ColorByNormal

Description¶

This example modifies the shader to color based on model normal To do this we have to modify the vertex shader to pass the normal in model coordinates through to the fragment shader. By default the normal is converted to View coordinates and then passed on. We keep that, but add a varying for the original normal. Then we modify the fragment shader to set the diffuse color based on that normal.

Cite

This example was inspired by the test TestUserShader.

Question

If you have a question about this example, please use the VTK Discourse Forum

Code¶

ColorByNormal.cxx

#include <vtkActor.h>
#include <vtkCamera.h>
#include <vtkNamedColors.h>
#include <vtkNew.h>
#include <vtkOpenGLPolyDataMapper.h>
#include <vtkProperty.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkRenderer.h>
#include <vtkSmartPointer.h>
#include <vtkTriangleMeshPointNormals.h>
#include <vtkVersion.h>

#include <vtkBYUReader.h>
#include <vtkOBJReader.h>
#include <vtkPLYReader.h>
#include <vtkPolyDataReader.h>
#include <vtkSTLReader.h>
#include <vtkSphereSource.h>
#include <vtkXMLPolyDataReader.h>
#include <vtksys/SystemTools.hxx>

#if VTK_VERSION_NUMBER >= 89000000000ULL
#define USE_SHADER_PROPERTIES 1
#include <vtkShaderProperty.h>
#endif

#include <sstream>
#include <string>

namespace {
vtkSmartPointer<vtkPolyData> ReadPolyData(const char* fileName);
}

//----------------------------------------------------------------------------
int main(int argc, char* argv[])
{
  auto polyData = ReadPolyData(argc > 1 ? argv[1] : "");

  vtkNew<vtkNamedColors> colors;

  vtkNew<vtkActor> actor;
  vtkNew<vtkRenderer> renderer;
  vtkNew<vtkOpenGLPolyDataMapper> mapper;
  renderer->SetBackground(colors->GetColor3d("SlateGray").GetData());

  vtkNew<vtkRenderWindow> renderWindow;
  renderWindow->SetSize(640, 480);
  renderWindow->AddRenderer(renderer);
  renderer->AddActor(actor);

  vtkNew<vtkRenderWindowInteractor> interactor;
  interactor->SetRenderWindow(renderWindow);

  vtkNew<vtkTriangleMeshPointNormals> norms;
  norms->SetInputData(polyData);

  mapper->SetInputConnection(norms->GetOutputPort());
  actor->SetMapper(mapper);
  actor->GetProperty()->SetAmbientColor(0.2, 0.2, 1.0);
  actor->GetProperty()->SetDiffuseColor(1.0, 0.65, 0.7);
  actor->GetProperty()->SetSpecularColor(1.0, 1.0, 1.0);
  actor->GetProperty()->SetSpecular(0.5);
  actor->GetProperty()->SetDiffuse(0.7);
  actor->GetProperty()->SetAmbient(0.5);
  actor->GetProperty()->SetSpecularPower(20.0);
  actor->GetProperty()->SetOpacity(1.0);

  // Modify the shader to color based on model normal
  // To do this we have to modify the vertex shader
  // to pass the normal in model coordinates
  // through to the fragment shader. By default the normal
  // is converted to View coordinates and then passed on.
  // We keep that, but add a varying for the original normal.
  // Then we modify the fragment shader to set the diffuse color
  // based on that normal. First lets modify the vertex
  // shader
#if USE_SHADER_PROPERTIES
  vtkShaderProperty* sp = actor->GetShaderProperty();
  sp->AddVertexShaderReplacement(
      "//VTK::Normal::Dec",  // replace the normal block
      true,                  // before the standard replacements
      "//VTK::Normal::Dec\n" // we still want the default
      "  varying vec3 myNormalMCVSOutput;\n", // but we add this
      false                                   // only do it once
  );
#else
  mapper->AddShaderReplacement(
      vtkShader::Vertex,
      "//VTK::Normal::Dec",  // replace the normal block
      true,                  // before the standard replacements
      "//VTK::Normal::Dec\n" // we still want the default
      "  varying vec3 myNormalMCVSOutput;\n", // but we add this
      false                                   // only do it once
  );
#endif
#if USE_SHADER_PROPERTIES
  sp->AddVertexShaderReplacement(
      "//VTK::Normal::Impl",                // replace the normal block
      true,                                 // before the standard replacements
      "//VTK::Normal::Impl\n"               // we still want the default
      "  myNormalMCVSOutput = normalMC;\n", // but we add this
      false                                 // only do it once
  );
#else
  mapper->AddShaderReplacement(
      vtkShader::Vertex,
      "//VTK::Normal::Impl",                // replace the normal block
      true,                                 // before the standard replacements
      "//VTK::Normal::Impl\n"               // we still want the default
      "  myNormalMCVSOutput = normalMC;\n", // but we add this
      false                                 // only do it once
  );
#endif
#if USE_SHADER_PROPERTIES
  sp->AddVertexShaderReplacement(
      "//VTK::Color::Impl", // dummy replacement for testing clear method
      true, "VTK::Color::Impl\n", false);
#else
  mapper->AddShaderReplacement(
      vtkShader::Vertex,
      "//VTK::Color::Impl", // dummy replacement for testing clear method
      true, "VTK::Color::Impl\n", false);
#endif
#if USE_SHADER_PROPERTIES
  sp->ClearVertexShaderReplacement("//VTK::Color::Impl", true);
#else
  mapper->ClearShaderReplacement(
      vtkShader::Vertex, // clear our dummy replacement
      "//VTK::Color::Impl", true);
#endif

