OpenGL VBO Indexing ( How to compute Index Array) - c++

Question

We are migrating OpenGL to newer version ( ES 2.0 ). Application actually renders Vector images ( i.e 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:
Delete the vertex at d.
Iterate over your indices and if an element has a value bigger than d, subtract one from it.
Set the value of the index at d to o.