Perhaps I am trying to use XNA in a way that it was not designed, but I would very much like to be able to draw my own primitives and save them as a bitmap, or a texture2D, or anything that will hold a few 2d lines for me. My goal is to program a game where the textures are created procedurally, instead of just being loaded from the content manager.
If it helps to know, I've made a Shape class that stores polygons, and I probably could just tell spriteBatch to draw each line out, but I'm trying to optimize by storing commonly used shapes somewhere, instead.
You can use a RenderTarget for your task. Something like this:
// var to store your drawing
Texture2D newShape;
// drawing will be on this target
RenderTarget2D rt = new RenderTarget2D(GraphicsDevice, width, height);
SpriteBatch sb = new SpriteBatch(GraphicsDevice);
// set to render all to render target
GraphicsDevice.SetRenderTarget(rt);
GraphicsDevice.Clear(Color.Transparent);
sb.Begin();
// Draw what you want here.
sb.End()
// Return to drawing on "main" buffer
GraphicsDevice.SetRenderTarget(null);
// Save the texture you just drawn
newShape = rt;