CG Compositing Series - 2.5 Material AOVs - Refractions & Reflections

In the first half of the video we are understanding the problem with refraction (transmission) and reflections (indirect specular) and the last half of the video we explore potential solutions.

The problem with Indirect Specular (Mirror Reflections) and Transmission (or Refraction) passes is they reflect or refract the beauty of the environment. There often seems there is not much we can do as compositors to separate those passes further.

We define Transparency, Transmission, Refraction, and Translucency. We dive deep into the science of Refractions and what causes light to bend and redirect, and how that can help us understand what the computer is mimicking.

We analyse the similarities between Refraction (Transmission) and Specular, why they are broken up into their own categories, and how the computer handles the Reflective or Transmissive Events. As well as how both are showing us "virtual images" of the surrounding environments

We investigate why most of the Transmission (Refraction) pass ends up in the Indirect section, unlike Specular pass where information is more evenly divided between direct and indirect.

We go over how we can incorporate the Transmission (Refraction) pass into our Material AOV Rebuild templates

We explore some Comp Tricks for Inverting / Mirroring Cameras to help with Planar Reflections (like ground plane reflection renders) including introducing my MirrorDimension Gizmo to make flipping the camera along a mirror plane easier. We also explore some ways to fake reflection passes in Nuke if you are in a pinch with Projection Geo or Position Point Clouds.

The last part of the video is a recording with Julius Ihle, Head of Lighting and LookDev at Trixter and a friend / colleague. Julius explains lighting solutions to the Transmission and Reflections limitations using Light Path Expressions ( LPE ) and Open Shading Language ( OSL ) where Julius has written his own reflection shader.

Please Check out my Blog Post on this Subject for References, Images, Renders, Gizmos, and Work Files:
https://compositingmentor.com/2024/04...

MirrorDimension gizmo:
https://github.com/CreativeLyons/Nuke...

Special Thanks to Adrian Pueyo and Julius Ihle for taking time, advising me, and providing renders!

Julius Ihle's Blog:
https://julius-ihle.de/?page_id=346

LPE Quick Tip #1: Light Path Splitting for Transmission
https://julius-ihle.de/?p=2619

Playing with OSL #5: Arnold Reflection Alpha + Utilities
https://julius-ihle.de/?p=2788

Timestamps:

00:00:00 - Intro
00:01:41 - Identifying Problems with Reflection and Refraction Passes
00:00:24 - Terminology
00:08:44 - What is Transparency?
00:09:31 - What is Transmission?
00:10:11 - What is Refraction?
00:11:44 - Refraction vs Transmission
00:12:52 - What is Light Redirected during Refraction?
00:13:27 - Light Wave Turning
00:14:32 - Color Spectrum Refraction
00:15:34 - Index of Refraction
00:16:50 - Light Re-Entering Original Medium
00:17:58 - Angle of Refraction
00:20:10 - Chromatic Aberration
00:20:26 - Caustics
00:21:06 - What is Translucency?
00:21:42 - Recap
00:22:06 - Similarities Between Reflection and Refraction
00:23:39 - Concave / Convex Reflections
00:25:10 - Concave / Convex Refractions
00:27:01 - There is No Spoon
00:28:05 - There is No Glass
00:29:05 - Bounce off vs Pass Through
00:29:12 - Transmission gets it’s Own Category
00:30:26 - Exploring Sub Categories of Refraction
00:31:51 - Why Everything Ends Up in Indirect
00:39:30 - Reducing Transmission Section
00:40:15 - Roughness makes Separation Redundant
00:41:22 - Recap
00:42:36 - Demonstrating Similarities of Specular and Transmission
00:44:03 - Adding Transmission into Our Rebuild Templates
00:48:41 - What Can We Do as Compositors?
00:49:53 - Solutions for Planar Reflections
00:52:01 - Mirroring the Camera for Ground Plane Reflections
00:56:25 - Mirror Dimension Tool
00:59:55 - Faking Reflections in Nuke with Geometry Projections
01:01:38 - Faking Reflections with Position to Points
01:04:03 - Mirror Dimension Tool works with Geo or Point Cloud
01:04:41 - Limitation to Comp Tricks
01:05:01 - Light Path Expression Render Examples
01:06:14 - Introducing Julius Ihle - Head of Lighting / LookDev
01:07:00 - Julius Ihle - How Renderers Handle Light Path Events
01:07:00 - Julius Ihle - Light Path Expressions Demo
01:27:21 - Julius Ihle - Solutions for Reflection Renders
01:42:05 - Outro