This appears to be tricky afterall. Shouldn't the vertex and direction z be affecting the tan1 as well?

Edit:
I changed the wave function to something more simple for now, I basically removed the power to 2.
So, the derivative of:

float wave1 = 30.0f * sin(fs * dir_vertex + t);

should now be:

float tan1y = cos(fs * dir.x * wave_vertex.x)
float bitan1y = cos(fs * dir.y * wave_vertex.z);

(is t in the derivative equation? I think not, as it's constant)

However this too doesn't give me a correct result.

This appears to be tricky afterall. Shouldn't the vertex and direction z be affecting the tan1 as well?

Edit:
I changed the wave function to something more simple for now, I basically removed the power to 2.
So, the derivative of:

float wave1 = 30.0f * sin(fs * dir_vertex + t);

should now be:

float tan1y = cos(fs * dir.x * wave_vertex.x)
float bitan1y = cos(fs * dir.y * wave_vertex.z);

(is t in the derivative equation? I think not, as it's constant)

However this too doesn't give me a correct result.

This appears to be tricky afterall. Shouldn't the vertex and direction z be affecting the tan1 as well?