Rotating the LookAt vector of gluLookAt - c++

Question

I'm a student new to opengl. Currently, I'm doing a project that creates a scene.
Right now, my team is using gluLookAt() for my camera. What I want to accomplish is to try and rotate the LookAt vector around a certain point, namely where the camera is looking at.
This accomplishes a sort of "swaying in a circle". I need this because I am making a dart game for the scene, and my camera stay still, but I need it to move in a circle, but still allow the user's mouse to influence it. I also need it to create a drunken movement. That is why I am not considering rotating the Up or Eye vectors.
Currently, my look at code is like this.
int deltax = x - mouse.mX;
int deltay = y - mouse.mY;
cameradart.mYaw -= ((deltax/360.0) * 3.142) * 0.5;
cameradart.mPitch -= deltay * 0.02;
mouse.mX = x;
mouse.mY = y;
cameradart.lookAt.x = sin (cameradart.mYaw);
cameradart.lookAt.y = cameradart.mPitch ;
cameradart.lookAt.z = cos (cameradart.mYaw);
gluLookAt (cameradart.eye.x, cameradart.eye.y, cameradart.eye.z,
cameradart.eye.x + cameradart.lookAt.x, cameradart.eye.y + cameradart.lookAt.y,
cameradart.eye.z + cameradart.lookAt.z,
cameradart.up.x, cameradart.up.y, cameradart.up.z);
I know that it could be done easier using a different camera, but I really don't want to mess with my team's code by not using gluLookAt().

Answer 1

There's a couple of solutions in my mind, I'll tell you the easiest to understand/implement as a new graphics student
Assuming at first you're looking at (0,0,1) -store that vector-:
Think of a point that's drawing a circle and you're looking at it,
-Do it first to turn right and left (2D on X & Z)
-Let HDiff be the horizontal difference between old mouse position and the new one
-Update the x = cos(HDiff)
-Update the z = sin(HDiff)
*I didn't try it but it should work :)

Answer 2

If you want to be able to manipulate the camera, using a camera matrix is a much more effective mechanism than working with gluLookat. GLM is a good library for matrix and vector math and includes a lookat mechanism you can use to initialize the matrix, or you can just initialize it with a series of operations. However, remember that lookat produces a view matrix, and the view matrix is the inverse of the camera matrix.
This piece of code has a demonstration of what I'm talking about. Specifically look at the player member variable and how it's manipulated
glm::mat4 player;
...
glm::vec3 playerPosition(0, eyeHeight, ipd * 4.0f);
player = glm::inverse(glm::lookAt(playerPosition, glm::vec3(0, eyeHeight, 0), GlUtils::Y_AXIS));
This approach lets you apply changes like rotation and translation directly to the player matrix
// Rotate on the Y axis
player = glm::rotate(player, angle, glm::vec3(0, 1, 0));
This is much more intuitive than manipulating the view matrix, since changes to the view matrix always have to be the inverse of what you'd do to the player matrix.
When you're ready to render you need to convert the player matrix to a view matrix by taking it's inverse. In my example it's done like this:
gl::Stacks::modelview().top() = riftOrientation * glm::inverse(player);
This is because I'm using an application based modelview matrix stack that gets applied to the Shader programs I'm running.
For an OpenGL 1.x program, you'd instead use LoadMatrix
glMatrixMode(GL_MODELVIEW);
glm::mat4 modelview = glm::inverse(player);
glLoadMatrixf(&modelview);