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

[Elvis Stansvik]

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}
)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://public.kitware.com/pipermail/vtkusers/attachments/20170209/ea950a84/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: result.png
Type: image/png
Size: 8300 bytes
Desc: not available
URL: <http://public.kitware.com/pipermail/vtkusers/attachments/20170209/ea950a84/attachment.png>