如果 OpenGL 3+ 请求，glfw3 不绘图？

Posted 2023-02-22

【中文标题】如果 OpenGL 3+ 请求，glfw3 不绘图？

【英文标题】：glfw3 not drawing if OpenGL 3+ requested?

【发布时间】：2017-10-09 07:30:21

【问题描述】：

下面我包含了glfw-quick-test.c，它基本上是从http://www.glfw.org/docs/latest/quick.html 逐字复制的——除了删除了glad 的使用、添加了背景颜色、几个定义，因此它可以在 Ubuntu 14.04（64 位）上编译，以及预处理器#ifdef 开关（宏DO_OPENGL_THREE）来更改请求的OpenGL 版本。

---

当我编译时：

gcc -g -o glfw-quick-test.exe glfw-quick-test.c -lglfw -lGLU -lGL -lm
./glfw-quick-test.exe

...然后我收到消息“GLFW Requesting OpenGL 2.1”并且绘图很好：

---

当我编译时：

gcc -g -DDO_OPENGL_THREE -o glfw-quick-test.exe glfw-quick-test.c -lglfw -lGLU -lGL -lm
./glfw-quick-test.exe

...然后我收到消息“GLFW Requesting OpenGL 3.2”并且根本没有绘制旋转三角形 - 只保留了背景颜色：

---

谁能解释为什么会这样？即使请求了 OpenGL 3.2，我能否以某种方式让 GLFW3 进行绘制，如果可以，如何绘制？

（我知道原始源代码显示“// NOTE: OpenGL error checks have been omitted for brevity”，但我不确定应该添加什么样的错误检查，看看 OpenGL 3.2 绘图会出现什么问题......）

代码，glfw-quick-test.c（编辑：现在有错误检查）：

// http://www.glfw.org/docs/latest/quick.html

#define UBUNTU14
#ifdef UBUNTU14 // assume Ubuntu 14.04
 // strange; Ubuntu 14 GLFW/glfw3.h doesn't have GLFW_TRUE, GLFW_FALSE, mentions GL_TRUE GL_FALSE
 #define GLFW_TRUE GL_TRUE
 #define GLFW_FALSE GL_FALSE
#endif

//~ #include <glad/glad.h> // "GL/GLES/EGL/GLX/WGL Loader-Generator based on the official specs."
#include <GLFW/glfw3.h>
#include "linmath.h"
#include <stdlib.h>
#include <stdio.h>
static const struct

  float x, y;
  float r, g, b;
 vertices[3] =

   -0.6f, -0.4f, 1.f, 0.f, 0.f ,
    0.6f, -0.4f, 0.f, 1.f, 0.f ,
     0.f,  0.6f, 0.f, 0.f, 1.f 
;
static const char* vertex_shader_text =
"uniform mat4 MVP;\n"
"attribute vec3 vCol;\n"
"attribute vec2 vPos;\n"
"varying vec3 color;\n"
"void main()\n"
"\n"
"  gl_Position = MVP * vec4(vPos, 0.0, 1.0);\n"
"  color = vCol;\n"
"\n";
static const char* fragment_shader_text =
"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);

void checkGLerrors(char *label) 
    // check OpenGL error
    GLenum err;
    while ((err = glGetError()) != GL_NO_ERROR) 
    char* errorstr = "";
    switch(err) 
      case GL_INVALID_OPERATION:  errorstr="INVALID_OPERATION";  break;
      case GL_INVALID_ENUM:       errorstr="INVALID_ENUM";     break;
      case GL_INVALID_VALUE:      errorstr="INVALID_VALUE";    break;
      case GL_OUT_OF_MEMORY:      errorstr="OUT_OF_MEMORY";    break;
      case GL_INVALID_FRAMEBUFFER_OPERATION:  errorstr="INVALID_FRAMEBUFFER_OPERATION";  break;
    
    printf("OpenGL error ('%s'): %d %s\n", label, err, errorstr);
    


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);
  // NB: Ubuntu will not draw if 3.2 (just black screen) - only if 2.0 or 2.1?
  #ifdef DO_OPENGL_THREE
  printf("GLFW Requesting OpenGL 3.2\n");
  glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
  glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
  glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE); // only 3.2+
  glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GLFW_TRUE); //only 3.0+
  glfwWindowHint(GLFW_RESIZABLE, GL_TRUE); // https://***.com/q/23834680/
  #else
  printf("GLFW Requesting OpenGL 2.1\n");
  glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2); // 2);
  glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 1); // 0);
  #endif
  checkGLerrors("post hint");
  window = glfwCreateWindow(640, 480, "Simple example", NULL, NULL);
  if (!window)
  
    glfwTerminate();
    exit(EXIT_FAILURE);
  
  checkGLerrors("post glfwCreateWindow");
  glfwSetKeyCallback(window, key_callback);
  glfwMakeContextCurrent(window);
  //~ gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
  glfwSwapInterval(1);
  // NOTE: OpenGL error checks have been omitted for brevity
  glGenBuffers(1, &vertex_buffer);
  checkGLerrors("post glGenBuffers");
  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);
  checkGLerrors("post glLinkProgram");
  mvp_location = glGetUniformLocation(program, "MVP");
  vpos_location = glGetAttribLocation(program, "vPos");
  vcol_location = glGetAttribLocation(program, "vCol");
  checkGLerrors("post gl locations");
  glEnableVertexAttribArray(vpos_location);
  checkGLerrors("post gl EnableVertexAttribArray");
  glVertexAttribPointer(vpos_location, 2, GL_FLOAT, GL_FALSE,
              sizeof(float) * 5, (void*) 0);
  checkGLerrors("post glVertexAttribPointer");
  glEnableVertexAttribArray(vcol_location);
  checkGLerrors("post glEnableVertexAttribArray");
  glVertexAttribPointer(vcol_location, 3, GL_FLOAT, GL_FALSE,
              sizeof(float) * 5, (void*) (sizeof(float) * 2));
  checkGLerrors("post glVertexAttribPointer");
  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);
    glClearColor(0.784314, 0.780392, 0.305882, 1.0); // add background color
    glClear(GL_COLOR_BUFFER_BIT);
    mat4x4_identity(m);
    mat4x4_rotate_Z(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_TRIANGLES, 0, 3);
    glfwSwapBuffers(window);
    glfwPollEvents();
  
  glfwDestroyWindow(window);
  glfwTerminate();
  exit(EXIT_SUCCESS);

【问题讨论】：

您检查过glGetError 告诉您的内容吗？在核心配置文件中，您必须使用 VAO。

谢谢@BDL - 不，不知道我应该这样做，现在会尝试这样做......编辑：做到了，凌晨3.2点得到“OpenGL error ('post glVertexAttribPointer'): 1282 INVALID_OPERATION”......

【参考方案1】：

感谢@BDL 和他对 VAO（顶点数组对象）的评论，我找到了How to use VBOs without VAOs with OpenGL core profile? - 通过尝试那里的东西，我发现上面 OP 代码的唯一更改是需要的，所以绘图显示在OpenGL 3.2，是这样的吗：

...
  //~ gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
  glfwSwapInterval(1);
  #ifdef DO_OPENGL_THREE
  // https://***.com/a/30057424/277826:
  // "You can however just create and bind a VAO and forget about it (keep it bound)."
  GLuint VAO;
  glGenVertexArrays(1, &VAO);
  glBindVertexArray(VAO);
  #endif
  // NOTE: OpenGL error checks have been omitted for brevity
  glGenBuffers(1, &vertex_buffer);
...

一旦这个带有 VAO 的部分进入，就不会再打印 OpenGL 错误，并且绘图就可以了。