如何在立方体上获得平坦的法线 [英] How to get flat normals on a cube

问题描述

我正在使用不推荐使用的OpenGL，并且我的光照计算是在片段着色器上完成的.所以，我正在做平滑着色.

I am using OpenGL without the deprecated features and my light calculation is done on fragment shader. So, I am doing smooth shading.

我的问题是，当我绘制一个立方体时，我需要平面法线.平面法线是指在面部中生成的每个片段都具有相同的法线.

My problem, is that when I am drawing a cube, I need flat normals. By flat normals I mean that every fragment generated in a face has the same normal.

到目前为止，我的解决方案是为每个面生成不同的顶点.因此，现在我有了24(6 * 4)个顶点，而不是有8个顶点.

My solution to this so far is to generate different vertices for each face. So, instead of having 8 vertices, now I have 24(6*4) vertices.

但这对我来说似乎是错误的，复制了顶点.有没有更好的方法来获得平坦的法线?

But this seems wrong to me, replicating the vertexes. Is there a better way to get flat normals?

更新:我使用的是OpenGL版本3.3.0，我还不支持OpenGL 4.

Update: I am using OpenGL version 3.3.0, I do not have support for OpenGL 4 yet.

推荐答案

如果在摄影机空间中进行照明，则可以使用dFdx/dFdy从顶点的摄影机空间位置计算面部法线.

If you do the lighting in camera-space, you can use dFdx/dFdy to calculate the normal of the face from the camera-space position of the vertex.

因此片段着色器看起来像这样.

So the fragment shader would look a little like this.

varying vec3 v_PositionCS; // Position of the vertex in camera/eye-space (passed in from the vertex shader)

    void main()
    { 
      // Calculate the face normal in camera space
      vec3 normalCs = normalize(cross(dFdx(v_PositionCS), dFdy(v_PositionCS)));

      // Perform lighting 
      ...
      ...
    }

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