OpenGL: How to update only a portion of a texture?

I dont want to update the whole texture every time i change a small p开发者_开发知识库art of it, what is the command for this?

And when i have mipmapping on, set with GL_GENERATE_MIPMAP, how optimized is that internally? will it calculate whole image again, or just the part i updated?

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glTexSubImage2D.

Mipmap generation process is implementation dependant, it probably will update the whole mipmap chain.