[vtkusers] Opacity discrepancy with downscaled volume despite opacity unit distance == spacing

[David Gobbi]

Hi Elvis,

Here is something you can do as a check: set the sample distance for the
ray casting to a very small value, so that it takes around 1000 steps for
the ray to traverse the volume from front to back.  The thickness of your
volume is 29*0.006 = 0.174, so you can try a sample spacing of 1.74e-04:

mapper->AutoAdjustSampleDistancesOff();
mapper->SetSampleDistance(1.74e-04);

If the two volumes look the same when you use a small sample distance, then
the difference you were seeing was probably just due to discretization.

 - David



On Thu, Feb 9, 2017 at 6:45 AM, Elvis Stansvik <elvis.stansvik at orexplore.com
> wrote:

> Hi all,
>
> The following example shows a 30x30x30 volume rendered, and next to it a
> downscaled version of it (but using an identical opacity/transfer
> function). Notice how in the result (attached), the downscaled version is
> less opaque. This is despite me using
>
>     SetScalarOpacityUnitDistance(image->GetSpacing()[0])
>
> to set the scalar opacity unit distance to the spacing of the respective
> image. I would thought calling SetScalarOpacityUnitDistance like this would
> ensure both volumes look the same (apart from resolution).
>
> How can I make both volumes look the same opacity-wise when using the same
> transfer functions?
>
> Thanks in advance,
> Elvis
>
> main.cpp:
>
>
> #include <algorithm>
>
> #include <vtkCamera.h>
> #include <vtkColorTransferFunction.h>
> #include <vtkGPUVolumeRayCastMapper.h>
> #include <vtkImageData.h>
> #include <vtkImageResize.h>
> #include <vtkPiecewiseFunction.h>
> #include <vtkRenderer.h>
> #include <vtkRenderWindow.h>
> #include <vtkRenderWindowInteractor.h>
> #include <vtkSmartPointer.h>
> #include <vtkVolume.h>
> #include <vtkVolumeProperty.h>
>
> // Create a volume from the given image
> vtkSmartPointer<vtkVolume> createVolume(vtkSmartPointer<vtkImageData>
> image) {
>     auto color = vtkSmartPointer<vtkColorTransferFunction>::New();
>     color->AddRGBPoint(0.0, 1.0, 0.0, 0.0);
>     color->AddRGBPoint(1.0, 1.0, 0.0, 0.0);
>
>     auto opacity = vtkSmartPointer<vtkPiecewiseFunction>::New();
>     opacity->AddPoint(0.0, 0.1);
>     opacity->AddPoint(1.0, 0.1);
>
>     auto property = vtkSmartPointer<vtkVolumeProperty>::New();
>     property->SetColor(color);
>     property->SetScalarOpacity(opacity);
>     property->SetInterpolationTypeToLinear();
>     property->ShadeOff();
>     property->SetScalarOpacityUnitDistance(image->GetSpacing()[0]);
>
>     auto mapper = vtkSmartPointer<vtkGPUVolumeRayCastMapper>::New();
>     mapper->SetInputData(image);
>
>     auto volume = vtkSmartPointer<vtkVolume>::New();
>     volume->SetMapper(mapper);
>     volume->SetProperty(property);
>
>     return volume;
> }
>
> int main(int argc, char *argv[]) {
>
>     // Create a 30x30x30 volume
>     auto image = vtkSmartPointer<vtkImageData>::New();
>     image->SetDimensions(30, 30, 30);
>     image->SetSpacing(0.006, 0.006, 0.006);
>     image->AllocateScalars(VTK_FLOAT, 1);
>
>     auto imagePtr = static_cast<float *>(image->GetScalarPointer());
>     std::fill_n(imagePtr, 30 * 30 * 30, 1.0);
>
>     auto volume = createVolume(image);
>
>     // Create a smaller (10x10x10) version
>     auto resize = vtkSmartPointer<vtkImageResize>::New();
>     resize->SetInputData(image);
>     resize->SetOutputDimensions(10, 10, 10);
>     resize->Update();
>
>     auto smallVolume = createVolume(resize->GetOutput());
>     smallVolume->SetPosition(0.2, 0, 0);
>
>     // Render the two volumes
>     auto renderer = vtkSmartPointer<vtkRenderer>::New();
>     renderer->SetBackground(1.0, 1.0, 1.0);
>     renderer->AddVolume(volume);
>     renderer->AddVolume(smallVolume);
>     renderer->ResetCamera();
>
>     auto camera = renderer->GetActiveCamera();
>     camera->SetParallelProjection(1);
>
>     auto window = vtkSmartPointer<vtkRenderWindow>::New();
>     window->AddRenderer(renderer);
>     window->SetSize(800, 400);
>
>     auto interactor = vtkSmartPointer<vtkRenderWindowInteractor>::New();
>     interactor->SetRenderWindow(window);
>     interactor->Start();
>
>     return 0;
> }
>
>
> CMakeLists.txt:
>
>
> cmake_minimum_required(VERSION 3.1)
>
> project(TestCase)
>
> set(CMAKE_CXX_STANDARD 11)
> set(CMAKE_CXX_STANDARD_REQUIRED ON)
>
> find_package(VTK COMPONENTS
>     vtkCommonCore
>     vtkCommonDataModel
>     vtkCommonExecutionModel
>     vtkInteractionStyle
>     vtkImagingCore
>     vtkRenderingCore
>     vtkRenderingOpenGL2
>     vtkRenderingVolume
>     vtkRenderingVolumeOpenGL2
> )
>
> add_executable(TestCase main.cpp)
>
> target_link_libraries(TestCase PUBLIC
>     vtkCommonCore
>     vtkCommonDataModel
>     vtkCommonExecutionModel
>     vtkImagingCore
>     vtkInteractionStyle
>     vtkRenderingCore
>     vtkRenderingOpenGL2
>     vtkRenderingVolume
>     vtkRenderingVolumeOpenGL2
> )
>
> target_include_directories(TestCase PUBLIC
>     ${VTK_INCLUDE_DIRS}
> )
>
>
> target_compile_definitions(TestCase PUBLIC
>     ${VTK_DEFINITIONS}
> )
>
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