[Interest] Porting Qt5 OpenGL app to Qt6 - QQuickRenderTarget and OpenGL frame buffer object

[Nuno Santos]

Laszlo,

I have a new problem regarding this question.

I have a QQuickRenderControl in order to do a side render and be able to extract the render output. I was reading from the FBO using the glReadPixels function.

But now the render target is a OpenGL Texture but apparently the function that reads from a GLTexture is glGetTexImage but that function is not on the qopenglfunctions.

How can I read the texture now? 

Thanks!

Best regards,

Nuno

> On 8 May 2024, at 16:41, Laszlo Agocs <laszlo.agocs at qt.io> wrote:
> 
> Hi,
> 
> Passing 0 as a renderbuffer object name is not going to work. Try using https://doc.qt.io/qt-6/qquickrendertarget.html#fromOpenGLTexture with the texture that is used as the color attachment of your OpenGL framebuffer object. (actually that FBO is not useful in Qt 6 as one will be created internally as needed)
> 
> Best regards,
> Laszlo
> 
> From: Interest <interest-bounces at qt-project.org> on behalf of Nuno Santos via Interest <interest at qt-project.org>
> Sent: Tuesday, April 16, 2024 4:21 PM
> To: interestqt-project. org <interest at qt-project.org>
> Subject: [Interest] Porting Qt5 OpenGL app to Qt6 - QQuickRenderTarget and OpenGL frame buffer object
>  
> Hi,
> 
> I have an application that was heavily written on top of OpenGL. Right now I want to port it to Qt 6 with the minimum effort trying to stick to the OpenGL backend until I get the grip on the new things of Qt 6 Graphics API abstraction.
> 
> I used to do this in order to assign a target FBO to a QuickWindow:
> 
> _quickWindow->setRenderTarget(_fboRender);
> 
> This is no longer possible and it requires a QQuickRenderTarget to be created and I seem to have three options:
> 
> (since 6.2) QQuickRenderTarget  fromOpenGLRenderBuffer(uint renderbufferId, const QSize &pixelSize, int sampleCount = 1)
> (since 6.4) QQuickRenderTarget  fromOpenGLTexture(uint textureId, uint format, const QSize &pixelSize, int sampleCount = 1)
> QQuickRenderTarget      fromOpenGLTexture(uint textureId, const QSize &pixelSize, int sampleCount = 1)
> 
> I’m trying this:
> 
> _renderTarget = QQuickRenderTarget::fromOpenGLRenderBuffer(0, QSize(_renderResolution.width(), _renderResolution.height()));
> _quickWindow->setRenderTarget(_renderTarget);
> 
> But nothing appears on the screen and I get a warning:
> 
> QQuickWindow: No render target (neither swapchain nor custom target was provided)
> 
> Can anyone tell me what am I failing here?
> 
> Any ideas suggestions are very welcome.
> 
> With my best regards,
> 
> Nuno
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