OpenGL glBufferSubData 偏移问题

Question

我在使用 glBufferSubData() 将所有数据复制/上传到缓冲区时遇到问题。我想逐块复制到缓冲区。所以我使用了这种方法。当我尝试渲染时，我看到一个空白屏幕。请在下面找到我的代码。您在计算缓冲区偏移量时看到任何问题吗？或者这不是将数据复制到缓冲区的方法吗？

下面是数据结构

struct DisplayIndexID {
        int idx;
        DrawStateT drawState;
        //Every display Index ID has its own draw models.
        std::vector<std::unique_ptr<vertexModel>> readytoDrawModels;
    };


void initVboData(std::vector<DisplayIndexID> & v)

{
    glBindBuffer(GL_ARRAY_BUFFER, geomVBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D) * 4096, NULL, GL_DYNAMIC_DRAW);

    std::vector<QVector3D> vecToDraw;
    GLintptr offset = 0;

    for (int i = 0; i < v.size(); i++) 
        for (auto& vModel : v[i].readytoDrawModels)
        {
            if (vModel) {
                vecToDraw = vModel->getVertices();
                glBufferSubData(GL_ARRAY_BUFFER, offset, sizeof(QVector3D) * vecToDraw.size(), &vecToDraw[0]);
                offset += sizeof(QVector3D) * vecToDraw.size(); // is this offset calculation fine?
            }
        }

 }

 //Below is my draw function

 void drawDisplayLists(std::vector<DisplayIndexID> & v) 
{
    initVboData(v);
    for (int i = 0; i < v.size(); i++)
    {
        //Make context current      
        makeCurrent();
        bool isTextureUsed = false;

        //Apply Projection Matrix.
        GLint mvp_mat = 0;
        GLint mvp_matText = 0;

        ///***********PRINT AREA***********************/
        for (auto& vModel : v[i].readytoDrawModels)
        {
         // Code related to Shaders
            ......


          glEnableVertexAttribArray(0);
          glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(QVector3D), nullptr);  how to get the offset and add offset here. 

switch (vModel->getDrawMode())
    {
                case 0: //GL_POINTS
                    glDrawArrays(GL_POINTS, 0, vModel->getVertices().size());
                    break;

                case 1: //GL_LINES
                    glDrawArrays(GL_LINES, 0, vModel->getVertices().size());
                    break;

                case 2: //GL_LINE_LOOP
                    glDrawArrays(GL_LINE_LOOP, 0, vModel->getVertices().size());
                    break;
              }  
        }
    }
}

Answer 1

考虑到您已经像这样初始化了持久 VBO：

  GLintptr offset = 0;
  for (int i = 0; i < v.size(); i++) {
    for (auto& vModel : v[i].readytoDrawModels) {
      if (vModel) {
        vecToDraw = vModel->getVertices();
        glBufferSubData(GL_ARRAY_BUFFER, offset, sizeof(QVector3D) * vecToDraw.size(), &vecToDraw[0]);
        offset += sizeof(QVector3D) * vecToDraw.size();
      }
    }
  }

那么您应该在绘图循环中复制相同的偏移量。 这可以通过将字节偏移量传递给glVertexAttribPointer() 来完成，其计算方式与 VBO 初始化期间完全相同：

  GLintptr offset = 0;
  for (int i = 0; i < v.size(); i++) {
    for (auto& vModel : v[i].readytoDrawModels) {
      if (vModel) {
        vecToDraw = vModel->getVertices();
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(QVector3D), offset);
        glDrawArrays(GL_LINES, 0, vecToDraw.size());
        offset += sizeof(QVector3D) * vecToDraw.size();
      }
    }
  }

或通过将顶点数量的偏移量传递给glDrawArrays()，以防所有模型具有与您的代码 sn-p (in vec3 Position) 中相同的顶点定义：

  GLintptr aFirstIndex = 0;
  glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(QVector3D), nullptr);
  for (int i = 0; i < v.size(); i++) {
    for (auto& vModel : v[i].readytoDrawModels) {
      if (vModel) {
        vecToDraw = vModel->getVertices();
        glDrawArrays(GL_LINES, aFirstIndex, vecToDraw.size());
        aFirstIndex += vecToDraw.size();
      }
    }
  }

确保分配的 VBO 具有适当的大小，并将 4096 替换为实际大小：

    glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D) * 4096, NULL, GL_DYNAMIC_DRAW);

顺便说一句，按GL_POINTS/GL_LINES 数组类型和相同的表示属性（颜色/材质/等）对模型进行分组将允许使用单个glDrawArrays() 调用来渲染每个组，而不是大量的glDrawArrays()对于每个模型，从性能的角度来看，这将是更优化的。