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POCan't set desired OpenGL version in QGLWidget
 

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  1. POCan't set desired OpenGL version in QGLWidget
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    <p>I'm trying to use QGLWidget in Qt 4.8.2. I noticed that the default context QGLWidget creates doesn't show any output for OpenGL above 3.1. <a href="http://qt-project.org/wiki/How_to_use_OpenGL_Core_Profile_with_Qt" rel="noreferrer">The Qt wiki has a tutorial</a> that demonstrates using OpenGL 3.3 to draw a simple triangle. When I try to run the tutorial, I get a blank screen. If I change the OpenGL version to 3.1, I get the expected output (a red triangle).</p> <p>My video card supports OpenGL 4.2, and calling <code>QGLFormat::openGLVersionFlags()</code> before creating the QGLWidget shows that Qt detects OpenGL 4.2 and all previous desktop versions.</p> <p>Here's another minimal example:</p> <pre><code>#include &lt;QApplication&gt; #include &lt;QGLWidget&gt; #include &lt;QDebug&gt; #include &lt;QtDeclarative/qdeclarativeview.h&gt; int main(int argc, char * argv[]) { QApplication app(argc, argv); qDebug() &lt;&lt; "OpenGL Versions Supported: " &lt;&lt; QGLFormat::openGLVersionFlags(); QGLFormat qglFormat; qglFormat.setVersion(4,2); // get expected output with (3,1) and below, else blank window qglFormat.setProfile(QGLFormat::CoreProfile); qglFormat.setSampleBuffers(true); QGLWidget* qglWidget = new QGLWidget(qglFormat); QString versionString(QLatin1String(reinterpret_cast&lt;const char*&gt;(glGetString(GL_VERSION)))); qDebug() &lt;&lt; "Driver Version String:" &lt;&lt; versionString; qDebug() &lt;&lt; "Current Context:" &lt;&lt; qglWidget-&gt;format(); QDeclarativeView mainView; mainView.setViewport(qglWidget); mainView.setSource(QString("helloworld.qml")); mainView.show(); return app.exec(); } </code></pre> <p>Here's the output:</p> <pre><code>OpenGL Versions Supported: QFlags(0x1|0x2|0x4|0x8|0x10|0x20|0x40|0x1000|0x2000|0x4000|0x8000|0x10000) Driver Version String: "4.2.0 NVIDIA 295.53" Current Context: QGLFormat(options QFlags(0x1|0x2|0x4|0x10|0x20|0x80|0x200|0x400) , plane 0 , depthBufferSize 24 , accumBufferSize 16 , stencilBufferSize 8 , redBufferSize 8 , greenBufferSize 8 , blueBufferSize 8 , alphaBufferSize -1 , samples 4 , swapInterval 0 , majorVersion 4 , minorVersion 2 , profile 1 ) </code></pre> <p>The <code>QFlags()</code> enum list on the first line describes the OpenGL versions supported. The list shows I support all variants except for OpenGL/ES versions. QFlags() on the third line describes format options (alpha channel, stencil buffer, etc).</p> <p>Anyone know why QGLWidget won't work with anything >= 3.1? I'm on Linux, have an Nvidia GT440, and glxinfo shows it supports OpenGL 4.2.0. The driver version is printed in the sample output above. I'm not too sure what else to try.</p> <p><em>Edit: I made some pretty bad mistakes/assumptions with my explanation of the problem before this edit. The issue is still similar, but hopefully makes a bit more sense now. Sorry for any confusion.</em></p>
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    1. COAre your shaders compiling/linking? If you are using a 4.2 context and incompatible/incorrect shaders, that is one cause for a blank window.
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      1. POCan't set desired OpenGL version in QGLWidget
      1. USStephan van den Heuvel
    2. COI haven't gotten that far yet (shaders). I wanted to use QGLWidget as a viewport for a QDeclarativeView and then add some 3d content that used shaders. But even a simple file with default components like a Rectangle{} renders nothing. It'll render as expected if I don't set the QGLFormat version, but the whole point of this question is figuring out why I can't set it in the first place. Plus, I'd get errors if I (or Qt internally) was using incompatible shaders. The only output is what I pasted in my question.
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    3. COHmm, the only reason why I suggest looking at the shader log, is that OpenGL 3.0 + requires that your own shaders are compiled and loaded, while OpenGL 2.0 and below does not. I have no experiance with using OpenGL through QT, so I do not know how much is handled for you. However, in the example you linked, the author prepares his or her own shaders. If you do not provide shaders to the graphics card using the core profile, you will not render anything.
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