 

 

 

 





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