Is it possible to in-place resize VBOs?

Question

The title says everything, but just to be clear I'll add some extra words.

In this case, resize means:

• getting more storage space at the end of the old vbo
• saving the old data at the front
• (hopefully not copying, but at least not on CPU side, meaning the driver should handle this)

EDIT

As to explain some more details and justify my question:
I will store data of (in forehand) unknown size to the VBO but I only know an upper limit that is a very rough estimation (10 - 100x as much or even more in unusual conditions).

Of course I know how much data I stored, when I am done with it, so it would be nice to store data until I find my VBO too small and resize it and then go on storing.

Here is why I don't want to copy(especially not on CPU side):
I am doing all this on the GPU to get interactive frame rates. When I have to copy it is very slow or even not possible, because there is not enough space. Worst of all is to copy the data over the CPU, hence passing everything over the bus, into a new memory region that has sufficient size, then glBufferDataing the VBO with new size and the new memory region as source. That would be the performance killer.

circumvented

I circumvented the problem with an exact estimation of the needed space. But I will let this question be unanswered for a week to see if someone has another hint on this as I am not very happy with the solution.

Answer 1

I think without doing a copy you won't get around this, because the only way to resize a buffer is to call glBufferData and there is IMO no way to tell the driver to keep the old data.

What you probably can do to at least not copy it to the CPU and back again, is creating some kind of auxiliary VBO for these purposes and copy directly from the VBO into the auxiliary VBO (using the ARB_copy_buffer extension), resize the VBO and copy its contents back.

But I think the best way is just to allocate a larger buffer beforehand, so the resize is not neccessary, but of course in this case you need to know approximately how much extra storage you need.

Answer 2

Revisiting this question after some years, the landscape has changed a bit with newer versions and extensions.

GPU Side Copying

The extension mentioned in Christian Rau's answer is core since 3.1 which allows us to copy contents (via glCopyBufferSubData) from one VBO to another. Hopefully, the driver does this on the GPU side!

Using this function we could create a larger buffer and copy the leading data over. This has the disadvantage of doubling the memory requirements because we need both buffers.

True resizing

The good news is: With sparse buffers an even better solution is on the horizon.

Given this extension we can allocate a virtual buffer with more than enough space for our data without ever paying for the unneeded space. We only have to "commit" the regions of memory we physically want to store data in. This means we can "grow" the VBO by committing new pages at the end of it.

The bad news is: As of the current OpenGL version (4.5) this is still an extension and not yet core, so adopting this might not be possible. You should also not that there are some details in the specification that are not yet worked out. For example, mapping of sparse buffers is disallowed in the current extension but support might be added in future versions.

I would be keen to hear about the availability of this extension if you have any data on that.