Metal · GPU compute · iOS / iPadOS · Research and development
GRAVITAS
Two million stars, real gravity, one finger — running at 120fps on an iPad.
GRAVITAS is a real-time gravity sandbox running on a purpose-built Metal particle engine. Roughly two million stars are generated on near-circular orbits with star-temperature coloring, and every one accelerates toward every attractor through a softened inverse-square force evaluated on a GPU compute shader each frame. Tap to place a black hole, drag to move it, and the disk deforms into streams that dissolve and reform. A built-in recorder captures MP4 clips straight to the iOS share sheet.
- Metal
- GPU compute shaders
- Swift
- iOS / iPadOS
- AVFoundation (recording)
Origin
How it started
I write graphics code that moves absurd numbers of points, which is a hard thing to sell into industrial markets and a genuinely fun thing to hand a person on a couch. The strategic logic was simple: in consumer spectacle the entire value is 'how is this running on a phone', and the output markets itself — a clip of a galaxy collapsing into a black hole is its own advertisement. That suits a solo builder who will not do outbound sales.
Features
What it does
Two million particles under real gravity
Not a particle effect — an actual n-body-style simulation against every attractor, softened and clamped for stability, computed on the GPU each frame at 60 to 120fps with ProMotion handling.
Tap to drop a black hole
Place an attractor anywhere and drag it through the field. Stars tear into gravitational streams that persist, dissolve, and reform. Reset re-bakes the galaxy from scratch.
Built-in recorder and share loop
MP4 capture with a free-tier watermark wired directly to the iOS share sheet — the self-marketing loop is part of the build rather than an afterthought, because for this kind of product the output is the distribution.
Under the hood
Engineering
What is real, stated plainly
The spectacle engine is done and verified running on an iPad: galaxy generation, gravity simulation, black hole placement, additive glow rendering, idle and ProMotion handling, recorder, share sheet, watermark. What does not exist is a game — there is no objective, no stakes, no levels, no win or lose condition, no score, no progression, no app icon, no in-app purchase, and nothing on the App Store. Listing this next to shipped products without that sentence would be dishonest.
A new engine, not the heritage one
This sandbox runs on a new, deliberately simple Metal particle engine written for it — comfortably two million particles, which is far past the point where the effect lands. It is not the separate heritage iOS engine that streams a hundred million LiDAR points via octree and 16-byte compression. That engine is an available upgrade path if the count ever needs to go higher, and conflating the two would overstate what is running here.
The open question is whether it is fun
The whole bet is that bending gravity is enjoyable for thirty seconds, and no amount of engine work answers that. The cheapest proof is one ninety-second level — protect three planets from incoming rogue objects, win or lose — wrapped around the existing sandbox. That is the next build step, and it comes before any art, monetization, or store work.