2026 · Case study
Fold & Squish
Paint one side. Squish the other.
Visit project- Contribution
- Design, paint simulation, Metal rendering and app development
- With
- Fountain Interactive
- Year
- 2026
- Built with
- Swift · SwiftUI · Metal · Metal compute · Apple Pencil · iPhone Duo
The project
Butterfly painting with real-feeling paint, simulated on the GPU with Metal for iPhone, iPad and iPhone Duo.
The folded paper painting from school, made digital. Paint on one half of the page, fold it shut, give it a rub, then peel it open. The paint has squished into a mirror print, and no two come out the same.
The paint is thick, wet and glossy, and it spreads when you press it. There's a brush, splats, drips, a finger to smudge with and a magic straw. It's easy for small hands and oddly calming for big ones, with no ads and no tracking.
Coming soon to the App Store for iPhone, iPad and iPhone Duo.
01 / Product
A butterfly every time
Every print is different, because the paint really moves. Even the app icon is a real print from the app, painted stroke by stroke, folded and opened exactly as a player would make it.
02 / iPhone Duo
The phone is the paper
Paint on the open screen, then close the iPhone Duo for real and the painting folds shut with it. Rub the outside screen to squish the paint, open the phone again, and there's your print. Colours mix where they touch, and the paper leaves fine veins in the paint as it peels apart.
03 / iPhone and iPad
A Fold button for every other device
On any iPhone or iPad, the Fold button folds the page for you, and the squish works just the same. Smudge the paint with a finger, or paint with Apple Pencil on iPad.
04 / Technical art
Ink physics
All the paint is simulated on the GPU using Metal compute. Each sheet is a stack of half-float textures holding the wet paint and its height, paint that has soaked into the paper, glitter, and the texture of the paper. Pigment is stored as absorbance, so yellow and blue make green. The paint flows and settles into rounded drops, pressing and rubbing squeeze it out along the grain of the paper, and peeling splits it between the two sheets in veins.
It carries on from the ink in INKS ten years ago. Even with dozens of compute passes and the full render, a frame takes a few milliseconds.
I've written up the details in a blog post, with live demos of the colour mixing, the settling and the glitter.
05 / Product
Glitter, a magic straw and a wall for every painting
Start from faint pencil outlines or a blank page. Paintings save as you go and get taped up on your own gallery wall.
It's free to try, with one optional purchase. The free version has every tool and keeps six paintings. The Sparkle Pack adds glitter, rainbow paint, every pencil guide and room for as many paintings as you like. No ads and no subscription.
