vtk points from display to world

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本文介绍如何将图像中的像素坐标从显示坐标系转换到世界坐标系，包括获取深度图、利用VTK库进行坐标变换的过程及关键步骤。文中详细解释了通过VTK窗口获取深度图、像素坐标到视图坐标的转换以及最终视图坐标到世界坐标的转换方法。

points from display to world

目的

已知条件
- 相机的位置　compose
- 相机的perspective transformation Matrix proj_mat
- 该视角下的一张深度图zbuffer
- 该视角下的关于mesh的shoot img
所求
- img中的每一个像素的显示坐标系的坐标　⇒ 世界坐标系下的坐标

过程（前提：mesh归一化）

init display position

# i,j为img的某个像素的位置，pixel_d是zbuffer的i,j位置对应的值
  display_pt_[0] = i;
  display_pt_[1] = j;
  display_pt_[2] =double(*pixel_d)/ -255.0 ;

##　深度图　z_buffer
    vtkSmartPointer<vtkWindowToImageFilter> filter = vtkSmartPointer<vtkWindowToImageFilter>::New();
    vtkSmartPointer<vtkXMLImageDataWriter> imageWriter = vtkSmartPointer<vtkXMLImageDataWriter>::New();
    vtkSmartPointer<vtkImageShiftScale> scale = vtkSmartPointer<vtkImageShiftScale>::New();
    filter->SetInput(renWin_);
    filter->SetMagnification(magnitude);
    filter->SetInputBufferTypeToZBuffer();        //Extract z buffer value
    double range[2];
    filter->GetOutput()->GetScalarRange(range);

    scale->SetOutputScalarTypeToDouble();
    scale->SetInputConnection(filter->GetOutputPort());
    scale->SetShift(0);
    scale->SetScale(-255);

    // Write depth map as a .vti image
    imageWriter->SetFileName(file_path.c_str());
    imageWriter->SetInputConnection(scale->GetOutputPort());
    imageWriter->Write();

displaytoview:191

void vtkViewport::DisplayToView()
{
  if ( this->VTKWindow )
  {
    double vx,vy,vz;
    int sizex,sizey;

    /* get physical window dimensions */
    const int *size = this->VTKWindow->GetSize();
    if (size == nullptr)
    {
      return;
    }
    sizex = size[0];
    sizey = size[1];

    vx = 0.0;
    if (sizex != 0.0)
    {
      vx = 2.0 * (this->DisplayPoint[0] - sizex*this->Viewport[0])/
        (sizex*(this->Viewport[2]-this->Viewport[0])) - 1.0;
    }

    vy = 0.0;
    if (sizey != 0.0)
    {
      vy = 2.0 * (this->DisplayPoint[1] - sizey*this->Viewport[1])/
        (sizey*(this->Viewport[3]-this->Viewport[1])) - 1.0;
    }

    vz = this->DisplayPoint[2];

    this->SetViewPoint(vx,vy,vz);
  }
}

viewtoworld:1375

void vtkRenderer::ViewToWorld()
{
  double result[4];
  result[0] = this->ViewPoint[0];
  result[1] = this->ViewPoint[1];
  result[2] = this->ViewPoint[2];
  result[3] = 1.0;
  this->ViewToWorld(result[0],result[1],result[2]);
  this->SetWorldPoint(result);
}

void vtkRenderer::ViewToWorld(double &x, double &y, double &z)
{
  double mat[16];
  double result[4];

  if (this->ActiveCamera == nullptr)
  {
    vtkErrorMacro("ViewToWorld: no active camera, cannot compute view to world, returning 0,0,0");
    x = y = z = 0.0;
    return;
  }

  // get the perspective transformation from the active camera
  vtkMatrix4x4 *matrix = this->ActiveCamera->
                GetCompositeProjectionTransformMatrix(
                  this->GetTiledAspectRatio(),0,1);

  // use the inverse matrix
  vtkMatrix4x4::Invert(*matrix->Element, mat);

  // Transform point to world coordinates
  result[0] = x;
  result[1] = y;
  result[2] = z;
  result[3] = 1.0;

  vtkMatrix4x4::MultiplyPoint(mat,result,result);

  // Get the transformed vector & set WorldPoint
  // while we are at it try to keep w at one
  if (result[3])
  {
    x = result[0] / result[3];
    y = result[1] / result[3];
    z = result[2] / result[3];
  }
}