How to correctly rotate an OBB ?

Jalil Habri — Fri, 03 Jan 2025 22:19:05 +0000

Hello everyone, I made, with OpenGL, a car simulator. For the car collision box, I used an OBB.The problem is when i wanna rotate the collision box. for this, I use this function :

std::vector<glm::vec3> rotateCollisionBox(float angle, glm::vec3 boxMin, glm::vec3 boxMax) {
    std::vector<glm::vec3> rotatedVertices;
    glm::mat4 rotationMatrix = glm::rotate(glm::mat4(1.0f), glm::radians(angle), glm::vec3(0.0, 1.0, 0.0));

    std::vector<glm::vec3> corners = {
        glm::vec3(boxMin.x, boxMin.y, boxMin.z),
        glm::vec3(boxMax.x, boxMin.y, boxMin.z),
        glm::vec3(boxMin.x, boxMax.y, boxMin.z),
        glm::vec3(boxMax.x, boxMax.y, boxMin.z),
        glm::vec3(boxMin.x, boxMin.y, boxMax.z),
        glm::vec3(boxMax.x, boxMin.y, boxMax.z),
        glm::vec3(boxMin.x, boxMax.y, boxMax.z),
        glm::vec3(boxMax.x, boxMax.y, boxMax.z)
    };

    for (auto& vertex : corners) {
        glm::vec3 rotatedVertex = glm::vec3(rotationMatrix * glm::vec4(vertex, 1.0f));
        rotatedVertices.push_back(rotatedVertex);
    }

    glm::vec3 xAxis = glm::normalize(glm::vec3(rotationMatrix[0]));
    glm::vec3 yAxis = glm::normalize(glm::vec3(rotationMatrix[1]));
    glm::vec3 zAxis = glm::normalize(glm::vec3(rotationMatrix[2]));

    // Calculer les projections des coins sur chaque axe
    float minX = std::numeric_limits<float>::max();
    float minY = std::numeric_limits<float>::max();
    float minZ = std::numeric_limits<float>::max();

    float maxX = std::numeric_limits<float>::min();
    float maxY = std::numeric_limits<float>::min();
    float maxZ = std::numeric_limits<float>::min();

    for (const auto& vertex : rotatedVertices) {
        float projX = glm::dot(vertex, xAxis);  
        float projY = glm::dot(vertex, yAxis);  
        float projZ = glm::dot(vertex, zAxis);  

        minX = std::min(minX, projX);
        minY = std::min(minY, projY);
        minZ = std::min(minZ, projZ);

        maxX = std::max(maxX, projX);
        maxY = std::max(maxY, projY);
        maxZ = std::max(maxZ, projZ);
    }

    glm::vec3 newMin = glm::vec3(minX, minY, minZ);
    glm::vec3 newMax = glm::vec3(maxX, maxY, maxZ);

    return {newMin, newMax};
}

the function, with min and max points of the OBB calculate all the corners and rotate them with a rotation matrix (calculated with angle). Then, the function determines the newMin and newMax points and return them. So, the problem I encountered is the function don't works with values of angle that are not equal to 0° or 180°. newMin and newMax don't match with the car.

at the base, to calculate the newMin and newMax values, I was just multiplying boxMin and boxMax with the rotationMatrix, but this method did not takes into account all the corners. To solve this problem, I made this function :

std::vector<glm::vec3> rotateCollisionBox(float angle, glm::vec3 boxMin, glm::vec3 boxMax) {
    std::vector<glm::vec3> rotatedVertices;
    glm::mat4 rotationMatrix = glm::rotate(glm::mat4(1.0f), glm::radians(angle), glm::vec3(0.0, 1.0, 0.0));

    std::vector<glm::vec3> corners = {
        glm::vec3(boxMin.x, boxMin.y, boxMin.z),
        glm::vec3(boxMax.x, boxMin.y, boxMin.z),
        glm::vec3(boxMin.x, boxMax.y, boxMin.z),
        glm::vec3(boxMax.x, boxMax.y, boxMin.z),
        glm::vec3(boxMin.x, boxMin.y, boxMax.z),
        glm::vec3(boxMax.x, boxMin.y, boxMax.z),
        glm::vec3(boxMin.x, boxMax.y, boxMax.z),
        glm::vec3(boxMax.x, boxMax.y, boxMax.z)
    };

    for (auto& vertex : corners) {
        glm::vec3 rotatedVertex = glm::vec3(rotationMatrix * glm::vec4(vertex, 1.0f));
        rotatedVertices.push_back(rotatedVertex);
    }

    float minX;
    float minY;
    float minZ;

    float maxX;
    float maxY;
    float maxZ;

    for (const auto& vertex : rotatedVertices) {
        minX = std::min(minX, vertex.x);
        minY = std::min(minY, vertex.y);
        minZ = std::min(minZ, vertex.z);

        maxX = std::max(maxX, vertex.x);
        maxY = std::max(maxY, vertex.y);
        maxZ = std::max(maxZ, vertex.z);
    }

    glm::vec3 newMin = glm::vec3(minX, minY, minZ);
    glm::vec3 newMax = glm::vec3(maxX, maxY, maxZ);

    return {newMin, newMax};
}

this function was better because it worked for all angles multiples of 90 but it was not enough.So, I made the first function but it's worst than the second because it don't works for those angles : 90° and 270°. Please help me.

DEV Community: Jalil Habri

How to correctly rotate an OBB ?