What are troughs and ridges?

[@paulbutler3638]

I learned a lot from this video - very well explained. One comment about the presentation: contrast is important when reading text. One segment of the presentation had black text over a dark purple background which made it difficult to read. Two solutions: 1) use light backgrounds with black text 2) use dark backgrounds with white text. Thanks...

[@ephraimbiblow7945]

This is very well-explained. We learned a lot. G&G