如何在 OpenGL 中创建网格并用线条绘制它

Question

我需要创建一个这样的网格：

  ----------------
  |  |  |  |  |  |
  ----------------
  |  |  |  |  |  |
  ----------------
  |  |  |  |  |  |
  ----------------

并仅用线条渲染它。这就是我创建顶点和索引的方式：

  std::vector<glm::vec3> vertices;
  std::vector<glm::uvec3> indices;

  for(int j=0; j<=slices; ++j) {
    for(int i=0; i<=slices; ++i) {
      float x = (float)i/(float)slices;
      float y = 0;
      float z = (float)j/(float)slices;
      vertices.push_back(glm::vec3(x, y, z));
    }
  }

  for(int j=0; j<slices; ++j) {
    for(int i=0; i<slices; ++i) {

      int row1 = j * (slices+1);
      int row2 = (j+1) * (slices+1);

      indices.push_back(glm::uvec3(row1+i, row1+i+1, row2+i+1));
      indices.push_back(glm::uvec3(row1+i, row2+i+1, row2+i));

    }
  }

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

  GLuint vbo;
  glGenBuffers( 1, &vbo );
  glBindBuffer( GL_ARRAY_BUFFER, vbo );
  glBufferData( GL_ARRAY_BUFFER, vertices.size()*sizeof(glm::vec3), glm::value_ptr(vertices[0]), GL_STATIC_DRAW );
  glEnableVertexAttribArray( 0 );
  glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nullptr );

  GLuint ibo;
  glGenBuffers( 1, &ibo );
  glBindBuffer( GL_ELEMENT_ARRAY_BUFFER, ibo );
  glBufferData( GL_ELEMENT_ARRAY_BUFFER, indices.size()*sizeof(glm::uvec3), glm::value_ptr(indices[0]), GL_STATIC_DRAW );

  glBindVertexArray( 0 );
  glBindBuffer( GL_ELEMENT_ARRAY_BUFFER, 0 );
  glBindBuffer( GL_ARRAY_BUFFER, 0 );

  lenght = (GLuint)indices.size()*3;

在这里我如何渲染它：

  glBindVertexArray(vao);

  glDrawElements(GL_LINES, lenght, GL_UNSIGNED_INT, NULL);

  glBindVertexArray(0);

结果不是我想要的。我也尝试使用 GL_LINE_LOOP 等我认为问题在于顶点和索引的创建

更新：

我做了Rabbit76建议的改动看起来很完美！！

这是创建顶点和索引的最终代码：

  std::vector<glm::vec3> vertices;
  std::vector<glm::uvec4> indices;

  for(int j=0; j<=slices; ++j) {
    for(int i=0; i<=slices; ++i) {
      float x = (float)i/(float)slices;
      float y = 0;
      float z = (float)j/(float)slices;
      vertices.push_back(glm::vec3(x, y, z));
    }
  }

  for(int j=0; j<slices; ++j) {
    for(int i=0; i<slices; ++i) {

      int row1 =  j    * (slices+1);
      int row2 = (j+1) * (slices+1);

      indices.push_back(glm::uvec4(row1+i, row1+i+1, row1+i+1, row2+i+1));
      indices.push_back(glm::uvec4(row2+i+1, row2+i, row2+i, row1+i));

    }
  }

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

  GLuint vbo;
  glGenBuffers( 1, &vbo );
  glBindBuffer( GL_ARRAY_BUFFER, vbo );
  glBufferData( GL_ARRAY_BUFFER, vertices.size()*sizeof(glm::vec3), glm::value_ptr(vertices[0]), GL_STATIC_DRAW );
  glEnableVertexAttribArray( 0 );
  glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, nullptr );

  GLuint ibo;
  glGenBuffers( 1, &ibo );
  glBindBuffer( GL_ELEMENT_ARRAY_BUFFER, ibo );
  glBufferData( GL_ELEMENT_ARRAY_BUFFER, indices.size()*sizeof(glm::uvec4), glm::value_ptr(indices[0]), GL_STATIC_DRAW);

  glBindVertexArray(0);
  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
  glBindBuffer(GL_ARRAY_BUFFER, 0);

  lenght = (GLuint)indices.size()*4;

这是渲染网格的代码：

  glEnable(GL_DEPTH_TEST);

  glBindVertexArray(vao);

  glDrawElements(GL_LINES, lenght, GL_UNSIGNED_INT, NULL);

  glBindVertexArray(0);

  glDisable(GL_DEPTH_TEST);

Answer 1

[...] I need to render a line raster [...]

There are a lot of possibilities. Based on this code, an option with very few changes is to create four line segments for each quad.

Change the type of the indices:

std::vector<glm::uvec4> indices;

添加线段：

indices.push_back(glm::uvec4(row1+i, row1+i+1, row1+i+1, row2+i+1));
indices.push_back(glm::uvec4(row2+i+1, row2+i, row2+i, row1+i));

在创建和初始化缓冲区的数据存储时注意更改的类型：

glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size()*sizeof(glm::uvec4),
             indices.data(), GL_STATIC_DRAW);

分别（甚至更优雅）

glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size()*sizeof(*indices.data()),
             indices.data(), GL_STATIC_DRAW);

最后计算适当的索引数

lenght = (GLuint)indices.size()*4;

绘制线段

glBindVertexArray(vao);
glDrawElements(GL_LINES, lenght, GL_UNSIGNED_INT, NULL);

另一种选择是在整个网格中渲染线段：

std::vector<glm::uvec2> indices;

int rowLen = slices+1;
int noVert = (int)vertices.size();
for(int j=0; j<slices; ++j) {
    indices.push_back(glm::uvec2(j * rowLen, (j+1) * rowLen - 1));
    indices.push_back(glm::uvec2(j, noVert - rowLen + j));
}

glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size()*sizeof(*indices.data()),
             indices.data(), GL_STATIC_DRAW);
lenght = (GLuint)indices.size()*2;