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Multithreading VBOs

Started by bobFritzelpuff Jun 29, 2008 at 1:48 PM 1 replies 900+ views

Original Post

Jun 29, 2008 at 1:48 PM

Hi. I'm using Vertex Buffer Objects in some openGL code. Update and Draw are running on different threads. I would like the update to be able to change the data in the VBOs that will then be drawn. Is there any way to do this without a whole lot of locking and concurrency issues? For example, I want one thread to be doing this: glBindBuffer(GL_ARRAY_BUFFER, aBuffer) glBufferData(GL_ARRAY_BUFFER, size, newdata, usage) at the same time that the draw is doing this: glBindBuffer(GL_ARRAY_BUFFER, anotherBuffer) glVertexPointer(num,type,offset,NULL) glDrawArrays(and so forth) Obviously, in order for this to work, there would have to be some way to: bind more than one buffer at once (in different threads) potentially draw a buffer while it is being modified Is there any way to do this? Will using glMapBuffer() in the update be any help, or will that fail too because it requires that a buffer be bound first? Thanks.

Lord Crc

Jun 29, 2008 at 2:31 PM

You could keep two buffer objects, one active which is used for drawing, and an inactive used for updating. When you need to update the data, use the inactive vbo, bind and map it in the rendering thread, then pass the pointer to the update thread.

Once it's done updating, the update thread would signal the rendering thread, which would unmap the updated vbo, and swap the active and inactive vbo handles.

You could go one step further if you want and use a pool of vbo's. You'd then dequeue, update, swap and enqueue.

10110

Jun 30, 2008 at 12:48 PM

Quote:
Original post by bobFritzelpuff
[...]
Obviously, in order for this to work, there would have to be some way to:
bind more than one buffer at once (in different threads)
potentially draw a buffer while it is being modified

[...]

No you can't bind two buffers because of OpenGL architecture. Binding a buffer means telling the GPU which object in video RAM you are working on.
So, by binding a buffer you call some graphic card hardware-coded function.
Now, the graphic card has no visibility on the context of the processes using it:
your graphic card doesn't know that process1 is working on buffer1 and process2 on buffer2.
In other words, the call to glVertexPointer(num,type,offset,NULL) refers to the actually bound buffer.

You cannot draw a buffer while modifing it: you modify it in RAM and then you have to recreate the buffer (guess there are some ways to update it..) in the video RAM. In this case, since you want to do it in multitasking you probably also need some locking system on the video ram buffer... guess opengl provides it.

You're just mixing a little things that happen in CPU/RAM and GPU/VRAM contexts.

Hope it helps...
LLL

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