Get cursor position on Z = 0 plane using glm::unproject()? - c++

Question

I'm trying to get the coordinates (x,y) of the grid (z = 0) using only the cursor coordinates. After a long search I found this way to do that using the glm::unproject.
First I'm getting the cursor coordinates using the callback:
void cursorCallback(GLFWwindow *window, double x, double y)
{
this->cursorCoordinate = glm::vec3(x, (this->windowHeight - y - 1.0f), 0.0f);
}
an then converting these coordinates:
glm::vec3 cursorCoordinatesToWorldCoordinates()
{
glm::vec3 pointInitial = glm::unProject(
glm::vec3(this->cursorCoordinate.x, this->cursorCoordinate.y, 0.0),
this->modelMatrix * this->viewMatrix,
this->projectionMatrix,
this->viewPort
);
glm::vec3 pointFinal = glm::unProject(
glm::vec3(this->cursorCoordinate.x, this->cursorCoordinate.y, 1.0),
this->modelMatrix * this->viewMatrix,
this->projectionMatrix,
this->viewPort
);
glm::vec3 vectorDirector = pointFinal - pointInitial;
double lambda = (-pointInitial.y) / vectorDirector.y;
double x = pointInitial.x + lambda * vectorDirector.x;
double y = pointInitial.z + lambda * vectorDirector.z;
return glm::vec3(x, y, 0.0f);
}
I use an ArcBall camera to rotate the world around specified axis, so that is how I generate the MVP matrixes:
this->position = glm::vec3(0.0f, 10.0f, 5.0f);
this->up = glm::vec3(0.0f, 1.0f, 0.0f);
this->lookAt = glm::vec3(0.0f, 0.0f, 0.0f);
this->fieldView = 99.0f;
this->farDistance = 100.0f;
this->nearDistance = 0.1f;
this->modelMatrix = glm::mat4(1.0f);
this->viewMatrix = glm::lookAt(this->position, this->lookAt, this->up) * glm::rotate(glm::degrees(this->rotationAngle) * this->dragSpeed, this->rotationAxis);
this->projectionMatrix = glm::perspective(glm::radians(this->fieldView), 1.0f, this->nearDistance, this->farDistance);
But something is going wrong because I'm not getting the right results. Look this print of the application:
each square is 1 unit, the cube is rendered at position (0, 0, 0). With rotationAngle = 0 when a put the cursor at (0,0), (1,1), (2,2), (3,3), (4,4), (5,5) I get (0, 5.7), (0.8, 6.4), (1.6, 6.9), (2.4, 7.6), (3.2, 8.2), (4.2, 8.8) respectivally. That's not expected.
Why y is delayed by 6 units?
It's necessary rotate the result cursorCoordinatesToWorldCoordinates based on rotationAngle isn't?
--
That I already did:
Checked if the viewport match with glViewport - OK
Checked the opengl coordinates (Y is up, not Z) - OK

Answer 1

You want to intersect the ray from glm::vec3(this->cursorCoordinate.x, this->cursorCoordinate.y, 0.0) to glm::vec3(this->cursorCoordinate.x, this->cursorCoordinate.y, 1.0) with the grid in world space, rather than model space (of the cuboid).
You've to skip this.modelMatrix:
glm::vec3 pointInitial = glm::unProject(
glm::vec3(this->cursorCoordinate.x, this->cursorCoordinate.y, 0.0),
this->viewMatrix,
this->projectionMatrix,
this->viewPort);
glm::vec3 pointFinal = glm::unProject(
glm::vec3(this->cursorCoordinate.x, this->cursorCoordinate.y, 1.0),
this->viewMatrix,
this->projectionMatrix,
this->viewPort);
In any case this->modelMatrix * this->viewMatrix is incorrect. If you eant to intersect the ray with an object in model space, then it has to be this->viewMatrix * this->modelMatrix. Matrix multiplication is not Commutative.