  // now modify the fragment shader
#if USE_SHADER_PROPERTIES
  sp->AddFragmentShaderReplacement(
      "//VTK::Normal::Dec",  // replace the normal block
      true,                  // before the standard replacements
      "//VTK::Normal::Dec\n" // we still want the default
      "  varying vec3 myNormalMCVSOutput;\n", // but we add this
      false                                   // only do it once
  );
#else
  mapper->AddShaderReplacement(
      vtkShader::Fragment,   // in the fragment shader
      "//VTK::Normal::Dec",  // replace the normal block
      true,                  // before the standard replacements
      "//VTK::Normal::Dec\n" // we still want the default
      "  varying vec3 myNormalMCVSOutput;\n", // but we add this
      false                                   // only do it once
  );
#endif
#if USE_SHADER_PROPERTIES
  sp->AddFragmentShaderReplacement(
      "//VTK::Normal::Impl",  // replace the normal block
      true,                   // before the standard replacements
      "//VTK::Normal::Impl\n" // we still want the default calc
      "  diffuseColor = abs(myNormalMCVSOutput);\n", // but we add this
      false                                          // only do it once
  );
#else
  mapper->AddShaderReplacement(
      vtkShader::Fragment,    // in the fragment shader
      "//VTK::Normal::Impl",  // replace the normal block
      true,                   // before the standard replacements
      "//VTK::Normal::Impl\n" // we still want the default calc
      "  diffuseColor = abs(myNormalMCVSOutput);\n", // but we add this
      false                                          // only do it once
  );
#endif

  renderWindow->Render();
  renderer->GetActiveCamera()->SetPosition(1.0, 1.0, -1.0);
  renderer->GetActiveCamera()->SetFocalPoint(0, 1, 0);
  renderer->GetActiveCamera()->SetViewUp(0, 1, 0);
  renderer->GetActiveCamera()->Elevation(30.0);
  renderer->ResetCamera();
  renderer->GetActiveCamera()->Zoom(1.3);
  renderWindow->Render();
  renderWindow->SetWindowName("ColorByNormal");
  renderWindow->Render();

  interactor->Start();

  return EXIT_SUCCESS;
}

namespace {
vtkSmartPointer<vtkPolyData> ReadPolyData(const char* fileName)
{
  vtkSmartPointer<vtkPolyData> polyData;
  std::string extension =
      vtksys::SystemTools::GetFilenameExtension(std::string(fileName));
  if (extension == ".ply")
  {
    vtkNew<vtkPLYReader> reader;
    reader->SetFileName(fileName);
    reader->Update();
    polyData = reader->GetOutput();
  }
  else if (extension == ".vtp")
  {
    vtkNew<vtkXMLPolyDataReader> reader;
    reader->SetFileName(fileName);
    reader->Update();
    polyData = reader->GetOutput();
  }
  else if (extension == ".obj")
  {
    vtkNew<vtkOBJReader> reader;
    reader->SetFileName(fileName);
    reader->Update();
    polyData = reader->GetOutput();
  }
  else if (extension == ".stl")
  {
    vtkNew<vtkSTLReader> reader;
    reader->SetFileName(fileName);
    reader->Update();
    polyData = reader->GetOutput();
  }
  else if (extension == ".vtk")
  {
    vtkNew<vtkPolyDataReader> reader;
    reader->SetFileName(fileName);
    reader->Update();
    polyData = reader->GetOutput();
  }
  else if (extension == ".g")
  {
    vtkNew<vtkBYUReader> reader;
    reader->SetGeometryFileName(fileName);
    reader->Update();
    polyData = reader->GetOutput();
  }
  else
  {
    vtkNew<vtkSphereSource> source;
    source->SetPhiResolution(25);
    source->SetThetaResolution(25);
    source->Update();
    polyData = source->GetOutput();
  }
  return polyData;
}
} // namespace

CMakeLists.txt¶

cmake_minimum_required(VERSION 3.12 FATAL_ERROR)

project(ColorByNormal)

find_package(VTK COMPONENTS 
  CommonColor
  CommonCore
  FiltersCore
  FiltersSources
  IOGeometry
  IOLegacy
  IOPLY
  IOXML
  InteractionStyle
  RenderingContextOpenGL2
  RenderingCore
  RenderingFreeType
  RenderingGL2PSOpenGL2
  RenderingOpenGL2
)

if (NOT VTK_FOUND)
  message(FATAL_ERROR "ColorByNormal: Unable to find the VTK build folder.")
endif()

# Prevent a "command line is too long" failure in Windows.
set(CMAKE_NINJA_FORCE_RESPONSE_FILE "ON" CACHE BOOL "Force Ninja to use response files.")
add_executable(ColorByNormal MACOSX_BUNDLE ColorByNormal.cxx )
  target_link_libraries(ColorByNormal PRIVATE ${VTK_LIBRARIES}
)
# vtk_module_autoinit is needed
vtk_module_autoinit(
  TARGETS ColorByNormal
  MODULES ${VTK_LIBRARIES}
)

Download and Build ColorByNormal¶

Click here to download ColorByNormal and its CMakeLists.txt file. Once the tarball ColorByNormal.tar has been downloaded and extracted,

cd ColorByNormal/build

If VTK is installed:

cmake ..

If VTK is not installed but compiled on your system, you will need to specify the path to your VTK build:

cmake -DVTK_DIR:PATH=/home/me/vtk_build ..

Build the project:

make

and run it:

./ColorByNormal

WINDOWS USERS

Be sure to add the VTK bin directory to your path. This will resolve the VTK dll's at run time.