想做一个波浪效果
float angleSpanH = wave * 3.14159265;
vec4 a_position = In.position;
float pz = amplitude * sin(cc_time.x * speed - (a_position.x - startPos.x + a_position.y - startPos.y) / textureWidth * angleSpanH);
vec4 position = vec4(a_position.x, a_position.y + pz, a_position.z, 1);
顶点的y值改变之后,怎么计算法线。。有什么思路吗