# PerspectiveTransform

VTKExamples/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.

### 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 2.8)

    PROJECT(PerspectiveTransform)

    find_package(VTK REQUIRED)
    include(${VTK_USE_FILE})

    add_executable(PerspectiveTransform MACOSX_BUNDLE PerspectiveTransform.cxx )

    target_link_libraries(PerspectiveTransform ${VTK_LIBRARIES})

### Download and Build PerspectiveTransform

Danger

The generation of tar files has not been ported to the new VTKExamples. Some tarballs may be missing or out-of-date.

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