Dear Qt Community,
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:
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:
QByteArray interpolationBufferData =
buildInterpolationMatrixBuffer(matInterpolationMatrix); //Builds the data array
from a large Eigen Matrix
m_pInterpolationMatBuffer = new
Qt3DRender::QBuffer(Qt3DRender::QBuffer::ShaderStorageBuffer);
m_pInterpolationMatBuffer->setData(interpolationBufferData);
m_pParameter ->setValue(QVariant::fromValue(m_pInterpolationMatBuffer.data()),
QStringLiteral("InterpolationMat"));
m_pInterpolationMatBuffer->setData(interpolationBufferData);
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.
QByteArray interpolationBufferData =
buildInterpolationMatrixBuffer(matInterpolationMatrix); //Builds the data array
from an incoming Eigen Matrix
m_pInterpolationMatBuffer->updateData(0, interpolationBufferData);
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 😊
Best from Germany and many thanks,
Lorenz
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