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UV-ink tracking for CG face replacement
Replacement face systems
Definition
A technique for driving a CG-generated mouth or face replacement on a stop-motion puppet without physically altering or marking it in any way visible to the finished frame: tracking points are drawn in UV-reactive invisible ink, and Dragonframe's multi-exposure capture records a normal "beauty" frame and a UV-lit tracking frame at every position, giving a VFX team a frame-locked motion track to key CG facial elements against.
Overview
For The Girl Who Cried Pearls (2025), directors Chris Lavis and Maciek Szczerbowski needed CG-generated mouth replacements on their stop-motion puppets without compromising the puppets themselves. The visual effects team's usual solutions for tracking a face for CG work -- cutting away the puppet's nose and ears, shrinking the head so it would not be occluded by CG geometry, or adding visible tracking markers -- were all refused by the directors as changes to the puppet's design. The production needed a way to give a CG team an accurate motion track for the mouth without altering, marking, or resculpting the physical puppet at all.
Technical director Eric Pouliot, working in the National Film Board of Canada's animation department, solved it with a material substitution rather than a new tool. Early tests with highlighters and glow-in-the-dark paints failed on the puppets' porous latex skin, which absorbed most inks too quickly, causing them to fade, or would not adhere at all and smeared onto the animators' fingers during handling. The fix that held was the OPTICZ Invisible Ink Pen, a UV-reactive marker designed for security and customs document marking rather than for animation: water-resistant, built for porous and textured surfaces, and invisible under standard lighting. Pouliot marked tracking points directly onto the puppets' latex heads with it; the dots reportedly still glow under UV light years after the shoot.
The on-set capture paired this invisible marking with Dragonframe's multi-exposure feature rather than a separate tracking pass. Each frame was captured twice: once under the crew's normal lighting for the final beauty image, and once under a DMX-automated UV light that revealed the invisible tracking dots. Because the two exposures were synchronized to the same frame inside Dragonframe, the production got a frame-locked tracking reference for every single beauty frame without adding a manual second pass to the animator's day.
The result is a third path alongside the two usual ways of animating a puppet's dialogue: continuously resculpting a single face by hand, or building an inventory of interchangeable pieces (see Replacement face systems). Here the physical puppet head never changes at all -- the performance lives entirely in a CG mouth composited on afterward, tracked from marks the camera could see but the audience never could. It trades a fabrication-heavy pipeline for a capture-and-compositing one, and sits alongside LAIKA's machine-learning roto tool (see Rig removal) as another case of a stop-motion production building a bespoke, invisible-to-camera workflow to move puppet-specific problems into a CG or compositing pipeline rather than solving them entirely on set.
Workflow
- Identify the constraint: a CG or VFX team needs a reliable motion track on a stop-motion puppet's face, but the director will not allow a physical redesign (cut-away features, a resized head, or visible markers) to get it.
- Test candidate marking materials against the puppet's actual skin material before committing -- highlighters and glow-in-the-dark paint were tried and rejected here because the latex surface absorbed or would not hold them.
- Mark the required tracking points with a UV-reactive invisible ink formulated for durability on porous, textured surfaces, choosing a marker built for that job (security/document marking) over an animation-specific product if none exists.
- Rig a DMX-controlled UV light and synchronize it to the capture software's multi-exposure feature so the tracking pass and the beauty pass are captured automatically at every frame, not as a separate manual pass.
- Hand the UV-exposure frame sequence to the VFX team as the motion-tracking reference for the CG element; the beauty-exposure sequence remains the plate that element is composited onto.