Codimate

Turn a running algorithm into an explainer video.

You write your algorithm as normal Python and say what each step looks like. Codimate works out the motion, the timing, and every frame.

import codimate as cm

@cm.trace()
def bubble_sort(values):
    for i in range(len(values)):
        for j in range(len(values) - 1 - i):
            cm.emit("compare", items=[values[j], values[j + 1]])
            if values[j] > values[j + 1]:
                values[j], values[j + 1] = values[j + 1], values[j]
                cm.emit("swap", items=[values[j], values[j + 1]])

def bars(frame):
    scene = cm.Scene()
    active = frame.items()

    for slot, item in cm.row(frame.state, gap=40):
        bar = scene.group(item.id, slot)
        bar.rect("bar", h=item.value * 70, at=cm.at(bottom=0)) \
           .fill("orange" if item in active else "blue")
        bar.text("label", item.value, size=32, at=cm.at(top=20))

    return scene

cm.explain(
    trace=bubble_sort(cm.items([3, 1, 4, 2])),
    view=bars,
    motion=[cm.Rule("*", position="lift_carry_drop", clearance=90)],
    timing=cm.Timing(default=0.55, events={"swap": 0.9}),
).render("results/bubble_sort.mp4")

That is the whole program. Run it, get results/bubble_sort.mp4.

Install

pip install codimate

Codimate is a Rust engine with a Python front end, but the wheels are prebuilt, so there is no Rust toolchain to install. One wheel covers every Python from 3.9 up, on Linux, macOS and Windows.

Working on the engine

Only if you are changing the Rust, rather than using Codimate:

git clone https://github.com/darhnoel/codimate && cd codimate
python3 -m venv .venv && .venv/bin/pip install maturin
.venv/bin/maturin develop --release
.venv/bin/python python/examples/bubble_sort/main.py

Use --release. Without it the Rust engine is unoptimized and renders roughly 17x slower — a scene that draws in 6ms takes 100ms. Only leave it off if you are debugging the engine itself.

What else you need

ffmpeg does the video encoding. Codimate uses the one on your PATH if you have it (brew install ffmpeg, apt install ffmpeg) and otherwise falls back to the copy that comes with imageio-ffmpeg, which pip installs for you. Point CODIMATE_FFMPEG at a binary to override both.

typst is needed only if you use scene.formula to typeset LaTeX maths (brew install typst). Everything else renders without it.

The four pieces

algorithm   your normal code, with emit() where something happens
view        what one moment looks like
motion      how things travel between moments
timing      how long each moment lasts

algorithm — write it the way you normally would. Call emit() after you change your data; Codimate snapshots the result for you.

view — a function from one moment to a picture. It receives frame.state (your data at that moment) and frame.event (what just happened). It runs once per event, not once per frame, so it can be as slow as you like.

motion — patterns matched against shape names. First match wins, and a straight line is the default, so most explanations need no rules at all.

motion=[cm.Rule("*", position="lift_carry_drop", clearance=90)]

A shape inside a group is named group/child, so "3/*" targets one group. A rule cannot make something move that did not move.

path
straight	a straight line, easing in and out — the default
linear	a straight line at constant speed, for things mid-journey at every event
lift_carry_drop	arcs up and over, then falls; takes clearance

Use linear when something turns or orbits: easing would make it accelerate and stop inside every segment.

Every path eases in and out. cm.ease(t) calls into the Engine if you need the curve itself — to draw it, or to pace something by hand — so you never keep a second copy that can drift.

timing — seconds per event, by event name.

cm.Timing(default=0.55, events={"swap": 0.9}, opening=0.8, final_hold=1.2)

The one thing to understand

Codimate has no idea what a "swap" looks like. It only knows this thing was here, and now it is there. The name you give each shape is how you tell it what moved.

scene.group(item.id, slot)    # the name follows the THING
scene.group(position, slot)   # the name follows the PLACE

Same algorithm, same data, completely different video:

	what you see
name follows the thing	bar "3" is in slot 0, then slot 1 — it slides across
name follows the place	slot 0 stays put and changes height — bars morph in place

Both are useful. Sorting wants the first. A grid, a heatmap or a matrix multiply wants the second — cells don't travel, they light up:

scene.group(("cell", row, col), slot)

Codimate cannot tell you which you meant, so this is the one decision worth thinking about.

cm.items() is how a plain value gets a name of its own. Two 3s in a list are two different bars, and only an identity can say so:

values = cm.items([3, 1, 4, 2])   # each gets an .id and a .value

Items compare by value, so your algorithm stays ordinary Python (values[j] > values[j + 1]), and they stay themselves across every moment of the trace. You only need them for things that move — a grid keyed by position doesn't.

