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POSharing between QOpenGLContext and QGLWidget
 

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    <p>The short form of the question: How can I draw in my QGLWidget using my FBO as a texture, without just getting a blank white image?</p> <p>And, some background and details... I am using Qt 5.1 for an app that does some image processing on the GPU. have a “compositor” class which uses a QOffscreenSurface, with a QOpenGLContext, and a QOpenGLFramebufferObject. It renders some things to the FBO. If the app is running in a render only mode, the result will get written to a file. Run interactively, it gets shown on my subclass of QGLWidget the “viewer.”</p> <p>If I make a QImage from the FBO and draw that to the viewer, it works. However, this requires round-tripping from GPU-> QImage-> Back to the GPU as a texture. In theory, I should be able to use the FBO as a texture directly, which is what I want.</p> <p>I am trying to share between my QOpenGLContext and the QGLWidget’s QGLContext like so:</p> <p>viewer = new tl::ui::glViewer(this); compositor = new tl::playback::glCompositor(1280, 720, this); viewer->context()->contextHandle()->setShareContext(compositor->context);</p> <p>Is it possible to share between the two types of contexts? Is this the way to do it? Do I need to do something else to draw in the viewer using the FBO in the compositor? I’m just getting solid white when I draw the FBO directly instead of the QImage, so I’m clearly doing something wrong. </p> <hr> <p>So I have figured out my problem. I was misinterpreting the documentation for setShareContext() which notes that it "Won't take effect until create() is called" which I mistakenly thought meant you had to share the context after it was created. Instead, sharing has to be established right before:</p> <pre><code>viewer = new tl::ui::glViewer(this); compositor = new tl::playback::glCompositor(512, 512, viewer-&gt;context()-&gt;contextHandle(), this); </code></pre> <p>and the new constructor for my glCompositor:</p> <pre><code>offscreenSurface = new QOffscreenSurface(); QSurfaceFormat format; format.setMajorVersion(4); format.setMinorVersion(0); format.setProfile(QSurfaceFormat::CompatibilityProfile); format.setSamples(0); offscreenSurface-&gt;setFormat(format); offscreenSurface-&gt;create(); context = new QOpenGLContext(); context-&gt;setShareContext(srcCtx); context-&gt;setFormat(format); context-&gt;create(); context-&gt;makeCurrent(offscreenSurface); QOpenGLFramebufferObjectFormat f; f.setSamples(0); f.setInternalTextureFormat(GL_RGBA32F); frameBuffer = new QOpenGLFramebufferObject(w, h, f); </code></pre> <p>Will create a new FBO in a new context which is sharing with the viewer's context. When it comes time to draw, I just bind glBindTexture(GL_TEXTURE_2D, frameBuffer->texture());</p> <p>I am marking the answer I got correct, even though it didn't directly solve my problem. It was very informative.</p>
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