in ..
out vec3 varyingNormalDirection;

void main(){
	vec3 a = gl_in[0].gl_Position.xyz - gl_in[2].gl_Position.xyz;
	vec3 b = gl_in[0].gl_Position.xyz - gl_in[1].gl_Position.xyz;
	varyingNormalDirection = normalize(cross(a, b));
}