add rendering notes (#1679)

* add rendering notes

Signed-off-by: Nick Porcino <meshula@hotmail.com>

---------

Signed-off-by: Nick Porcino <meshula@hotmail.com>

docs/tutorials/otio-timeline-structure.md

@@ -5,6 +5,26 @@ clips, gaps, and transitions. This document is meant to clarify how these
 objects nest within each other, and how they work together to represent an
 audio/video timeline.
 
+## Rendering
+
+Rendering of the image tracks in a timeline is done in painter order. The layers
+in a stack are iterated from the bottom (the first entry in the stack) towards
+the top (the final entry in the stack). Images in a stack overlay lower images
+using an alpha composite operation respecting any alpha in the source materials.
+All compositing is assumed to occur over a background of zero values in color components, and 100% values for alpha components.
+Within a track, clips may overlap via a transition. In that case, the 
+contribution of track is the linear blend of the elements joined by the
+transition.
+
+If there are effects on a clip, OpenTimelineIO does not say anything about the
+impact of the effect and deviation from the base behavior is application
+specific.
+
+Rendering of the audio tracks is additive. It is strongly advised, but not
+required, that the summed audio is summed as floating point, and that it is 
+processed through a compression filter in order to prevent clipping and 
+distortion. 
+
 ## Simple Cut List
 
 Let’s start with a simple cut list of a few clips. This is stored as a