cursor orientation openGL c++

Question

I want my 2D sprite to rotate while always facing my cursor.

I am using glad, SDL2 & glm for the math.

the "original" way I tried was to calculate the angle between my Front and my desired LookAt vector of the object and put that angle in degrees into an glm::rotate matrix. That did not work out for some strange reason. The other way was to do it within a quat and apply the quat to my model matrix, which did not help either.

My object is rendered with its center at the origin (0,0,0) - so no translation needs to be done for the rotation.

I draw 2 triangles to make a rectangle on which I load my texture.

My model matrix looks like that in theory:

model = rotate * scale;

I then plug it into the shader (where Position is my vec3 Vertex)

position = projection * view * model * vec4(Position, 1.0f);

The first strange thing is, if I hardcode 90.0f as an angle into glm::rotate, my object is actually rotated about 120° clockwise. If I plug in 80° it actually rotates about ~250° clockwise. If I plug in 45° it's a perfect 45° clockwise rotation. All rotations are around the z-axis, eg.:

model = glm::rotate(model, 90.0f, glm::vec3(0.0f,0.0f,1.0f);

If I use a quaternion to simulate an orientation, it gives me angles between 0,2&0,9 radians and my object seems only to rotate between 0.0° & 45° clockwise, no matter where I put my cursor.

If I calculate the angle btw. my two vectors (ObjectLookAt & MousePosition) and store them I get also quite correct angles, but the glm::rotate function does not work as I'd expected.

Finally if I simply code the angle as:

float angle = static_cast<float>(SDL_GetTicks()/1000);

Which starts by one it actually rotates even more weird. I'd expect it to start to rate by 1° (as it starts with 1 second) and then rotate a full circle around the z axis until there are 360 seconds over. However it rotates full 360° in about 6 second. so ever "1" that is added on the angle and plugged into glm::rotate as a degree represents 60°?

Is this now a flaw in my logic? Do I not rotate a sprite around the z-axis if it is drawn on a x-y plane? I also tried the x & y axis just to be safe here, but that didn't work. I am really stuck here, I (think I) get how it should work in theory, especially as it all happens in "2D", but I cannot get it work..

Answer 1

The first strange thing is, if I hardcode 90.0f as an angle into glm::rotate, my object is actually rotated about 120° clockwise. If I plug in 80° it actually rotates about ~250° clockwise. If I plug in 45° it's a perfect 45° clockwise rotation.

This is, because the function glm::rotate expects the angle in radians (since GLM version 0.9.6).

Adapt your code like this:

model = glm::rotate(model, glm::radians(angel_degrees), glm::vec3(0.0f,0.0f,1.0f);

see also:

• glm rotate usage in Opengl
• GLM::Rotate seems to cause wrong rotation?
• GLM: function taking degrees as a parameter is deprecated (WHEN USING RADIANS)