What you can draw

scene.rect(name, h=, w=, at=)
scene.circle(name, r=, at=)
scene.text(name, content, size=, at=)
scene.polygon(name, points)                             # cm.ngon, cm.star
scene.curve(name, points, w=)                           # smooth, through the points
scene.svg(name, "logo.svg", size=)                       # vector art, as real geometry
scene.line(name, start=slot_or_point, end=slot_or_point, w=)
scene.formula(name, r"\frac{a}{b}", size=, at=)         # LaTeX, needs `typst`
cm.measure(text, size) -> (w, h)                        # to size a box around text
scene.group(name, slot)      # a place to draw a thing made of several shapes

Each takes only what decides what the shape is. Everything else — colour, outline, rotation, layer, opacity — is said afterwards on the handle it hands back, so no call grows past five arguments:

scene.circle("bob", r=28, at=(x, y)).fill("orange").on(layer=4)
scene.rect("card", h=120, w=200).fill("#243046", edge="grey").round(8)
scene.polygon("tri", cm.ngon(3, r=60)).grow(1.8).turn(12)

Place things by whichever edge you actually mean. at= takes a point, a Slot, or cm.at(...) when an edge is what you mean:

scene.rect("bar", h=200, w=90, at=(640, 460))               # a point
scene.rect("bar", h=200, w=90, at=cm.at(x=640, bottom=560)) # sits on a line
scene.text("label", 3, size=32, at=cm.at(x=640, top=580))   # under something

No baselines, no y - h/2. Give both y and top and you get an error, not a silent winner.

Colors are names (white, black, red, orange, blue, green, grey, yellow, cyan) or #rrggbb. Higher layer draws on top.

Shapes that appear or disappear between moments fade, and shapes that change size, colour or position tween. You do not ask for any of that.

Where things sit

You should not be inventing layout arithmetic. cm.row() and cm.column() divide the canvas and hand back one slot per item; scene.group() puts a thing in one:

for slot, item in cm.row(frame.state, gap=40):
    bar = scene.group(item.id, slot)
    bar.rect("bar", h=item.value * 70, at=cm.at(bottom=0))
    bar.text("label", item.value, at=cm.at(top=20))

Inside a group, 0 is the group's own point. cm.at(bottom=0) stands the bar on it; cm.at(top=20) puts the label 20 below it. w defaults to the group's width and x to its centre, so the only thing left to say is the one thing a bar and a label disagree about.

A name can be built from other names. Nested keys flatten, so you never concatenate tuples by hand — and a line takes Slots directly rather than making you pull .x and .y out of them:

scene.line(("edge", src, dst), start=at[src], end=at[dst])   # -> "edge/0/1/1/2"

Everything on a group moves as one thing. The bar and its label cannot come apart, because the Engine sees them as 3/bar and 3/label — one name, two shapes. Groups nest, so a thing made of a thing made of a thing still travels as a unit.

A slot is a place, not a shape — nothing draws it. Hand one to group() and you never take it apart; read slot.x, slot.bottom, slot.left, slot.top, slot.right, slot.w, slot.h for the odd case that needs it.

cm.column() stacks instead of spreading — layers of a network, levels of a tree — and its slots anchor at their centre rather than a baseline:

for slot in cm.column(4, gap=44, at=cm.at(x=640)):
    scene.group(("neuron", 1, i), slot).circle("body", r=30)

Widths, spacing and the baseline come from the canvas unless you override them (size=, at=, within=). The canvas is 1280x720 by default:

cm.canvas(1920, 1080)     # everything below follows
cm.width(), cm.height()   # for the odd thing you place by hand

Output resolution is separate from your coordinates. scale only changes how many pixels each coordinate becomes, so nothing in your view has to move:

.render("out.mp4", fps=60, scale=1.5)   # 1080p60 from the default canvas

Frames are rasterized at the larger size rather than upscaled afterwards, so 1080p is genuinely drawn at 1080p.

How it works

Python (once per event)          Rust (once per frame)
──────────────────────           ─────────────────────
algorithm + emit()
view → a Scene per event
motion rules, durations
        │
        └─ hands over ONCE ───▶  pair shapes by item
                                 build tweens
                                 rasterize
                                 pipe to ffmpeg
                                        │
                                        ▼
                                    out.mp4

One handover, not one per frame. A frame is never played forward — it is computed from scratch at time t, so any moment can be rendered in any order, and twice the same way.

Under the hood

The Rust crates are the Engine. You do not need to read them to use Codimate, and the Rust API is not a second way to author explanations — see ADR 0008.

crates/codimate-core/      pure animation model — f(t) → Scene
crates/codimate-animation/ duration and composition
crates/codimate-render/    tiny-skia CPU raster
crates/codimate-export/    raw RGBA → ffmpeg
crates/codimate-glyph/     text → glyph outlines
crates/codimate-py/        the bindings — the diff lives here
python/codimate/           the Python package

More

All of this is also a site — https://darhnoel.github.io/codimate/ — with the API reference generated from the docstrings alongside it. Build it locally with python docs/build_site.py.

The guide, in order. Four chapters; read them front to back the first time.

1. Writing Your First Animation — build one from an empty file, meeting all four pieces on the way.
2. What You Have to Work With — the shapes, what they are enough for, and a car built out of them.
3. How Codimate Thinks — why motion is derived rather than authored, and the one decision you have to make.
4. Reference — every call and parameter, on one page.

Then python/examples/, eight worked examples with notes, and the decisions behind the design.

cargo test                                  # the Engine
.venv/bin/python python/tests/run.py             # the Authoring Surface