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<p class="MsoNormal">Dear Qt Community,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">I am writing my first application using a compute shader based on Qt3D in C++. My question: What is the correct way of updating the data needed by the compute shader during run-time? This is what I tried so far:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I create a QBuffer object (in form of a QPointer) and fill it with data. The pointer is kept as a member in a class. I set the buffer via .data() to a QVariant and set it as data to the corresponding QParameter. The following
 is a code snippet showing the basic idea:<o:p></o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:purple;mso-fareast-language:DE"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:purple;mso-fareast-language:DE">QByteArray</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:silver;mso-fareast-language:DE">
</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:#092E64;mso-fareast-language:DE">interpolationBufferData</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:silver;mso-fareast-language:DE">
</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE">=<span style="color:silver">
</span><i><span style="color:#00677C">buildInterpolationMatrixBuffer</span></i>(<span style="color:#092E64">matInterpolationMatrix</span>);
</span><span lang="EN-US" style="color:green">//Builds</span><span lang="EN-US" style="color:silver">
</span><span lang="EN-US" style="color:green">the</span><span lang="EN-US" style="color:silver">
</span><span lang="EN-US" style="color:green">data array</span><span lang="EN-US" style="color:silver">
</span><span lang="EN-US" style="color:green">from a large Eigen Matrix </span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE"><o:p></o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:maroon;mso-fareast-language:DE">m_pInterpolationMatBuffer</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:silver;mso-fareast-language:DE">
</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE">=<span style="color:silver">
</span><span style="color:olive">new</span><span style="color:silver"> </span><span style="color:purple">Qt3DRender</span>::<span style="color:purple">QBuffer</span>(<span style="color:purple">Qt3DRender</span>::<span style="color:purple">QBuffer</span>::<span style="color:purple">ShaderStorageBuffer</span>);<o:p></o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:maroon;mso-fareast-language:DE">m_pInterpolationMatBuffer</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE">-><span style="color:#00677C">setData</span>(<span style="color:#092E64">interpolationBufferData</span>);<o:p></o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:maroon;mso-fareast-language:DE">m_pParameter</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE"> ->setValue(<span style="color:purple">QVariant</span>::<span style="color:#00677C">fromValue</span>(<span style="color:maroon">m_pInterpolationMatBuffer</span>.<span style="color:#00677C">data</span>()),<span style="color:silver">
</span><span style="color:navy">QStringLiteral</span>(<span style="color:green">"InterpolationMat"</span>));<o:p></o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:maroon;mso-fareast-language:DE">m_pInterpolationMatBuffer</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE">-><span style="color:#00677C">setData</span>(<span style="color:#092E64">interpolationBufferData</span>);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Everything works fine until here. The visualization is doing what it is supposed to do and the compute shader works fine. After a new input matrix arrives I create a new QByteArray and try to load to the QBuffer object
 via updateData (I also tried setData). Once I try to update the data in the QBuffer the application crashes.<o:p></o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:purple;mso-fareast-language:DE">QByteArray</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:silver;mso-fareast-language:DE">
</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:#092E64;mso-fareast-language:DE">interpolationBufferData</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:silver;mso-fareast-language:DE">
</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE">=<span style="color:silver">
</span><i><span style="color:#00677C">buildInterpolationMatrixBuffer</span></i>(<span style="color:#092E64">matInterpolationMatrix</span>);
</span><span lang="EN-US" style="color:green">//Builds</span><span lang="EN-US" style="color:silver">
</span><span lang="EN-US" style="color:green">the</span><span lang="EN-US" style="color:silver">
</span><span lang="EN-US" style="color:green">data array</span><span lang="EN-US" style="color:silver">
</span><span lang="EN-US" style="color:green">from an incoming Eigen Matrix</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";mso-fareast-language:DE"><o:p></o:p></span></p>
<p class="MsoListParagraph"><span lang="EN-US" style="color:maroon;mso-fareast-language:DE">m_pInterpolationMatBuffer</span><span lang="EN-US" style="mso-fareast-language:DE">-><span style="color:#00677C">updateData</span>(<span style="color:navy">0</span>,<span style="color:silver">
</span><span style="color:#092E64">interpolationBufferData</span>);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:purple"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Am I missing something? Is there a better way to update data used by the compute shader? Should I pay attention to the usage type (StreamDraw, StaticDraw, etc.)? Also, is the complete QByteArray internally copied to the
 GPU when setting only the pointer to the QVariant? Maybe I should create a new buffer every time I receive new data? Questions over questions
</span><span lang="EN-US" style="font-family:"Segoe UI Emoji",sans-serif">😊</span><span lang="EN-US">
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Best from Germany and many thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal">Lorenz<o:p></o:p></p>
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