Slice rotation plan problem

[nesifao]

Hello,

I wanted to know if it was possible to slice a plan for example in 2 in Y coordinate and apply a different rotation for each new plan? I managed to almost do that with that code in a vertex shader:

uniform float Angle;
// simple vertex shader

vec4 rotate (vec4 p, float t)
{
	if (t<0.5)
	{
		float	a = smoothstep(0.0, 1.0, Angle * 1.0) * (3.14159 / 2.0);
		float	ca = cos(a);
		float	sa = sin(a);

		return vec4( p.x * sa + p.z * ca,
				  p.y,
				  -p.x * ca + p.z * sa,
				  p.w);
	}
	
	else
	{
		float	a = smoothstep(0.0, 1.0, Angle * 2.0) * (3.14159 / 2.0);
		float	ca = cos(a);
		float	sa = sin(a);

		return vec4( p.x * sa + p.z * ca,
				  p.y,
				  -p.x * ca + p.z * sa,
				  p.w);
	}
	
}


void main()
{
	gl_TexCoord[0] = gl_MultiTexCoord0;
	vec4 pos          = rotate(gl_Vertex, gl_TexCoord[0].y);
	gl_Position       = gl_ModelViewProjectionMatrix * pos;
	gl_FrontColor    = gl_Color;
	
}

And I obtain something like that:

imageshack.us/photo/my-images/7/capturedcran20130408165.png

As you can see the two rectangles are linked and I want to avoid that. It works correctly if I create two rectangles and I apply the shader with different parameters for each but I wanted to know if it was possible to split a plan directly in two and just declare one shader directly and how if you have an idea?

Thanks a lot!

[imported_tonyo_au]

You could do it with a geometry shader by creating 2 rectangles in it.

[nesifao]

Ok thanks for the tip but do you think it is possible to do that in avoiding to use the geometry shader and only use the vertex shader?

[imported_tonyo_au]

You can’t create vertices in the vertex shader and you want to split your rectangle which requires creating a vertex