In online procedural generation, content is generated as the game is running on the consumers computer. Our GPU-based Noise Modeler composites noise and other functions through a flow-graph editor similar to the ones used by procedural shader editors and offline terrain generators. Our framework enables non-programmers to edit models for procedural terrain while observing the effect of changes immediately in a real-time preview. Each time a change is made to the model, a corresponding GLSL shader function is automatically generated. The shader is then compiled, and used to render a real-time terrain preview.