Drawing lines in QPainter takes a lot of time

[xcxl]

As you said, I currently use the classical :

// Setup our vertex buffer object.
m_myVbo.create();
m_myVbo.bind();
m_myVbo.allocate(m_vertexData.constData(), m_floatCount * static_cast<int>(sizeof(GLfloat)));
m_myVbo.release();

// Setup our vertex buffer object.
m_myVbo.create();
m_myVbo.bind();
m_myVbo.allocate(m_vertexData.constData(), m_floatCount * static_cast<int>(sizeof(GLfloat)));
m_myVbo.release();

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Sorry I don't actually have the skills to understand the second option with the CPU/GPU mapping

The full project, configured for the GPU test (test 6) is available here : sources.zip
The VBO setup is in myGLwidget_vertAndFrag.cpp at line 269.

Could you send me the modif to do to test this option ? I'm interested too.

[xcxl]

Ok, I take my courage to understand the new method, and it was finally not the hard.
I replaced :

m_myVbo.bind();
m_myVbo.write(0, m_vertexData.constData(), m_floatCount * static_cast<int>(sizeof(GLfloat)));
glDrawArrays(GL_TRIANGLES, 0, m_vertexCount);
m_myVbo.release();

m_myVbo.bind();
m_myVbo.write(0, m_vertexData.constData(), m_floatCount * static_cast<int>(sizeof(GLfloat)));
glDrawArrays(GL_TRIANGLES, 0, m_vertexCount);
m_myVbo.release();

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by :

m_myVbo.bind();
auto ptr = m_myVbo.mapRange(0, m_vertexData.size()*int(sizeof(GLfloat)),
                                QOpenGLBuffer::RangeInvalidateBuffer | QOpenGLBuffer::RangeWrite);
memcpy(ptr, m_vertexData.data(), size_t(m_vertexData.size())*sizeof(GLfloat));
m_myVbo.unmap();
 
glDrawArrays(GL_TRIANGLES, 0, m_vertexCount);
m_myVbo.release();

m_myVbo.bind();
auto ptr = m_myVbo.mapRange(0, m_vertexData.size()*int(sizeof(GLfloat)),
                                QOpenGLBuffer::RangeInvalidateBuffer | QOpenGLBuffer::RangeWrite);
memcpy(ptr, m_vertexData.data(), size_t(m_vertexData.size())*sizeof(GLfloat));
m_myVbo.unmap();

glDrawArrays(GL_TRIANGLES, 0, m_vertexCount);
m_myVbo.release();

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But the performances were quite the same ; can't see the difference on GPU monitoring (load of around 90% for 50 000 lines).