This box makes it easy to use start/end/current frame values 
from a rib file, returning a value between 0 and 1. 
0 returns when current frame is start frame, 1 when it's end frame.
If the current frame is outside the start/end frame, values will
fall outside of the 0 to 1 range. This is not recommended, but possible.

This is the start frame for the frame range.
This value should be less than the end frame, but
CAN be greater than the start frame for rendering frames in reverse.

This is the end frame for the frame range.
This value should be greater than the start frame, but
CAN be less than the start frame for rendering in reverse.

This is the 'current' frame. It SHOULD be within the range specified 
by start and end, for a predictable output of 0.0 - 1.0. 
Use the slider value for testing, else make it an argument to the shader
to make the value actually follow the values from the renderer.

Returns a fractional value between 0.0 and 1.0 representing the
current frame within the start/end frame range.
OUT is 0.0 when frame is the start frame, 1.0 when it's the end frame.
If the current frame falls outside the start/end frame range, OUT will
fall outside of the 0.0 to 1.0 range.