Rotating a light around a stationary object in openGL/glsl - opengl

Question

So, I'm trying to rotate a light around a stationary object in the center of my scene. I'm well aware that I will need to use the rotation matrix in order to make this transformation occur. However, I'm unsure of how to do it in code. I'm new to linear algebra, so any help with explanations along the way would help a lot.
Basically, I'm working with these two right now and I'm not sure of how to make the light circulate the object.
mat4 rotation = mat4(
vec4( cos(aTimer), 0.0, sin(aTimer), 0.0),
vec4( 0, 1.0, 0.0, 0.0),
vec4(-sin(aTimer), 0.0, cos(aTimer), 0.0),
vec4( 0.0, 0.0, 0.0, 1.0)
);
and this is how my light is set up :
float lightPosition[4] = {5.0, 5.0, 1.0, 0};
glLightfv(GL_LIGHT0, GL_POSITION, lightPositon);
The aTimer in this code is a constantly incrementing float.

Answer 1

Even though you want the light to rotate around your object, you must not use a rotation matrix for this purpose but a translation one.
The matrix you're handling is the model matrix. It defines the orientation, the position and the scale of your object.
The matrix you have here is a rotation matrix, so the orientation of the light will change, but not the position, which is what you want.
So there is two problems to fix here :
1.Define your matrix properly. Since you want a translation (circular), I think this is the matrix you need :
mat4 rotation = mat4(
vec4( 1.0, 0.0, 0.0, 0.0),
vec4( 0.0, 1.0, 0.0, 0.0),
vec4( 0.0, 0.0, 1.0, 0.0),
vec4( cos(aTimer), sin(aTimer), 0.0, 1.0)
);
2.Define a good position vertex for your light. Since it's a single vertex and it's the job of the model matrix (above) to move the light, the light vector 4D should be :
float lightPosition[4] = {0.0f, 0.0f, 0.0f, 1.0f};
//In C, 0.0 is a double, you may have warnings at compilation for loss of precision, so use the suffix "f"
The forth component must be one since it's thanks to it that translations are possible.
You may find additional information here
Model matrix in 3D graphics / OpenGL
However they are using column vectors. Judging from your rotation matrix I do belive you use row vectors, so the translation components are in the last row, not the last column of the model matrix.