OpenGL，尽管按逆时针顺序排列，但一些法线还是反转了

Question

在下面的代码中，我不明白为什么有些面孔的法线颠倒了。 三角形看起来按逆时针方向排列，但有些面仍然是黑色的。

当我使用 color = -vnormal 修改片段着色器时;两个黑色面孔渲染正确，但显然其他面孔没有

。感谢您提供的任何帮助

// minimalist but fonctional code using glew, glfw, glm
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <glm/vec3.hpp>
#include <glm/vec4.hpp>
#include <glm/mat4x4.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtc/type_ptr.hpp>
#include "shaders.h"
GLuint myVAO;
void createCube() {
//    v6----- v5
//   /|      /|
//  v1------v0|
//  | |     | |
//  | |v7---|-|v4
//  |/      |/
//  v2------v3
const GLfloat cube_vertices[] = {
1, 1, 1,  -1, 1, 1,  -1,-1, 1,      // v0-v1-v2 (front)
-1,-1, 1,   1,-1, 1,   1, 1, 1,      // v2-v3-v0
1, 1, 1,   1,-1, 1,   1,-1,-1,      // v0-v3-v4 (right)
1,-1,-1,   1, 1,-1,   1, 1, 1,      // v4-v5-v0
1, 1, 1,   1, 1,-1,  -1, 1,-1,      // v0-v5-v6 (top)
-1, 1,-1,  -1, 1, 1,   1, 1, 1,      // v6-v1-v0
-1, 1, 1,  -1, 1,-1,  -1,-1,-1,      // v1-v6-v7 (left)
-1,-1,-1,  -1,-1, 1,  -1, 1, 1,      // v7-v2-v1
-1,-1,-1,   1,-1,-1,   1,-1, 1,      // v7-v4-v3 (bottom)
1,-1, 1,  -1,-1, 1,  -1,-1,-1,      // v3-v2-v7
1,-1,-1,  -1,-1,-1,  -1, 1,-1,      // v4-v7-v6 (back)
-1, 1,-1,   1, 1,-1,   1,-1,-1 };    // v6-v5-v4
// normal array
const GLfloat cube_normalsI[] = {
0, 0, 1,   0, 0, 1,   0, 0, 1,      // v0-v1-v2 (front)
0, 0, 1,   0, 0, 1,   0, 0, 1,      // v2-v3-v0
1, 0, 0,   1, 0, 0,   1, 0, 0,      // v0-v3-v4 (right)
1, 0, 0,   1, 0, 0,   1, 0, 0,      // v4-v5-v0
0, 1, 0,   0, 1, 0,   0, 1, 0,      // v0-v5-v6 (top)
0, 1, 0,   0, 1, 0,   0, 1, 0,      // v6-v1-v0
-1, 0, 0,  -1, 0, 0,  -1, 0, 0,      // v1-v6-v7 (left)
-1, 0, 0,  -1, 0, 0,  -1, 0, 0,      // v7-v2-v1
0,-1, 0,   0,-1, 0,   0,-1, 0,      // v7-v4-v3 (bottom)
0,-1, 0,   0,-1, 0,   0,-1, 0,      // v3-v2-v7
0, 0,-1,   0, 0,-1,   0, 0,-1,      // v4-v7-v6 (back)
0, 0,-1,   0, 0,-1,   0, 0,-1 };    // v6-v5-v4
// Upload per-vertex positions
GLuint positionVBO = 0;
glGenBuffers(1, &positionVBO);
glBindBuffer(GL_ARRAY_BUFFER, positionVBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(cube_vertices) * sizeof(GLfloat), cube_vertices, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);
// Upload per-vertex normals
GLuint normalVBO = 0;
glGenBuffers(1, &normalVBO);
glBindBuffer(GL_ARRAY_BUFFER, normalVBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(cube_normalsI) * sizeof(GLfloat), cube_normalsI, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);

// Hook up vertex/normals buffers to a "vertex array object" (VAO)
glGenVertexArrays(1, &myVAO);
glBindVertexArray(myVAO);
// Attach position buffer as attribute 0
glBindBuffer(GL_ARRAY_BUFFER, positionVBO);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(float) * 3, 0);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
// Attach normal buffer as attribute 1
glBindBuffer(GL_ARRAY_BUFFER, normalVBO);
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(float) * 3, 0);
glEnableVertexAttribArray(1);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
}
int main(int argc, char** argv) {
glfwInit();
GLFWwindow* window = glfwCreateWindow(768, 768, "", NULL, NULL);
glfwMakeContextCurrent(window);
glewInit();
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE); // same problem with glEnable(GL_FRONT_AND_BACK);
glClearColor(0.8f, 0.7f, 0.5f, 1.0f);
unsigned int program = shaders::CreateShader("simple.vert", "simple.frag");  
createCube();
while (glfwWindowShouldClose(window) == GL_FALSE) {            
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glm::mat4 model = glm::translate(glm::mat4(1.0f), glm::vec3(0.0, 0.0, -4.0));
glm::mat4 view = glm::lookAt(glm::vec3(-2.0, -2.0, 0.0), glm::vec3(0.0, 0.0, -4.0), glm::vec3(0.0, 1.0, 0.0));
glm::mat4 projection = glm::perspective(45.0f, 1.0f, 0.1f, 10.0f);
glm::mat4 mvp = projection * view * model;
glUseProgram(program);
GLuint imvp = glGetUniformLocation(program, "mvp");
glUniformMatrix4fv(imvp, 1, false, glm::value_ptr(mvp));
glBindVertexArray(myVAO);
glDrawArrays(GL_TRIANGLES, 0, 36);
glBindVertexArray(0);
glUseProgram(0);
glfwSwapBuffers(window);
}
}

顶点着色器：

#version 330 core
layout (location = 0) in vec3 in_position;
layout (location = 1) in vec3 in_normal;
uniform mat4 mvp;
out vec3 vnormal;
void main() {
vnormal = in_normal;
gl_Position = mvp * vec4(in_position,1); 
}

片段着色器：

#version 330 core
in vec3 vnormal;
out vec3 color;
void main() {
color= vnormal;
}

Answer 1

输出颜色被钳制到 0.0-1.0 范围。

因此，像-1, 0, 0这样的负法线最终在颜色缓冲区中显示为 RGB(0,0,0(。