Every shape
is real.

Hola PDF, in the browser

Hola PDF on the web

holapdf.app does the PDF work on any computer, with nothing to install: merge, split, organize, sign, fill in forms, compress, and turn images into PDFs and back. Hola PDF Pro on the web, US$20 once, adds redaction, editing text, comparing versions, OCR and passwords.

Sharp, in the browser

Sharp on the web

sharpmerge.com puts several shots of the same scene together into one, on any computer, with nothing to install: focus stacks sharp from front to back, bursts averaged clean, passers-by taken out, star trails kept, exposure brackets blended, panoramas stitched, and the sharpest shot of a burst picked — from photos, RAW files or a video's frames. Sharp Pro for the web, US$10 once, lifts the 12-frame limit, adds the retouch brush and 16-bit TIFFs, and has tools of its own: a long exposure from a burst, a motion sequence, a night sky over a sharp landscape, super-resolution, a time-slice from day into night, and cinemagraphs.

Mathematics and physics, on the web

ConvergerLab

Twenty courses in the browser, from geometry and calculus to complex analysis. Every lesson has a lab beside it: drag a point, slide a parameter, predict what happens, then check. Answers are graded as mathematics, and practice comes back when it is due.

Shaders, a playground and a Mac app

Ray Kiln

A new Metal shader in every episode. A short video on its page builds it from nothing, beside the full source, every parameter explained, and the sound it makes, which the shader generates too.

The playground runs every episode in the browser to change and share, with exercises that ask you to rebuild each stage by eye; the gallery keeps what people make. The free Mac app turns a shader into a video, a still or a loop, and runs SwiftUI's shader effects ready for Xcode. Ray Kiln Plus, US$10 once, adds ProRes and transparent video, a live wallpaper and screen saver, sound and MIDI, keyframes, your own camera and pictures as inputs, snapshots of your work, a performance mode for a screen on stage, performance hints in the editor as you type, and GPU frame analysis.

Ray Kiln, for Metal developers

The shader linter

Paste a Metal shader and it shows what costs it time on Apple GPUs: half math that a float turns into 32-bit math, discards and dependent texture reads, square roots nothing needs, integer division, each on its line with the fix. It is free and runs in the browser, so the shader never leaves the page, and every episode opens in it.

With Ray Kiln Plus, the Mac app runs the same rules as you type, marking each line in its editor. GPU frame analysis goes further: it renders a frame of your shader and reads it with Apple's own tools, for its GPU time, what limits it and its registers. It also checks a frame of your own Metal app for loads, stores and passes it doesn't need, each with the change in Swift. It needs macOS 27.

Hindcast, in the browser

Hindcast on the web

hindcastweather.com blends seven forecast models, says how much they agree, and checks every forecast against what the nearest weather station actually measured, so each place has a track record. Free, with US radar and alerts. Hindcast Pro, US$9.99 a year (or US$1.99 a month or US$19.99 once), adds every model's own line, how each day's forecast changed as it approached, an almanac, trip forecasts, and satellite and forecast maps, and the same account signs in the Hindcast app.