OpenGL VBO within multiple threads
I am developing a program in C++/OpenGL which draws terrain of the entire world. I have a database of altitude heights stored as tiles. Every time I start the program, a tile is loaded. Then as the person moves, another tile should load, this does not happen every frame, maybe once every 5 minutes.
I load the initial tile in the video card's memory:
glBindBufferARB(GL_ARRAY_BUFFER_ARB, VertexBuffer[idx]);
glBufferDataARB(GL_ARRAY_BUFFER_ARB, VBOsz * 3 * sizeof(float), tile_data, GL_STATIC_DRAW_ARB);
...There are normals, color and index buffers
And I draw them:
glBindBufferARB(GL_ARRAY_BUFFER_ARB, VertexBuffer[idx]);
glBufferDataARB(GL_ARRAY_BUFFER_ARB, VBOsz * 3 * sizeof(float), tile_data, GL_STATIC_DRAW_ARB);
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glEnableClientState(GL_NORMAL_ARRAY);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, VertexBuffer[idx]);
glVertexPointer(3, GL_FLOAT, 0, BUFFER_OFFSET(0));
...
glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, IndexBuffer[idx]);
glDrawElements(GL_TRIANGLES, IndexBuffersz, GL_UNSIGNED_INT, BUFFER_OFFSET(0));
glDisableClientState(GL_NORMAL_ARRAY);
glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);
Since I want the program to be as smooth as possible, I can't calculate the vertex+color+normal+ other textures the same frame, as it takes about 20 seconds to create a tile.
So I decided to make a loader thread that would check for when a new tile needs to be loaded and then load it. When it would be all done, it should just swap the VBO(hence the [idx].
So for a loader thread, I know that I need a second OpenGL context, I created one and I share the lists between them. The idea is working, but in the loader thread, when I send the new VBO data, I need this function: wglMakeCurrent
Only when all the data has been loaded, I can set the context to the rendering thread(main program's thread). This causes nothing to be drawn for that amount of time, which makes the program useless.
Do you have any ideas on a solution? Do I need to change the concept?
I am using OpenGL 2.1. Will upgrading to OpenGL 3 solve the problem?
Answer
This is really not that complicated.
You just make two buffer objects: one that you're using for current rendering and one that you will be using in the future. When the unused buffer is full of data, the rendering thread switches to rendering from that one. The previously used buffer becomes the unused one.
You can upload the data in one of two ways. One way is for your data creation thread to create an array of data that the rendering thread will upload to the buffer object with glBufferData. Obviously this will require some synchronization, but it's sync code that you need: your render thread must ultimately be informed when data is ready so that it can render with the new data.
The other way is for your rendering thread to be told that data needs to start being generated. At which point, it will map the unused buffer object and pass the mapped pointer to the data creation thread. That thread will generate data directly into that mapped pointer. When it finishes, it informs the render thread, which will unmap the buffer and then render with the data.
Neither method requires multiple contexts or threading through OpenGL code.
Note that performance will be best served by not making the buffers bigger and smaller. Pick a size and stick with it; you don't want to resize buffers with glBufferData.