0

I'm recoding the rendering pipeline in c++ and have some trouble to make it works. I mean, it's working but not in the way i expected. To make it simple, i transform my 3D point to 2D point with a perspective projection matrix builds as follow :

My draw_edge function simply take 2 points in world space and make the calls to transform those point to screen coordinates :

void Scene::draw_edge(const geometry::Point<float,4>& p1, const geometry::Point<float,4>& p2, gui::Color c) const
{

   geometry::LineSegment ls(p1.ReduceToPoint3DAsPoint(), p2.ReduceToPoint3DAsPoint());
   std::optional<geometry::LineSegment> cls = camera.VisiblePart(ls);

   if(cls != std::nullopt)
     {
        auto p1Persp = Perspective((*cls).GetBegin().ExtendedPoint4DAsPoint());
        auto p2Persp = Perspective((*cls).GetEnd().ExtendedPoint4DAsPoint());

        om::Vector<2,float> p1Projection2D;
        om::Vector<2,float> p2Projection2D;

        p1Projection2D[0] = p1Persp.At(0);
        p1Projection2D[1] = p1Persp.At(1);
        p2Projection2D[0] = p2Persp.At(0);
        p2Projection2D[1] = p2Persp.At(1);

        gui->render_line(p1Projection2D,p2Projection2D,c);
     }
}

This function is use to actually transform my points by applying the perspective matrix and then divide my coordinates by 'w'.

geometry::Point<float,3> Scene::Perspective(const geometry::Point<float,4>& point) const
  {
    geometry::Transform perspectiveTransform = camera.GetPerspectiveProjection();
    geometry::Point<float,4> pointHomogenous = perspectiveTransform.TransformTo(point);
    om::Vector<3,float> normalized;
    for(int i = 0; i < 3; i++)
      {
    normalized[i] = pointHomogenous.At(i) / pointHomogenous.At(3);
      }

    return geometry::Point<float,3>(normalized);
  }

This function is use to build my perspective projection matrix

geometry::Transform Camera::GetPerspectiveProjection() const
  {
    float nearValue = position.Dist(frustum.nearPlane);
    float farValue = position.Dist(frustum.farPlane);
    float rightValue = nearValue / focaleDistance;
    float leftValue = -nearValue / focaleDistance;
    float topValue = (aspectRatio * nearValue) / focaleDistance;
    float bottomValue = -(aspectRatio * nearValue) / focaleDistance;

    auto perspective = geometry::Transform(nearValue, farValue, rightValue, leftValue, topValue, bottomValue);
    //auto perspectiveInverse = perspective.GetMatrix().Reverse();
    //perspective = geometry::Transform(perspectiveInverse);

    return perspective;
  }

As you can see, i have comment the inversion and that's the point of this post i'll come back to it later.

So finally, the function that build my perspective matrix :

Transform(float near, float far, float right, float left, float top, float bottom)
{
  transform[0][0] = (2*near)/(right - left);
  transform[0][1] = 0.0f;
  transform[0][2] = (right + left)/(right - left);
  transform[0][3] = 0.0f;

  transform[1][0] = 0.0f;
  transform[1][1] = (2*near)/(top - bottom);
  transform[1][2] = (top + bottom)/(top - bottom);
  transform[1][3] = 0.0f;

  transform[2][0] = 0.0f;
  transform[2][1] = 0.0f;
  transform[2][2] = -((far + near)/(far - near));
  transform[2][3] = -((2*near*far)/(far - near));

  transform[3][0] = 0.0f;
  transform[3][1] = 0.0f;
  transform[3][2] = -1.0f;
  transform[3][3] = 0.0f;
}

So, now that all the code needed is exposed, here is my problem. When i use the inversion of perspective projection (by uncommenting the 2 lines), the result looks correct as follow :

inversed matrix

But if I comment my inversion, the resultat is as follow :

no inversed matrix

So my problem is that I don't understand why i need to use the inverse matrix to have a resultat that's look correct ? I mean, all document i have read about perspective projection, no one was speaking about inversing the perspective projection matrix.

Thanks for reading and don't hesitate to ask for more informations

LenweSeregon
  • 132
  • 1
  • 10
  • What does the `.Reverse` function do? It looks like you are using a custom math library. It would be helpful to at least post a link to a repo. – meowgoesthedog Dec 09 '18 at 17:05
  • Thanks for your answer, the reverse function simply computing the inverse of matrix and i can't post the custom math library because this is a library made by myself for scholl project. But i have unit tests and i'm pretty sure that my computation is correct. I have even test it with my runtime computed perspective projection matrix and the result is correct. – LenweSeregon Dec 09 '18 at 17:22

0 Answers0