how to use glm::rotate with a eulerangle?

Question

I have a vec3 to represent my object's orientation/rotation but the glm::rotate method expects a quaternion. If I just convert it to a quaternion like this:

glm::quat rot = rotation;

The W value will just be zero, right? And then there won't be any changes in rotation.

In my code I just want to be able to do rotation.x += 5.0f; in the update method of an object.

This is the method I'm using for my transformations:

glm::mat4 GameObject::Transform(glm::mat4 model, glm::vec3 position, glm::vec3 scale, glm::quat angleAxis)
{
    model = glm::translate(model, position);
    if (angleAxis.w > 360.0f) 
    { 
         angleAxis.w -= 360.0f; 
    }
    else if (angleAxis.w < 0.0f) 
    { 
         angleAxis.w += 360.0f; 
    }
    model = glm::rotate(model, angleAxis.w * toRadians, glm::vec3(angleAxis.x, angleAxis.y, angleAxis.z));
    model = glm::scale(model, scale);
    return model;
}

Currently I'm just passing on that rotation vec3 to that angleAxis quaternion parameter but that obviously doesn't work.

This is how i currently calculate my front, up, and right vectors:

void GameObject::calculateCameraView()
{       
    front.x = cos(glm::radians(rotation.x)) * cos(glm::radians(rotation.y));
    front.y = sin(glm::radians(rotation.y));
    front.z = sin(glm::radians(rotation.x)) * cos(glm::radians(rotation.y));
    front = glm::normalize(front);

    right = glm::normalize(glm::cross(front, worldUp));
    up = glm::normalize(glm::cross(right, front));

    front.y = invertMouse ? front.y * -1 : front.y;
}

Note that an angle-axis representation (specifying a unit vector axis and an angle to rotate about that axis) is not the same as an Euler angle representation (which specifies a sequence of three angles to rotate about, on a fixed sequence of axes). It looks like your code is trying to use an angle-axis form, so I'm not sure why the input is typed as a quaternion, or why your question mentions Euler angles. Can you clarify? — DMGregory
– DMGregory ♦, Commented Aug 18, 2018 at 15:12
@DMGregory in another part of my program i calculate the front, up, and right vectors by taking the object's yaw/pitch which are just the rotation.x and rotation.y . I couldn't calculate those with a quaternion (at least i couldn't) so i decided to use rotation as a euler angle and thought i could later "convert" it to pass it as a quaternion to the glm::rotate function. — Thomas Linssen
– Thomas Linssen, Commented Aug 18, 2018 at 15:29
You can make your front, up, and right vectors from a quaternion q by just transforming the local basis vectors like (0, 0, 1) by q. — DMGregory
– DMGregory ♦, Commented Aug 18, 2018 at 15:56
@DMGregory Sorry im confused by what you mean, could you maybe show it in code? I've edited my post so you can see how i calculate front, up and right. — Thomas Linssen
– Thomas Linssen, Commented Aug 18, 2018 at 17:36

DMGregory · Accepted Answer · 2018-08-18 14:47:09Z

2

in my code i just want to be able to do rotation.x += 5.0f; in the update method of an obj

A quaternion representing a rotation of angle radians in the x axis is:

q.x = sin(angle / 2f);
q.y = 0;
q.z = 0;
q.w = cos(angle / 2f);

Or in general for any angle-axis rotation:

q.xyz = axis.xyz * sin(angle / 2f);
q.w = cos(angle / 2f);

answered Aug 18, 2018 at 14:47

DMGregory♦

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