OpenGL glBufferSubData Offset issue - c++

Question

I have a problem copying/upload all the data into the buffer using glBufferSubData(). I want to copy chunk by chunk to the buffer. So I have used this approach. I am seeing a blank screen when I try to render. Please find my code below. do you see any problem in calculating the buffer offset? Or is this not the way to copy data into the buffer?
Below is the data structure
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

Considering that you have persistent VBO initialized like that:
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();
}
}
}
then you should replicate the same offsets within drawing loop.
This can be done either by passing offset in bytes to glVertexAttribPointer() computed exactly in the same way as during VBO initialization:
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();
}
}
}
or by passing offset in number of vertices to glDrawArrays(), in case if all models have the same vertex definition as in your code snippet (in vec3 Position):
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();
}
}
}
Make sure that allocated VBO has a proper size and 4096 is replaced by a real size:
glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D) * 4096, NULL, GL_DYNAMIC_DRAW);
By the way, grouping models by GL_POINTS/GL_LINES array type and same presentation attributes (color/material/etc.) would allow rendering each group with a single glDrawArrays() call, instead of numerous glDrawArrays() for every model, which would be more optimal from performance point of view.