How to obtain the transformation matrix of VTK?

Development
1

The matrix I obtained through the following method shows that the variable “wcdc” is different from the variable “MCDCMatrix” in the shader

	vtkMatrix4x4* wcdc;
	vtkMatrix4x4* wcvc;
	vtkMatrix3x3* norms;
	vtkMatrix4x4* vcdc;
	vtkOpenGLCamera* cam = (vtkOpenGLCamera*)(renderer->GetActiveCamera());
	cam->GetKeyMatrices(renderer, wcvc, norms, vcdc, wcdc);
2

Hello @Lincac

If your actor has been translated/scaled/rotated, you’ll need to fetch it’s model-to-world transformation matrix and multiply that with the world-to-display transform in order to get the correct model-to-display matrix.

vtkMatrix4x4* mcdc = nullptr;

vtkMatrix4x4* wcdc = nullptr;
vtkMatrix4x4* wcvc = nullptr;
vtkMatrix3x3* norms = nullptr;
vtkMatrix4x4* vcdc = nullptr;
auto* camera = renderer->GetActiveCamera();
auto* oglCamera= vtkOpenGLCamera::SafeDownCast(camera);
cam->GetKeyMatrices(renderer, wcvc, norms, vcdc, wcdc);
if (!actor->GetIsIdentity())
{
  auto* oglActor = vtkOpenGLActor::SafeDownCast(actor);
  oglActor ->GetKeyMatrices(mcwc, norms);
  vtkMatrix4x4::Multiply4x4(mcwc, wcdc, mcdc);
}
else
{
  mcdc = wcdc;
}
3

Thank you very much for your reply. In my program, I did not transform the Actor. However, when I used RenderDoc to capture the rendering call of VTk, I found that the data passed into the vertex shader was different from the data I read. Did it perform any other operations internally

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4

It could be possible that the mapper shifted and scaled your coordinates to prevent precision issues in shaders. You can disable that with vtkOpenGLVertexBufferObject::SetGlobalCoordShiftAndScaleEnabled(false) and check the vertexMC values again to confirm the shift scale is causing it.

When a shift scale is applied, the mapper pre-multiplies an internal 4x4 matrix called VBOShiftScale on the mcwc

vtkMatrix4x4::Multiply4x4(VBOShiftScale, mcwc, mcwcShiftedAndScaled);

Unfortunately, there is no easy way to get a pointer to the VBOShiftScale matrix outside the mapper. Here’s a way to compute it yourself.

if (auto* oglMapper = vtkOpenGLPolyDataMapper::SafeDownCast(mapper))
{
  vtkNew<vtkMatrix4x4> VBOShiftScale;
  auto* vbos = oglMapper->GetVBOs();
  posVBO = vbos->GetVBO("vertexMC");
  if (posVBO && 
      posVBO->GetCoordShiftAndScaleEnabled() &&
      !oglMapper->GetPauseShiftScale())
  {
    const auto& shift = posVBO->GetShift();
    const auto& scale = posVBO->GetScale();
    vtkNew<vtkTransform> transform;
    transform->Identity();
    transform->Translate(shift[0], shift[1], shift[2]);
    transform->Scale(1.0 / scale[0], 1.0 / scale[1], 1.0 / scale[2]);
    transform->GetTranspose(VBOShiftScale);
  }
}
5

Thank you very much for your prompt！Based on your advice, I have found the following interface, which will disable VBO offset

	auto mapper = vtkSmartPointer<vtkOpenGLPolyDataMapper>::New();
	mapper->SetVBOShiftScaleMethod(false);

After using this interface, VBO offset will no longer occur in VTK