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c++algorithmopengldata-structuresopengl-3

VBO/VAO memory management practices when dealing with creation/destruction of possibly dependent vertices

I'm messing with a simple concept of creating 2D walls with C++/OpenGL. Each wall is described by 4 vertices and either extends horizontally or vertically, and also has a color. Walls interlink if they collide. Perimeters formed by walls are filled by simple rectangles.

original

My idea would be to store many walls' vertex data in a single VBO. Or multiple for each attribute then place in a VAO? Interleave in one VBO and access via VAO? Regardless, many walls in one object. If whatever I'm storing in runs out of space, make another VBO (or whatever I'm using) to store another group of walls. I also wanted to have walls that meet, such as at a right angle, share vertices to save space.

However, the walls are mutable, so vertices can be added, moved, or removed at any time. This would require keeping track of the free blocks in the VBO and then allocating wall data to them as needed. It would also mean that if I have multiple VBOs, 2 walls that share vertices must be in the same VBO, possibly introducing all sorts of headaches. But at the same time, I don't want to make a VBO for every single wall, which is nothing but a very simple 4 vertex thing at most.

Example of some alterations occurring to the original image: altered

How is this situation typically handled? Does it require rolling my own memory allocation scheme to work with the VBOs, or is there a suitably fast solution that doesn't require so much work?

Solution

  • Why do you think that a VBO could possibly run out of space? I mean, for most GPUs the maximum size of a VBO is of the order of 16-32-64Mb, that's a lot! (check also here)

    If you can fit all your datas in a single VBO the problem is much simpler. I'd just re-buffer all your datas when they need to be changed (and that, I believe, wouldn't happen often, unless you want to add/remove a wall every frame).

    If this is not accetable in your case, I suggest you to check out some popular implementation from open source codes, here for example there is osg's GLBufferObject.

    Take a look at the GLBufferObject::compileBuffer() function; in your case each osg::BufferEntry can be a group of connected walls. To render the scene you will have to query this class to get the offsets of each data.

    Hope this helps.