"Step-Printing" Test • Wong Kar-Wai Image Smearing
Several months ago I was asked to digitally simulate the Step-Printing effect made famous by Wong Kar-Wai in his early films such as Chungking Express, and Happy Together. A video compilation of this effect in his movies can be found HERE.
The Director felt this effect would visually portray the character as a fish out-of-water; lonely in a crowded landscape. To help educate our creative decision process, I devised this test comparing several frame rates and shutter angles which directly influence the amount of smearing and strobing in the image sequence.
camera • ARRI ALEXA MINI
codec • PRORES 4:4:4:4
resolution • 4:3 2.8K: 2880x2160
lens • CANON K-35
Shot 1: 12fps - 180⁰ shutter (aka 1/48 shutter speed)
Shot 2: 12fps - 360⁰ shutter (aka 1/24 shutter speed)
*shots placed in 24fps timeline and slowed down 50% to achieve “normal” speed (frame sample on)
Shot 3: 6fps - 180⁰ shutter (aka 1/48 shutter speed)
Shot 4: 6fps - 360⁰ shutter (aka 1/24 shutter speed)
*shots placed in 24fps timeline and slowed down 25% to achieve “normal” speed (frame sample on)
Shot 5: 3fps - 180⁰ shutter (aka 1/48 shutter speed)
Shot 6: 3fps - 360⁰ shutter (aka 1/24 shutter speed)
*shots placed in 24fps timeline and slowed down 12.5% to achieve “normal” speed (frame sample on)
What is Step-Printing ?
Step-Printing is basically the duplication of a film frame. When applied to a frame sequence the duplication of multiple film frames stretches the running time of the sequence which creates a sense of slow-motion. Traditionally Step-Printing was achieved with an optical printer which had the ability to print each motion picture frame individually. A famous example of this was Tarantino’s opening to Reservoir Dogs. Notice how duplicating the film frames seemingly slows down the motion.
When filming a standard shot, the motion picture camera captures the scene at 24 frames-per-second (fps). And the resulting film frame sequence looks like this:
When Step-Printing, those same film frames are individually duplicated to slow down their perceived movement. And the resulting film frame sequence looks like this:
Using this technique, a Director could duplicate each film frame as many times as necessary to achieve the desired amount of slow-motion. In the above opening sequence from Reservoir Dogs the individual film frames were in fact tripled.
A noticeable side effect of the Step-Printing process is the strobing or jitteriness in movement. This strobing effect becomes heavier the more each film frame is duplicated in the sequence. Step-Printing and it’s strobing effect are not to be confused with the more customarily used technique of smoother slow-motion filming achieved by over-cranking (filming faster than 24fps).
How is smearing motion achieved ?
Wong Kar-Wai added one more technique before Step-Printing his image sequences.
To create the smearing effect one has to think in terms of a still photographer. The amount of time a shutter is open directly effects the amount of smear created by movement in the frame.
When filming at 24fps, the shutter is open for 1/48 of a second. This creates the “normal” motion blur we are familiar with in movies. If one were to under-crank at 12fps this leaves the shutter open for 1/24 of a second. That means the image is exposed twice the amount of time resulting in smeared object movement. Now consider filming at 6fps or 3 fps; notice how lowering the frame rate increases the smearing of movement.
So lets film a shot under-cranked at 6fps. Not only is there smeared movement, but it will also look sped up. The film image sequence looks like this:
To slow it down to “normal” 24fps speed, utilize Step-Printing to quadruple each film frame. The film image sequence becomes this:
And that’s how Wong Kar-Wai created his signature smeared imagery. He combined the techniques of Step-Printing and under-cranking. By combining film frame duplication and various frame-rates, one can create infinite variations within this look.
How does Step-Printing apply to Digital Image capture ?
In this particular test I was asked to simulate this film effect using a digital camera. Under-cranking the camera is straightforward with the Arri Alexa. It allows one to shoot at lower frame rates. Starting from 24fps, I chose to test at 12, 6, and 3 as they were halves of each other.
Because the Arri Alexa can also adjust shutter angle, I chose to shoot each frame rate at standard 180⁰ shutter (aka 1/48 shutter speed), and an open 360⁰ shutter (aka 1/24 shutter speed) to see how that might add to the effect.
Once each test shot was completed, I imported the individual files into Premiere and created a 24fps timeline. Here is where the digital version of Step-Printing comes into play - for the shots captured at 12fps I slowed down the footage 50% to achieve “normal” speed. In essence this digitally duplicated each frame. For the shots captured at 6fps each frame quadrupled when slowed down to 25% for “normal” speed. And the shots captured at 3fps; octupled at 12.5% for “normal” speed.
*** When changing speeds in the timeline, Premiere gives the option to use frame blending or frame sampling. I chose to frame sample***
In each shot I wanted to showcase the differences in smear and strobe between two characters who were instructed to move at different speeds. The man is walking at hurried pace. Notice the smearing of his movement increases when shooting at a lower frame rate. The woman was instructed to minimize her motions and move slowly as she tries to hand out fliers. The smearing is much less noticeable.
Ultimately, which fps and shutter angle to use relies solely on individual preference and the mood you are wanting to create for your story. I hope this test helps guide you in your creative endeavors.