Real Actual Magic

Film, VFX, and Mary Poppins

Magic

Mary Poppins (1964)

  • "In turn of the century London, a magical nanny employs music and adventure to help two neglected children become closer to their father." (IMDb)

Motivation

First: some background

This is analog tech

It's Chemicals™

What is visible light?

  • Electromagnetic radiation
  • "Visible" is a specific range of wavelengths
  • Rods are mostly for low-light sensing; cones are for colour imaging
  • Different cone types respond to different frequencies

So colour isn't real, actually

  • We perceive a continuous spectrum of colour, but we don't have perfect sensors
  • Hues perceived based on the inputs from these three types of cones
    • We don't have a special yellow sensor; we perceive yellow when green/red sensors are both active
    • "Fuchsia" doesn't have a frequency

So what does film see?

So, you want colour film?

Fake colour: Tinted B/W

  • e.g. most of Phantom of the Opera (1925)
  • B/W film dyed a different hue
  • Tinting: White is not preserved; black is

Early colour: Technicolor Process 1

  • Only one film: The Gulf Between (1917)
  • Two frames of film exposed at once: one with a green filter, one red
  • The positives of each are tinted to the colours the filters let through
  • Both frames are projected at once, combining colours additively
    • Start from black → combine to get white

Early colour: Technicolor Process 1

  • Doesn't accurately reproduce blue/green components

Early colour: Technicolor Process 2 and 3

  • Toning: White is preserved, black is not preserved
  • The positives of each channel are toned to opposites of filters
  • Toned positives are combined subtractively onto a single frame of film
    • Start with white → subtract to get black
  • It's just one frame! We can hire any random teenager to project this!

Early colour: Technicolor Process 2 and 3

Modern colour: Three-Strip Technicolor (Process 4)

So, you want to put a character on a different background?

Option 1: Rear Projection

Option 1: Rear Projection

Option 2: In-Camera Double Exposure

Option 3: Printed Double Exposure

Option 3a: Rotoscoping

Option 3b: Luma Matte

  • Idea: adjust the exposure of your film until bright parts become pure white and dark parts pure black
  • Basically: thresholding
  • Not very good at separating foreground/background usually
a: b:

Option 3c: Single-Channel Luma Matte

Option 3d: Chroma Matte

  • Keep areas where the other channels are stronger than the matte channel
  • Subtract matte channel from other channels
    • e.g. for a greenscreen: a = r + b - g > 0
    • + implies mixing light; - implies mixing dye
  • Can basically only key out green or blue; too much red in skin tones
  • Getting a clear edge or transparency is hard (have to remove green bleeding from edges after)
a: b:

Wait, no transparency? No fine edges?

How??

RGB channels are not good enough

What if...

How would we do that?

  1. Pack a fourth reel of film in the camera
  2. Split just that narrow frequency band onto it
  3. Light your backdrop with just that band
  4. Use this channel directly as a matte

The Sodium Vapour Process: Luma Matte on Y Channel

But where's the magic?

How would we do that?

  1. ✅ Pack a fourth reel of film in the camera
  2. 🤔 Split just that narrow frequency band onto it
  3. ✅ Light your backdrop with just that band
  4. ✅ Use this channel directly as a matte
    • Without (2), you get the same problem of the colour bleeding into other channels

Missing link: the beam splitter

And here's the magic

Every Sodium Vapour Process film after Mary Poppins

Other lost technologies

Why haven't we re-invented this?

tl;dr

Thanks