Instead of rendering/drawing/painting directly to the screen, the example code uses a technique called "render to texture". In order to render to a texture, you'll need to set up a framebuffer and bind a texture to it. Multiple textures can be bound to a single framebuffer.
The textures bound to the framebuffer hold the vector(s) returned by the fragment shader. Typically these vectors are color vectors (r, g, b, a) but they could also be position or normal vectors (x, y, z, w). For each bound texture, the fragment shader can output a different vector. For example you could output a vertex's position and normal in a single pass.
Most of the example code dealing with Panda3D involves setting up framebuffer textures. To keep things straightforward, nearly all of the fragment shaders in the example code have only one output. However, you'll want to output as much as you can each render pass to keep your frames per second (FPS) high.
There are two framebuffer texture setups found in the example code.
The first setup renders the mill scene into a framebuffer texture using a variety of vertex and fragment shaders. This setup will go through each of the mill scene's vertexes and corresponding fragments.
In this setup, the example code performs the following.
The second setup is an orthographic camera pointed at a screen-shaped rectangle. This setup will go through just the four vertexes and their corresponding fragments.
In this second setup, the example code performs the following.
I like to think of this second setup as using layers in GIMP, Krita, or Inkscape.
In the example code, you can see the output of a particular framebuffer texture by using the Tab key or the Shift+Tab keys.
(C) 2019 David Lettier
lettier.com