Object Rotation

[SpiderPig]

Getting the rotation of an object doesn't seem to go past 90 degrees on any one axis.  E.g. I turn the box along the x axis but it only just gets to 90 degrees then starts counting down again and the y and z axis flip to 180 at around 45 degrees as it counts down on the x.  This probably has something to do with quaternions but I do not know for sure.

#include "Engine.h"
using namespace UltraEngine;

int main(int argc, const char* argv[])
{
    auto displays = GetDisplays();
    auto window = CreateWindow("Ultra Engine", 0, 0, 800, 600, displays[0]);
    auto world = CreateWorld();
    auto framebuffer = CreateFramebuffer(window);

    auto camera = CreateCamera(world);
    camera->SetClearColor(0.0f, 0.0f, 1.0f);
    camera->SetFov(70);
    camera->SetRange(0.01f, 1000.0f);
    camera->SetPosition(0, 1, -3);

    auto light = CreateDirectionalLight(world);
    light->SetColor(5.0f);
    light->SetRotation(35, 45, 0);

    auto font = LoadFont("Fonts/arial.ttf");
    auto ui = CreateInterface(world, font, framebuffer->size);
    ui->SetRenderLayers(2);
    ui->root->SetColor(0, 0, 0, 0);

    auto ui_cam = CreateCamera(world, PROJECTION_ORTHOGRAPHIC);
    ui_cam->SetPosition(framebuffer->size.x / 2, framebuffer->size.y / 2, 0);
    ui_cam->SetRenderLayers(2);
    ui_cam->SetClearMode(CLEAR_DEPTH);

    auto label = CreateLabel("", 10, 10, 200, 30, ui->root);

    auto box = CreateBox(world);

    while (window->Closed() == false and window->KeyDown(KEY_ESCAPE) == false)
    {
        box->Turn(0.25f, 0.0f, 0.0f);

        auto local = box->GetRotation();
        auto global = box->GetRotation(true);
        label->SetText("Local : " + String(local.x) + ", " + String(local.y) + ", " + String(local.z) + "\n"
            "Global : " + String(global.x) + ", " + String(global.y) + ", " + String(global.z));

        world->Update();
        world->Render(framebuffer, false);
    }
    return 0;
}

 

[Josh]

Yes, you answered your own question.

Euler angles are easy for humans to understand, but they cannot accurately describe all rotations. When the pitch approaches 90 degrees, there is no way to describe the roll accurately, because at that angle the pitch and roll are the same. This is called Gimbal lock:

This has even caused problems in space missions. I can't find the exact clip, but they talk about this in the movie "Apollo 13".

Quaternions are an exact description, but you need a degree in mathematics to understand them. In fact there is an entire book I hope to some day read named simply "Visualizing Quaternions":
https://www.elsevier.com/books/visualizing-quaternions/hanson/978-0-12-088400-1

Each entity stores its rotation in both Euler and quaternion form, and these are kept in sync whenever rotation changes. The Turn method uses quaternions to accurately turn the entity, and then fills in the entity's Euler rotation value by converting it from the resulting quaternion.

So everything is correct.

[SpiderPig]

Okay thanks.  I need to read that book too.  I don't know enough about them.  

3 hours ago, Josh said:

Each entity stores its rotation in both Euler and quaternion form, and these are kept in sync whenever rotation changes. The Turn method uses quaternions to accurately turn the entity, and then fills in the entity's Euler rotation value by converting it from the resulting quaternion.

 I want to rotate an object back and forth on the x axis between 0 and 90 degrees.  I thought GetRotation() should return Euler angles?

[Josh]

If you are only rotating on one axis you can use SetRotation instead of Turn and the Euler angles you set won't be modified. Or you can just store the pitch in a variable, and increment that and call SetRotation every frame using that variable for the pitch.