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# PerspectiveTransform

VTKEx/Cxx/SimpleOperations/PerspectiveTransform

### Description

The perspective transform automatically appends the "homogeneous 1" to the point before it is multiplied by the matrix. It then divides the first 3 coordinates of the result by the 4th coordinate of the result. For comparison, the standard projection vtkTransform) is also performed with the same matrix and the same point. Note that the results are much different.

Other languages

See (Java), (CSharp)

Question

If you have a simple question about this example contact us at VTKExProject If your question is more complex and may require extended discussion, please use the VTK Discourse Forum

### Code

PerspectiveTransform.cxx

    #include <vtkSmartPointer.h>
    #include <vtkPerspectiveTransform.h>
    #include <vtkTransform.h>
    #include <vtkMatrix4x4.h>

    int main(int, char *[])
    {
      vtkSmartPointer<vtkMatrix4x4> m =
        vtkSmartPointer<vtkMatrix4x4>::New();
      m->SetElement(0,0,1);
      m->SetElement(0,1,2);
      m->SetElement(0,2,3);
      m->SetElement(0,3,4);
      m->SetElement(1,0,2);
      m->SetElement(1,1,2);
      m->SetElement(1,2,3);
      m->SetElement(1,3,4);
      m->SetElement(2,0,3);
      m->SetElement(2,1,2);
      m->SetElement(2,2,3);
      m->SetElement(2,3,4);
      m->SetElement(3,0,4);
      m->SetElement(3,1,2);
      m->SetElement(3,2,3);
      m->SetElement(3,3,4);

      vtkSmartPointer<vtkPerspectiveTransform> perspectiveTransform =
          vtkSmartPointer<vtkPerspectiveTransform>::New();
      perspectiveTransform->SetMatrix(m);

      vtkSmartPointer<vtkTransform> transform =
          vtkSmartPointer<vtkTransform>::New();
      transform->SetMatrix(m);

      double p[3];
      p[0] = 1.0;
      p[1] = 2.0;
      p[2] = 3.0;

      double normalProjection[3];
      transform->TransformPoint(p, normalProjection);

      std::cout << "Standard projection: "
        << normalProjection[0] << " "
        << normalProjection[1] << " "
        << normalProjection[2] << std::endl;

      double perspectiveProjection[3];
      perspectiveTransform->TransformPoint(p, perspectiveProjection);
      std::cout << "Perspective projection: "
        << perspectiveProjection[0]
        << " " << perspectiveProjection[1] << " "
        << perspectiveProjection[2] << std::endl;

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(PerspectiveTransform)

    find_package(VTK COMPONENTS
      vtkCommonCore
      vtkCommonMath
      vtkCommonTransforms QUIET)
    if (NOT VTK_FOUND)
      message("Skipping PerspectiveTransform: ${VTK_NOT_FOUND_MESSAGE}")
      return ()
    endif()
    message (STATUS "VTK_VERSION: ${VTK_VERSION}")
    if (VTK_VERSION VERSION_LESS "8.90.0")
      # old system
      include(${VTK_USE_FILE})
      add_executable(PerspectiveTransform MACOSX_BUNDLE PerspectiveTransform.cxx )
      target_link_libraries(PerspectiveTransform PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(PerspectiveTransform MACOSX_BUNDLE PerspectiveTransform.cxx )
      target_link_libraries(PerspectiveTransform PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS PerspectiveTransform
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### Download and Build PerspectiveTransform

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

    cd PerspectiveTransform/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:

    ./PerspectiveTransform

WINDOWS USERS

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