OpenGL VBO 索引（如何计算索引数组）

Question

We are migrating OpenGL to newer version ( ES 2.0 ). Application actually renders Vector images ( ie CGM files). I have successfully rendered the graphics using vertices using VBO. But the problem is performance. DisplayList performance is way better than VBO. So I am thinking using VBO indexing. How to come up the indices array? Will Indexing improve performance? Please find my code below

//This is my data structure

struct DisplayIndexID {
        int idx;
        DrawStateT drawState;

        //Every display Index ID has its own draw models.
        std::vector<std::unique_ptr<vertexModel>> readytoDrawModels;
    };

//Initializing the VBO 
void initVbo(std::vector<DisplayIndexID> & v)

{
    glBindVertexArray(geomVAO);
    glBindBuffer(GL_ARRAY_BUFFER, geomVBO);
    std::vector<QVector3D> vecToDraw;
    std::vector<QVector3D> finalVecToDraw;
    for (int j = 0; j < v.size(); j++)
        for (auto& vModel : v[j].readytoDrawModels)
        {
            if (vModel) {
                vecToDraw = vModel->getVertices();
                finalVecToDraw.insert(finalVecToDraw.end(), vecToDraw.begin(), vecToDraw.end());
    
            }
        }
    
    glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D) * finalVecToDraw.size(), &finalVecToDraw[0],GL_STATIC_DRAW );
glBindBuffer(GL_ARRAY_BUFFER, 0);
}

//Rendering function 
void drawDisplayLists(std::vector<DisplayIndexID> & v)
{
    GLintptr offset = 0;
    
    initVbo(v);
    
    for (int i = 0; i < v.size(); i++)
    {

        //***********PRINT AREA***********************/
        for (auto& vModel : v[i].readytoDrawModels)
        {
           glBindBuffer(GL_ARRAY_BUFFER, geomVBO);
           glEnableVertexAttribArray(0);
           glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(QVector3D), (GLvoid*)offset); 
           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 3: // GL_TRIANGLE
                     ...
        }
                  offset += sizeof(QVector3D) * vModel->getVertices().size();
    }  



}

Answer 1

I never worked with vector graphics but this is just on how to create indices from some vertices. So if I read your code right, your vertices are stored in the finalVecToDraw vector. First create a vector for the indices, the supported types are unsigned char up to unsigned int .

Then resize that vector to the size of your vertex array (this will save you much time resizing the vector).

Fill the vector, so that the element at index n has the value n. So for example with this code:

for(int i = 0; i < indices.size(); i++) {
    indices[i] = i;
}

This will do what you already do, but using indices.

Now we need to remove the duplicate entries in the vertices vector and adjust the indices. I don't know what algorithm would be the best to find duplicates for your case, but you need to do this. The duplicate is at the index d and the first time this value showed up was at o . d must be bigger than o :

1. Delete the vertex at d.
2. Iterate over your indices and if an element has a value bigger than d, subtract one from it.
3. Set the value of the index at d to o.