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

[Elvis Stansvik]

2017-02-12 17:31 GMT+01:00 David Gobbi <david.gobbi at gmail.com>:

> 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.
>

That indeed did the trick, thanks David!

Elvis


>
>  - 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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