OpenGL，为什么这不是一个立方体？

Question

我正在学习 openGL，但无法正确渲染立方体。我有我认为正确的多维数据集数据，但它并不完全是多维数据集。我不确定发生了什么。任何人都可以提供一些建议吗？

#include <glad/gl.h>
#define GLFW_INCLUDE_NONE
#include <GLFW/glfw3.h>

#include "linmath.h"

#include <stdlib.h>
#include <stdio.h>

static const struct
{
  float x, y, z;
  float r, g, b;
} vertices[36] =
    {
        {-1.0f,-1.0f,-1.0f, 0.f, 1.f, 0.f}, // triangle 1 : begin
        {-1.0f,-1.0f, 1.0f, 0.f, 1.f, 0.f},
        {-1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f}, // triangle 1 : end
        {1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f}, // triangle 2 : begin
        {-1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f}, // triangle 2 : end
        {1.0f,-1.0f, 1.0f,0.f, 1.f, 0.f},
        {-1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {-1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {-1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f, 1.0f,0.f, 1.f, 0.f},
        {-1.0f,-1.0f, 1.0f,0.f, 1.f, 0.f},
        {-1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {-1.0f,-1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f,-1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {-1.0f, 1.0f, 1.0f,0.f, 1.f, 0.f},
        {1.0f,-1.0f, 1.0f,0.f, 1.f, 0.f},

    };

static const char* vertex_shader_text =
    "#version 110\n"
    "uniform mat4 MVP;\n"
    "attribute vec3 vCol;\n"
    "attribute vec3 vPos;\n"
    "varying vec3 color;\n"
    "void main()\n"
    "{\n"
    "    gl_Position = MVP * vec4(vPos, 1.0);\n"
    "    color = vCol;\n"
    "}\n";

static const char* fragment_shader_text =
    "#version 110\n"
    "varying vec3 color;\n"
    "void main()\n"
    "{\n"
    "    gl_FragColor = vec4(color, 1.0);\n"
    "}\n";

static void error_callback(int error, const char* description)
{
  fprintf(stderr, "Error: %s\n", description);
}

static void key_callback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
  if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
    glfwSetWindowShouldClose(window, GLFW_TRUE);
}

int main(void)
{
  GLFWwindow* window;
  GLuint vertex_buffer, vertex_shader, fragment_shader, program;
  GLint mvp_location, vpos_location, vcol_location;

  glfwSetErrorCallback(error_callback);

  if (!glfwInit())
    exit(EXIT_FAILURE);

  glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2);
  glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);

  window = glfwCreateWindow(640, 480, "Simple example", NULL, NULL);
  if (!window)
  {
    glfwTerminate();
    exit(EXIT_FAILURE);
  }

  glfwSetKeyCallback(window, key_callback);

  glfwMakeContextCurrent(window);
  gladLoadGL(glfwGetProcAddress);
  glfwSwapInterval(1);

  // NOTE: OpenGL error checks have been omitted for brevity

  glGenBuffers(1, &vertex_buffer);
  glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer);
  glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

  vertex_shader = glCreateShader(GL_VERTEX_SHADER);
  glShaderSource(vertex_shader, 1, &vertex_shader_text, NULL);
  glCompileShader(vertex_shader);

  fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
  glShaderSource(fragment_shader, 1, &fragment_shader_text, NULL);
  glCompileShader(fragment_shader);

  program = glCreateProgram();
  glAttachShader(program, vertex_shader);
  glAttachShader(program, fragment_shader);
  glLinkProgram(program);

  mvp_location = glGetUniformLocation(program, "MVP");
  vpos_location = glGetAttribLocation(program, "vPos");
  vcol_location = glGetAttribLocation(program, "vCol");

  glEnableVertexAttribArray(vpos_location);
  glVertexAttribPointer(vpos_location, 3, GL_FLOAT, GL_FALSE,
                        sizeof(vertices[0]), (void*) 0);
  glEnableVertexAttribArray(vcol_location);
  glVertexAttribPointer(vcol_location, 3, GL_FLOAT, GL_FALSE,
                        sizeof(vertices[0]), (void*) (sizeof(float) * 3));

  while (!glfwWindowShouldClose(window))
  {
    float ratio;
    int width, height;
    mat4x4 m, p, mvp;

    glfwGetFramebufferSize(window, &width, &height);
    ratio = width / (float) height;

    glViewport(0, 0, width, height);
    glClear(GL_COLOR_BUFFER_BIT);

    mat4x4_identity(m);
    mat4x4_rotate_Z(m, m, (float) glfwGetTime());
    mat4x4_rotate_Y(m, m, (float) glfwGetTime());
    mat4x4_ortho(p, -ratio, ratio, -1.f, 1.f, 1.f, -1.f);
    mat4x4_mul(mvp, p, m);

    glUseProgram(program);
    glUniformMatrix4fv(mvp_location, 1, GL_FALSE, (const GLfloat*) mvp);
    glDrawArrays(GL_POLYGON, 0, 36);

    glfwSwapBuffers(window);
    glfwPollEvents();
  }

  glfwDestroyWindow(window);

  glfwTerminate();
  exit(EXIT_SUCCESS);
}

我只是缺少几个三角形吗？有没有更有效的方法来创建立方体？

Answer 1

主要问题是primitive 类型。由于您创建了一个由 12 个三角形组成的立方体，因此基本类型必须是而不是 GL_POLYGON：

glDrawArrays(GL_POLYGON, 0, 36);

glDrawArrays(GL_TRIANGLES, 0, 36);

---

无论如何，我建议构建一个有 6 个 6 面有 6 种不同颜色的立方体：

struct TVetex
{
    float x, y, z;
    float r, g, b;
};

TVetex vertices[36];

struct TVetex2
{
    float x, y;
};

struct TColor
{
    float r, g, b;
};

int main(void)
{
    TVetex2 v[]{ {-1, -1}, {1, -1}, {1, 1}, {-1, -1}, {1, 1}, {-1, 1} };
    TColor c[]{ {1, 0, 0}, {1, 1, 0}, {0, 0, 1}, {0, 1, 0}, {1, 0, 1},  {1, 0.5, 0} };
    for (int i = 0; i < 6; ++i)
    {
        for (int j = 0; j < 6; ++j)
        {
            int k = i*6 + j;
            switch (i)
            {
                case 0: vertices[k] = { -1, v[j].x, v[j].y, c[i].r, c[i].g, c[i].b }; break;
                case 1: vertices[k] = {  1, v[j].x, v[j].y, c[i].r, c[i].g, c[i].b }; break;
                case 2: vertices[k] = { v[j].y, -1, v[j].x, c[i].r, c[i].g, c[i].b }; break;
                case 3: vertices[k] = { v[j].y,  1, v[j].x, c[i].r, c[i].g, c[i].b }; break;
                case 4: vertices[k] = { v[j].x, v[j].y, -1, c[i].r, c[i].g, c[i].b }; break;
                case 5: vertices[k] = { v[j].x, v[j].y,  1, c[i].r, c[i].g, c[i].b }; break;
            }
        }
    }

    // [...]
}

---

进一步增加观看量，以看到整个立方体。比如

mat4x4_ortho(p, -ratio*2.f, ratio*2.f, -2.f, 2.f, 2.f, -2.f);