Technical art · Games

Paint That Squishes

How the paint in Fold & Squish splats, mixes, squishes and sparkles, and how Metal keeps it all running every frame.

Topics: Metal, GPU compute, Paint simulation, Shaders, Rendering
Related work: Fold & Squish, INKS

In Fold & Squish you paint half a butterfly, fold the page shut, give it a rub and peel it open. All the paint is simulated on the GPU using Metal, and this post walks through how that works. It's mostly pictures.

The app icon painting itself. It’s a real print from the app, painted, folded and opened like any other.

Ten years after INKS

In INKS (2016, Apple Design Award) a pinball left a trail of splats behind it. Fold & Squish comes from the same place, but this time the paint is thick and has some weight to it.

A Fold & Squish butterfly print with squished, glossy paint

	INKS · 2016	Fold & Squish · 2026
Ink is	flat, see-through colour	paint with height, gloss and glitter
A pixel holds	colour + wetness	pigment, height, wetness, glitter, a dry layer, drift
It moves by	a fluid sim	thin-film flow, pressure, rubbing, blowing
Built on	Unity render textures	Metal compute, 12+ passes a frame

INKS pushed the phones of 2016 about as far as they would go. Ten years on, Apple's GPUs, Metal compute and unified memory leave room for a lot of things I wanted back then and couldn't fit in.

2026 experiments

Pink ink splats up close: comet tails, spray and rings of satellite droplets — **Thirteen small experiments** came before the app. Each one tested a single idea against a time budget in milliseconds.

A distance field around merged blobs, drawn as contour lines like a map — **Thirteen small experiments** came before the app. Each one tested a single idea against a time budget in milliseconds.

The sheet

Each sheet is a stack of 1024×858 textures. There's wet paint that can still move, dry paint that has soaked into the paper, glitter, and the texture of the paper itself.

Splats

Each splat is built from a few simple shapes: a body, some lobes, little satellite drops that get smaller the further out they land, and stretched spikes. The idea comes from Lei et al. 2013. Adding noise to the edges lets the shapes run into each other the way real paint does.

● 1 body● 3 lobes● 13 satellites● 5 spikes

Kernel by kernel, then settling

Colour mixes like paint

If you blend yellow and blue the way most painting apps do, you get grey. The app stores how much light each pigment soaks up (its absorbance), so yellow and blue make green, like they did in school.

Blending pixels

yellow + blue = grey

Mixing paint (the app)

yellow + blue = green

Crossing strokes mixing into green, orange and purple

Settling

Wet paint flows from where it's thick to where it's thin. Right at the edge, any paint thinner than a set grip height stays where it is. That lets a drop round off and come to rest, and it's how the app fakes surface tension. When you squeeze the paint between the sheets, the grip lets go and it spreads again.

edge grip strongspread 0 texels

Side view of two drops, using the app's real flow rule. The dashed outline is where they started.

With grip: rounds off and stops

Without: keeps creeping

Fold, squish, rub

When you fold, the paint on both halves meets in the middle. When you rub, it squishes out under the paper.

The app on iPad, painting, folding, rubbing and opening a print.

Under the paper, the squished paint follows the texture of the paper, which breaks it up into veins.

On top: what you see

Under the paper

On smooth paper

The same rub, three ways. On top of the folded sheet, then underneath, with and without the paper’s texture.

On iPhone Duo the phone is the paper. Close it and rub the outside: the paint soaks through where you press.

The peel

When you peel the sheets apart, the paint splits between them in streaks that run from thin paint towards thick. That's what gives you the fingers reaching in from every edge.

Which sheet keeps the paint: red right, blue left

The magic straw

Every puff of air breaks up into a few separate streams, and each one drags the thick paint out into a spidery finger.

The air

With streams of air

Without: one smooth tongue

Glitter

The glitter lives in the paint. Every bit of paint remembers where it was first put down and how far it has been pushed since then. Each flake is drawn at that remembered spot and moves with it, so when you smudge glittery paint the flakes stay sharp and travel along with it.

The glitter

What the flakes ride on

Same moment, two views. On the right, a grid drawn where the paint was put down bends wherever the finger pushes it. The flakes ride that grid.

Each flake is a tiny tilted mirror. Move over it (or tilt your phone in the app) to move the light, and the flakes that line up flash.

Drawing it

A single shader draws the paint, working up from its height one layer at a time.

The render, built up step by step: 1 · Flat

Why Metal

All of this runs every frame, and there's still plenty of the frame left over.

GPU time per frame, from Metal’s own timestamps (M2 Max). The frame budget is 16.7 ms.

0.56 ms

GPU per painting frame, 3 flow steps + blur

2.4 ms

heaviest frame: straw + full render, of 16.7

1.4 µs

cost of one more dispatch in an encoder

0.90 ms

to draw the whole sheet at 2853×2007

A few things in Metal made a big difference. I can chain dozens of small compute passes in a frame and each extra one only costs a few microseconds. Function constants let me compile the flow kernel twice, as a full version and a lighter one for older devices. Mipmaps give me cheap blurs for the darker rims and the peel. Storing paint in half floats halves the memory, and because the CPU and GPU share memory, saving a painting just means reading the paint straight back.

Reading

Lei et al. Interactive Physics-based Ink Splattering Art Creation, 2013.
Chu, Tai. MoXi: Real-Time Ink Dispersion in Absorbent Paper, 2005.
Adobe Research. Dripping Thin Films for Real-time Digital Painting, 2026.
Curtis et al. Computer-Generated Watercolor, 1997.
Harris. Fast Fluid Dynamics Simulation on the GPU, GPU Gems.
Splats: the ink behind INKS.