CODEWITHSUNDEEP
openglopengl-es
J.Doe
J.Doe

Reputation: 1552

glBufferData set to null for constantly changing vbo

I have a huge vbo, and the entire thing changes every frame.

I have heard of different methods of quickly changing buffer data, however only one of them seems like a good idea for my program. However I dont understand it and cant find any code samples for it.

I have heard people claim that you should call glBufferData with "null" as the data then fill it with your real data each frame. What is the goal of this? What does this look like in code?

Upvotes: 1

Views: 2118

Answers (1)

Dietrich Epp
Dietrich Epp

Reputation: 213338

It's all in the docs.

https://www.opengl.org/sdk/docs/man/html/glBufferData.xhtml

If you pass NULL to glBufferData(), it looks something like this:

int bufferSize = ...;
glBufferData(GL_ARRAY_BUFFER, bufferSize, NULL, GL_DYNAMIC_DRAW);
void *ptr = glMapBuffer(GL_ARRAY_BUFFER, GL_WRITE_ONLY);
...

Ignore most of that function call, the only two important parts are bufferSize and NULL. This tells OpenGL that the buffer has size bufferSize and the contents are uninitialized / undefined. In practice, this means that OpenGL is free to continue using any previous data in the buffer as long as it needs to. For example, a previous draw call using the buffer may not have finished yet, and using glBufferData() allows you to get a new piece of memory for the buffer instead of waiting for the implementation to finish using the old piece of memory.

This is an old technique and it works fairly well. There are a couple other common techniques. One such technique is to double buffer, and switch between two VBOs every frame. A more sophisticated technique is to use a persistent buffer mapping, but this requires you to manage memory fences yourself in order for it to work correctly.

Note that if you are uploading data with glBufferData() anyway, then calling glBufferData() beforehand with NULL doesn't actually accomplish anything.

Upvotes: 7

Related Questions