Function ScreenPosToWorldRay

src/Physics/Physics.cpp:8–61

#include "Physics.h"

void ScreenPosToWorldRay(
        int mouseX, int mouseY,             // Mouse position, in pixels, from bottom-left corner of the window
        int screenWidth, int screenHeight,  // Window size, in pixels
        glm::mat4 ViewMatrix,               // Camera position and orientation
        glm::mat4 ProjectionMatrix,         // Camera parameters (ratio, field of view, near and far planes)
        glm::vec3 &out_origin,              // Ouput : Origin of the ray. /!\ Starts at the near plane, so if you want the ray to start at the camera's position instead, ignore this.Reall
        glm::vec3 &out_direction            // Ouput : Direction, in world space, of the ray that goes "through" the mouse.
) {

    // The ray Start and End positions, in Normalized Device Coordinates (Have you read Tutorial 4 ?)
    glm::vec4 lRayStart_NDC(
            ((float) mouseX / (float) screenWidth - 0.5f) * 2.0f,
            ((float) mouseY / (float) screenHeight - 0.5f) * 2.0f,
            -1.0, // The near plane maps to Z=-1 in Normalized Device Coordinates
            1.0f
    );
    glm::vec4 lRayEnd_NDC(
            ((float) mouseX / (float) screenWidth - 0.5f) * 2.0f,
            ((float) mouseY / (float) screenHeight - 0.5f) * 2.0f,
            0.0,
            1.0f
    );


    // The Projection matrix goes from Camera Space to NDC.
    // So inverse(ProjectionMatrix) goes from NDC to Camera Space.
    glm::mat4 InverseProjectionMatrix = glm::inverse(ProjectionMatrix);

    // The View Matrix goes from World Space to Camera Space.
    // So inverse(ViewMatrix) goes from Camera Space to World Space.
    glm::mat4 InverseViewMatrix = glm::inverse(ViewMatrix);

    glm::vec4 lRayStart_camera = InverseProjectionMatrix * lRayStart_NDC;
    lRayStart_camera /= lRayStart_camera.w;
    glm::vec4 lRayStart_world = InverseViewMatrix * lRayStart_camera;
    lRayStart_world /= lRayStart_world.w;
    glm::vec4 lRayEnd_camera = InverseProjectionMatrix * lRayEnd_NDC;
    lRayEnd_camera /= lRayEnd_camera.w;
    glm::vec4 lRayEnd_world = InverseViewMatrix * lRayEnd_camera;
    lRayEnd_world /= lRayEnd_world.w;


    // Faster way (just one inverse)
    //glm::mat4 M = glm::inverse(ProjectionMatrix * ViewMatrix);
    //glm::vec4 lRayStart_world = M * lRayStart_NDC; lRayStart_world/=lRayStart_world.w;
    //glm::vec4 lRayEnd_world   = M * lRayEnd_NDC  ; lRayEnd_world  /=lRayEnd_world.w;

    glm::vec3 lRayDir_world(lRayEnd_world - lRayStart_world);
    lRayDir_world = glm::normalize(lRayDir_world);


    out_origin = glm::vec3(lRayStart_world);
    out_direction = glm::normalize(lRayDir_world);
}

btCollisionShape *Physics::buildFrustumShape() {
    float m_ScreenHeight = 768;