Real Actual Magic
Film, VFX, and Mary Poppins
Magic
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Oxford:
- n. "a special quality or ability that somebody/something has, that seems too wonderful to be real"
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)
This is analog tech
- Remember: Mary Poppins was filmed in the 60s
- How do you do VFX?
- What even is film, anyway?
It's Chemicals™
- B/W: intensity of light hitting film
- Parts that were exposed more end up darker (a negative)
- Negative is used to expose a second copy, making a positive
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?
- Depends on the film
- Black-and-white film originally was most sensitive to blue/green
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
- e.g. 17 minutes in the middle of Phantom
Modern colour: Three-Strip Technicolor (Process 4)
- To capture a much closer* colour representation, three frames of film exposed at once: red, green, blue
- *still fine-tuned for good skin tones on white actors mostly
- Red filtered light printed in cyan, green in magenta, blue in yellow
- Used in Wizard of Oz (1939) through to The Godfather Part II (1974)
So, you want to put a character on a different background?
Option 1: Rear Projection
- e.g. 2001: A Space Odyssey (1968)
- Place a screen behind actors, light it from behind with the background you want
- Transparent stuff works! Reflections work!
Option 1: Rear Projection
- Modern updates: LED stage
- r e a l t i m e r e n d e r i n g
Option 2: In-Camera Double Exposure
- Block out the light from one area while filming your foreground
- Block out the opposite area and film again over the same frames to fill the background
- This is masking, a key concept
- Used as early as A Trip to the Moon (1902)
Option 3: Printed Double Exposure
- Film foreground/background separately
- Combine them using masks using a special compositing printer
- Key benefit: You can make your mask film however you want!
- But how?
Option 3a: Rotoscoping
- Get someone to trace over the outlines of shapes manually
- This takes a lot of work
- We still do this! (But mostly to clean up other techniques)
Option 3c: Single-Channel Luma Matte
- Idea: use just one colour channel for the luma matte
- Problem: white things appear on all channels
- We really want things that appear in one channel but no others to disappear
Wait, no transparency? No fine edges?
RGB channels are not good enough
- RGB channels capture too many frequencies
- Objects are visible to some extent in every channel
How would we do that?
- Pack a fourth reel of film in the camera
- Split just that narrow frequency band onto it
- Light your backdrop with just that band
- Use this channel directly as a matte
The Sodium Vapour Process: Luma Matte on Y Channel
- Sodium vapour lights emit at a narrow yellow band
- That's why street lamps look so yellow
- Light the background with those
- This won Mary Poppins a VFX oscar
How would we do that?
- ✅ Pack a fourth reel of film in the camera
- 🤔 Split just that narrow frequency band onto it
- ✅ Light your backdrop with just that band
- ✅ 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
- Uses a prism with thin-film interference to be a highpass or lowpass filter for a target frequency ("dichroic" beam splitting)
- Combine a highpass and a lowpass to create a bandpass splitter
- Hard part: precision manufacturing to get the frequencies Just Right
And here's the magic
- They didn't figure out how to manufacture this reliably
- They managed to manufacture exactly one such prism
- Every film using the Sodium Vapour Process was made using the same camera
Every Sodium Vapour Process film after Mary Poppins
- The Birds (1963)
- Bedknobs and Broomsticks (1971)
- Island at the Top of the World (1974)
- The Apple Dumpling Gang (1975)
- Escape to Witch Mountain (1975)
- Gus (1976)
- Pete's Dragon (1977)
- ...
- Dick Tracy (1990)
Other lost technologies
- Roman concrete
- Damascus steel
- Saturn V rocket
Why haven't we re-invented this?
- Good dichroic filters are expensive
- We got better at analog greeenscreens (more careful chemical processes)
- We can edit digitally now
- Accuracy of chemical processes no longer a factor
- Manual cleanup is suddenly easier
- We can use ✨machine learning✨
tl;dr
- We can get good results cheaply without Sodium Vapour
- But it's not as clean a process