c++ - 使用 Emscripten 将 OpenGL ES 转换反馈数据返回给主机

Question

使用 GLFW，下面的 C++ OpenGL ES 代码计算五个数字的平方根并在命令行上输出它们。该代码使用了转换反馈。当我在 Ubuntu 17.10 上编译时，使用以下命令，我得到了我期望的结果:

$ g++ tf.cpp -lGL -lglfw

但是，如果我使用 Emscripten，则会抛出异常，这表明glMapBufferRange 仅在访问为 MAP_WRITE|INVALIDATE_BUFFER 时才受支持。我确实想阅读而不是写作，所以也许我不应该使用 glMapBufferRange，但是我可以使用什么来代替呢？我在 Firefox 和 Chromium 上都试过了。我使用 Emscripten 编译的命令是:

$ em++ -std=c++11 tf.cpp -s USE_GLFW=3 -s USE_WEBGL2=1 -s FULL_ES3=1 -o a.out.html

代码如下:

#include <iostream>

#define GLFW_INCLUDE_ES3
#include <GLFW/glfw3.h>

#ifdef __EMSCRIPTEN__
#include <emscripten.h>
#endif

static const GLuint WIDTH  = 800;
static const GLuint HEIGHT = 600;
static const GLchar *vertex_shader_src =
"#version 300 es\n"
"precision mediump float;\n"
"in  float inValue;\n"
"out float outValue;\n"
"void main() {\n"
"  outValue = sqrt(inValue);\n"
"}\n";

// Emscripten complains if there's no fragment shader
static const GLchar *fragment_shader_src =
"#version 300 es\n"
"precision mediump float;\n"
"out vec4 colour;\n"
"void main() {\n"
"  colour = vec4(1.0, 1.0, 0.0, 1.0);\n"
"}\n";

static const GLfloat vertices[] = {
   0.0f,  0.5f, 0.0f,
   0.5f, -0.5f, 0.0f,
  -0.5f, -0.5f, 0.0f,
};

GLint get_shader_prog(const char *vert_src, const char *frag_src = "")
{
  enum Consts { INFOLOG_LEN = 512 };
  GLchar infoLog[INFOLOG_LEN];
  GLint fragment_shader, vertex_shader, shader_program, success;
  shader_program = glCreateProgram();

  auto mk_shader = [&](GLint &shader, const GLchar **str, GLenum shader_type) {
    shader = glCreateShader(shader_type);
    glShaderSource(shader, 1, str, NULL);
    glCompileShader(shader);
    glGetShaderiv(shader, GL_COMPILE_STATUS, &success);
    if (!success) {
      glGetShaderInfoLog(shader, INFOLOG_LEN, NULL, infoLog);
      std::cout << "ERROR::SHADER::COMPILATION_FAILED\n" << infoLog << '\n';
    }
    glAttachShader(shader_program, shader);
  };

  mk_shader(vertex_shader,   &vert_src, GL_VERTEX_SHADER);
  mk_shader(fragment_shader, &frag_src, GL_FRAGMENT_SHADER);

  const GLchar* feedbackVaryings[] = { "outValue" };
  glTransformFeedbackVaryings(shader_program, 1, feedbackVaryings,
                              GL_INTERLEAVED_ATTRIBS);

  glLinkProgram(shader_program);
  glGetProgramiv(shader_program, GL_LINK_STATUS, &success);
  if (!success) {
    glGetProgramInfoLog(shader_program, INFOLOG_LEN, NULL, infoLog);
    std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << '\n';
  }
  glUseProgram(shader_program);

  glDeleteShader(vertex_shader);
  glDeleteShader(fragment_shader);
  return shader_program;
}

int main(int argc, char *argv[])
{
  glfwInit();
  glfwWindowHint(GLFW_CLIENT_API, GLFW_OPENGL_ES_API);
  glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
  GLFWwindow *window = glfwCreateWindow(WIDTH, HEIGHT, __FILE__, NULL, NULL);
  glfwMakeContextCurrent(window);

