Shiver Development: Shaders in PyOpenGL

After a turbulent weekend trying to get shaders working with PyOpenGL - I finally got my first vertex and fragment files compiled and working inside Shiver. Without any documentation to bail me out of my usual jams with new code - I hacked away through the weekend until I stumbled upon a way of changing C-type function arguments, then converted variables into C-compliant data to be fed into the altered function in question. In English: I needed to convert data so it could be read and processed correctly.


Fig 1.1: Globe with atmospheric shader applied

So - in the lab I am often asked: "why are shaders important"? Shaders can provide surface appearance variance in regard to color, opacity, reflection, refraction, etc... even physical alterations to the geometry itself. In relation to what I am attempting to achieve - it is vital to be able to replicate light and physical phenomena that exists when viewing the earth from space.

My first goal was to get a representation of the atmospheric layer that wraps around the earth - a thin haze of a light blue hue. The trick is - the haze should not rotate along with the earth as the camera moves around the globe. To achieve this; a shader was created that calculates how light is hitting the surface at each given frame - then simply finds where the light drop-off occurs on the surface (the edges) and colors those areas a varying hue of blue based on how much light exists on a given fragment of the geometry. Since I applied the shader on a sphere and the lighting is both hitting the sphere straight on and is static in all attributes over time - we can assume perfect symmetry in the light distribution across the sphere at any given time, which allows for the Lambertian Reflection calulation of:



Where:
I0 = reflected intensity
Ld = light diffuse color,
Md = material diffuse coefficient

Once we have the reflected intensity of each fragment, simple logic can be installed within the shader that only allows for color to be applied a certain areas of intensity - and, since we are dealing with a sphere with light shooting right at it (symmetrical light reflection!) - it becomes simple to create a halo light effect.


Fig 1.2: Closer look of the glow-like effect of the atmospheric shader

The next steps with using shaders are to create a fog shader for the globe, as well as the potential for a very subtle bump-map, as well as other effects as time permits.