VTK blend color in wierd way with vtkassembly

Question

When I update VTK to version 8.2.0, I encounter a specical bug that VTK will blend color with 'vtkAssembly' in wierd way comparing to behavior by vtkActor.

While blend color with vtkAssembly, it seems that VTK just add color by (r,g,b), thus white color appears offen.

In the following example, left is drawn by vtkActor, and right is added to scene by vtkAssembly

I reproduce bug with following simple code :

#include "vtkActor.h"
#include "vtkNew.h"
#include "vtkCamera.h"
#include "vtkCellArray.h"
#include "vtkFloatArray.h"
#include "vtkPointData.h"
#include "vtkPoints.h"
#include "vtkPolyData.h"
#include "vtkPolyDataMapper.h"
#include "vtkRenderWindow.h"
#include <vtkGenericOpenGLRenderWindow.h>
#include "vtkRenderWindowInteractor.h"
#include "vtkRenderer.h"
#include <vtkLookupTable.h>
#include <vtkProperty.h>
#include <vtkCellData.h>
#include <vtkInteractorStyleTrackballCamera.h>
#include <vtkAssembly.h>

#include <vtkAutoInit.h>

VTK_MODULE_INIT(vtkRenderingOpenGL2)
VTK_MODULE_INIT(vtkInteractionStyle)
VTK_MODULE_INIT(vtkRenderingFreeType)

void createScene(vtkActor* actor)
{
    int i;
    //
    static float x[10][3]={{ 0, 0, 1}, { 0, 0,-1}
                          , { 0, 1, 0}, { 1, 1, 0}
                          , { 1, 0, 0}, { 1,-1, 0}
                          , { 0,-1, 0}, {-1,-1, 0}
                          , {-1, 0, 0}, {-1, 1, 0}};
    //
    static vtkIdType pts[8][4]={{0,1,2}, {0,1,3}, {0,1,4}, {0,1,5}
                                , {0,1,6}, {0,1,7}, {0,1,8}, {0,1,9}};
    //
    vtkPolyData *cube = vtkPolyData::New();
    vtkPoints *points = vtkPoints::New();
    vtkCellArray *polys = vtkCellArray::New();
    //
    vtkFloatArray *scalars = vtkFloatArray::New();
    //
    for(i=0;i<10;i++)points->InsertPoint(i,x[i]);
    //
    for(i=0;i<8;i++)polys->InsertNextCell(4,pts[i]);
    //
    for(i=0;i<8;i++)scalars->InsertTuple1(i,i);
    //
    cube->SetPoints(points);
    //
    cube->SetPolys(polys);
    //
    cube->GetCellData()->SetScalars(scalars);
    points->Delete();
    polys->Delete();
    scalars->Delete();
    //
    vtkLookupTable *pColorTable=vtkLookupTable::New();
    //
    pColorTable->SetNumberOfColors(6);
    pColorTable->SetTableValue(0, 1.0, 0.0, 0.0, 1.0);
    pColorTable->SetTableValue(1, 0.0, 1.0, 0.0, 1.0);
    pColorTable->SetTableValue(2, 1.0, 1.0, 0.0, 1.0);
    pColorTable->SetTableValue(3, 0.0, 0.0, 1.0, 1.0);
    pColorTable->SetTableValue(4, 1.0, 0.0, 1.0, 1.0);
    pColorTable->SetTableValue(5, 0.0, 1.0, 1.0, 1.0);
    pColorTable->Build();

    //
    vtkPolyDataMapper *cubeMapper = vtkPolyDataMapper::New();
    cubeMapper->SetScalarModeToUseCellData();
    cubeMapper->SetInputData(cube);
    cubeMapper->SetScalarRange(0,7);
    cubeMapper->SetLookupTable(pColorTable);
    actor->SetMapper(cubeMapper);
    actor->GetProperty()->SetOpacity(0.5);
}

int main(int argc, char** argv)
{
    vtkCamera *camera = vtkCamera::New();
    camera->SetPosition(1,1,1);
    camera->SetFocalPoint(0,0,0);

    vtkRenderer *renderer = vtkRenderer::New();

    vtkRenderWindow *renWin = vtkRenderWindow::New();
    renWin->AddRenderer(renderer);

    vtkRenderWindowInteractor *iren = vtkRenderWindowInteractor::New();
    iren->SetRenderWindow(renWin);
    iren->SetInteractorStyle(vtkInteractorStyleTrackballCamera::New());

    vtkNew<vtkActor> actor1, actor2;
    createScene(actor1);
    renderer->AddActor(actor1);

    createScene(actor2);
    vtkNew<vtkAssembly> assembly;
    assembly->AddPart(actor2);
    assembly->SetPosition(3,0,0);
    renderer->AddActor(assembly);

    renderer->SetActiveCamera(camera);
    renderer->ResetCamera();
    renderer->SetBackground(0,0,0);

    renWin->SetSize(600,600);
    renWin->Render();
    iren->Start();
    return 0;
}

Answer 1

See answer in the vtk discourse thread here.