  GLuint shader_prog = get_shader_prog(vertex_shader_src, fragment_shader_src);
  GLint inputAttrib = glGetAttribLocation(shader_prog, "inValue");

  glViewport(0, 0, WIDTH, HEIGHT);

  GLuint vao;
  glGenVertexArrays(1, &vao);
  glBindVertexArray(vao);

  GLfloat data[] = { 1.0f, 2.0f, 3.0f, 4.0f, 5.0f };

  GLuint vbo;
  glGenBuffers(1, &vbo);
  glBindBuffer(GL_ARRAY_BUFFER, vbo);
  glBufferData(GL_ARRAY_BUFFER, sizeof(data), data, GL_STATIC_DRAW);

  glVertexAttribPointer(inputAttrib, 1, GL_FLOAT, GL_FALSE, 0, 0);
  glEnableVertexAttribArray(inputAttrib);

  GLuint tbo;
  glGenBuffers(1, &tbo);
  glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, tbo);
  glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER, sizeof(data),
               nullptr, GL_STATIC_READ);
  glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, tbo);
  glEnable(GL_RASTERIZER_DISCARD);

  glBeginTransformFeedback(GL_POINTS);
  glDrawArrays(GL_POINTS, 0, 5);
  glEndTransformFeedback();

  glDisable(GL_RASTERIZER_DISCARD);
  glFlush();

  GLfloat feedback[5]{1,2,3,4,5};
  void *void_buf = glMapBufferRange(GL_TRANSFORM_FEEDBACK_BUFFER,0,
                                    sizeof(feedback), GL_MAP_READ_BIT);
  GLfloat *buf = static_cast<GLfloat *>(void_buf);
  for (int i = 0; i < 5; i++)
    feedback[i] = buf[i];
  glUnmapBuffer(GL_TRANSFORM_FEEDBACK_BUFFER);

  for (int i = 0; i < 5; i++)
    std::cout << feedback[i] << ' ';
  std::cout << std::endl;

  glDeleteBuffers(1, &vbo);
  glDeleteBuffers(1, &tbo);
  glDeleteVertexArrays(1, &vao);
  glfwTerminate();

  return 0;
}

Answer 1

正如另一个答案所提到的，WebGL 2.0 接近于 OpenGL ES 3.0，但令人困惑的是没有定义 glMapBufferRange() 函数，因此 Emscripten 尝试仅模拟该函数的部分功能。

然而，Real WebGL 2.0 公开了 glGetBufferSubData() 模拟，它在 OpenGL ES 中不存在，但在桌面 OpenGL 中存在。该方法可以通过 EM_ASM_ 包装在代码中:

void myGetBufferSubData (GLenum theTarget, GLintptr theOffset, GLsizeiptr theSize, void* theData)
{
#ifdef __EMSCRIPTEN__
  EM_ASM_(
  {
    Module.ctx.getBufferSubData($0, $1, HEAPU8.subarray($2, $2 + $3));
  }, theTarget, theOffset, theData, theSize);
#else
  glGetBufferSubData (theTarget, theOffset, theSize, theData);
#endif
}

取回 VBO 数据在多平台代码中很麻烦:

1. OpenGL 1.5+
   • glGetBufferSubData():是
   • glMapBufferRange():是
2. OpenGL ES 2.0
   • glGetBufferSubData():否
   • glMapBufferRange():否
3. OpenGL ES 3.0+
   • glGetBufferSubData():否
   • glMapBufferRange():是
4. WebGL 1.0
   • glGetBufferSubData():否
   • glMapBufferRange():否
5. WebGL 2.0
   • glGetBufferSubData():是
   • glMapBufferRange():否

这样:

• 桌面 OpenGL 提供了最大的灵 active 。
• OpenGL ES 2.0 和 WebGL 1.0 没有机会检索数据。
• OpenGL ES 3.0+ 仅提供映射缓冲区。
• WebGL 2.0 仅提供 getBufferSubData()。