Question

I am having an OpenGL VBO problem. I downloaded the old VBO example called Lesson45 from NeHe and I modified it to check something.

My end result is to create about 9 tiles, one of them being the origin. Then as the player moves on the screen, the top/bottom rows/columns update the data. But for now I want something basic:

I create one VBO and then I want to update the data in another thread. While the data is being uploaded, I do not want to draw the VBO because that would cause problems.

Here I create the VBO:

glGenBuffersARB( 1, &m_nVBOVertices );
glBindBufferARB(GL_ARRAY_BUFFER_ARB, m_nVBOVertices);
glBufferDataARB(GL_ARRAY_BUFFER_ARB, m_nVertexCount*3*sizeof(float), m_pVertices, GL_DYNAMIC_DRAW_ARB);

I create a thread, I set up an OpenGL context, I share lists. Then I process the data, when the user presses “R” on the keyboard:

    while(TerrainThreadRun)
    {
        //look for R
        if(window.keys->keyDown[82] == TRUE && keyactivated == false)
        {
            keyactivated = true;
            window.keys->keyDown[82] = FALSE;
        }

        if(keyactivated)
        {
            for(int i = 0; i < g_pMesh->m_nVertexCount; i++)
            {
                g_pMesh->m_pVertices[i].y = 800.0f;
            }
            while(!wglMakeCurrent(window.hDCThread,window.hRCThread))//This was removed
                Sleep(5);//This was removed
            glBindBufferARB(GL_ARRAY_BUFFER_ARB, g_pMesh->m_nVBOVertices);      
            glBufferSubDataARB(GL_ARRAY_BUFFER_ARB, 0, g_pMesh->m_nVertexCount*3*sizeof(float), g_pMesh->m_pVertices);
            keyactivated = false;
        }
    }

To draw the data:

if(!keyactivated)
{
    glEnableClientState( GL_VERTEX_ARRAY );

    glBindBufferARB(GL_ARRAY_BUFFER_ARB, g_pMesh->m_nVBOVertices);
    glVertexPointer(3, GL_FLOAT, 0, (char*)NULL);


    glDrawArrays(GL_TRIANGLES, 0, g_pMesh->m_nVertexCount);

    glDisableClientState(GL_VERTEX_ARRAY);
}

I know that using ARB extensions is not recommended, but this is just for a quick basic example.

The problem is that when I first press “R”, the data does not get updated. The VBO draws the same. The second time that I press “R”, it updates the data. What can I do to force the draw. Am I doing something wrong?

Does the data need to be forced to the video card? Am I missing something?

Update: I looked over my code and now I use wglMakeCurrent only once, when the context is initialized. In the thread, I use it after sharing the lists and on the main thread as soon as the lists are shared, like this:

window->hRC = wglCreateContext (window->hDC);
if (window->hRC ==0)
{
    // Failed
}

TerrainThreadRun = true;
TerrainThread = CreateThread(NULL, NULL,(LPTHREAD_START_ROUTINE)TerrainThreadProc, 0, NULL, NULL);

while(!sharedContext)
    Sleep(100);

if (wglMakeCurrent (window->hDC, window->hRC) == FALSE)

And in the thread:

    if (!(window.hRCThread=wglCreateContext(window.hDCThread)))
    {
//Error
    }

while(wglShareLists(window.hRC, window.hRCThread) == 0)
{
    DWORD err = GetLastError();
    Sleep(5);
}
sharedContext = true;
int cnt = 0;

while(!wglMakeCurrent(window.hDCThread,window.hRCThread))
    Sleep(5);
while(TerrainThreadRun)
{
    //look for R

Second update: I tried using glMapBuffer instead of glBuferSubData, but the application behaves the same. Here is the code:

        void *ptr = (void*)glMapBuffer(GL_ARRAY_BUFFER_ARB, GL_READ_WRITE_ARB);
        if(ptr)
        {
            memcpy(ptr, g_pMesh->m_pVertices, g_pMesh->m_nVertexCount*3*sizeof(float));
            glUnmapMapBuffer(GL_ARRAY_BUFFER_ARB);
        }

Update three:

I was doing some things wrong, so I modified them, but the problem remains the same. Here is how I do everything now:

When the application loads, I create two windows, each with its own HWND. Based on them, I create two device contexts.

Then I share the lists between them:

wglShareLists(window.hRC, window.hRCThread);

This is done only once when I initialize.
After that I show the OGL window, which renders; I make the context active. Then I load the function pointers and create the VBO.
After the main rendering OGL is done, I create the thread. When the thread is loaded, I make its device context active.

Then we do normal stuff.

So my question is: Do I need to update the function pointers for each device context? Could this be my problem?

As an update, if I run my test app in gDEBugger and I first press “R” and then pause, it doesn’t display correctly. I take a look at the memory (Textures, Buffers and Image Viewers) and GLContext1(I think the main rendering thread) device context has the OLD data. While GLContext2 (Shared-GL1) (I think the thread context) has the correct data.

The odd part, if I look back at GLContext1, with the program still in pause mode, now it displays the new data, like it “refreshed” it somehow. And then if I press play, it starts drawing correctly.

Answer 1

I found the solution, I need to call glFinish() in the worker thread after calling glUnmapBuffer. This will solve the problem and everything will render just fine.