Hi Andras,
Sorry to bother you again. I found some interesting things about the RGBA volume rendering and can’t figure out the reason.
Right now, I have around two modes to achieve the task. Mode 1 uses the regular color transfer function with one channel and Mode 2 uses the RGBA volume rendering without independentcomponentoff(). The task actually requires colorful rendering, but I only use (255,255,255) here for convenience.
// Data generation
maskgrid->SetDimensions( dims ); // dims = 500x500x50
vtkNew<vtkInformation> info;
maskgrid->SetScalarType(VTK_UNSIGNED_CHAR, info);
maskgrid->SetSpacing(1.0, 1.0, 1.0);
maskgrid->SetOrigin(0.0, 0.0, 0.0);
vtkNew<vtkUnsignedCharArray> scalars;
scalars->SetNumberOfComponents(1); //mode 1
scalars->SetNumberOfComponents(4); //mode 2
scalars->SetNumberOfTuples( dims[0] * dims[1] * dims[2] );
scalars->Fill(0);
// ---------Set Data---------
// scalars.setTuple1(pointIdx, 255); //mode 1
// scalars.setTuple4(pointIdx, 255, 255, 255, 1); //mode 2
// .....
// ---------Set Data---------
maskgrid->GetPointData()->SetScalars(scalars);
// Rendering
vtkNew<vtkPiecewiseFunction> opacityFunc;
opacityFunc2->AddPoint(0, 0);
opacityFunc2->AddPoint(0.1, 1);
vtkNew<vtkColorTransferFunction> colorTransferFunction;
colorTransferFunction2->AddRGBPoint(0.1, 0.0, 0.0, 0.0);
colorTransferFunction2->AddRGBPoint(255, 1.0, 1.0, 1.0);
vtkNew<vtkVolumeProperty> volumeProperty;
volumeProperty2->SetColor(colorTransferFunction); // only for mode 1 with independent components on
volumeProperty2->SetScalarOpacity(opacityFunc); // for both mode 1&2
volumeProperty2-> IndependentComponentsOff(); // only for mode 2
// An extra sphere with a radius of 100 and an outline are rendered for reference.
//The rest of the pipeline is the same, including actor and renderer
All the above are the only difference between the two modes. I suppose the results from the two paths are mostly the same, but it actually differs a lot.
The result from mode 1 looks as I expected:
However, the result from mode 2 looks weird:
A lot of structures below the surface disappear except the one in the corner (and even that one seems incomplete). Also, if I look from bottom to top, most of the structures disappear.
This looks like an out-of-range error in the data array to me, but I don’t see any issue here. I’ve checked the number of tuples. It is dim[0] x dim[1] x dim[2], and the number of components is 4. Do you know if you have any other clues about the issue?
Thanks.
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Update:
Something weird happened again. I switch the vtkFixedPointVolumeRayCastMapper to vtkGPUVolumeRayCastMapper and it comes up with a very light white, just like with a very low alpha value. Then I change the alpha value to 255 and everything is fixed up.
I don’t understand why the alpha value would change that, as my opacity transfer function should already reach 1. Also, I thought the vtkFixedPointVolumeRayCastMapper should have a similar performance as the vtkGPUVolumeRayCastMapper, but obviously, there’s some difference.
I’ll keep working with real RGBA value instead of all-white color.