VisuBezier

Provides a preview when hovering CSS easing functions in Sublime Text.

A Sublime Text port of the VisuBezier VS Code extension by Chris Kirk-Nielsen.

Features

Easing functions are underlined in the buffer. Hover one to see a plot of the curve alongside an animation comparing it against a reference easing (linear by default).

Supported easings:

• Keywords — linear, ease, ease-in, ease-out, ease-in-out, step-start, step-end
• cubic-bezier(), including curves that overshoot
• steps() with every jumpterm — start, end, jump-start, jump-end, jump-both, jump-none
• linear(), with explicit, implicit and doubled stop positions

Requirements

Sublime Text 4. The package uses the 3.8 plugin host, declared in .python-version.

Installation

Package Control

Open the command palette, run Package Control: Install Package, and pick VisuBezier.

Manually

Clone into your Packages directory:

cd "$(python3 -c 'import sublime; print(sublime.packages_path())' 2>/dev/null || echo ~/Library/Application\ Support/Sublime\ Text/Packages)"
git clone https://github.com/mrcgrtz/visubezier.git VisuBezier

The directory must be named VisuBezier — note the capitalisation. The menu and command palette entries reference ${packages}/VisuBezier/, so a clone named visubezier loads and previews fine but has no Settings entry under Preferences → Package Settings.

Changes to files under core/ are picked up on the next reload of visubezier.py; if a pull ever seems to have no effect, restarting Sublime Text is the reliable reset.

Settings

Open them from Preferences → Package Settings → VisuBezier → Settings.

selectors defaults to:

["source.css", "source.scss", "source.sass", "source.less", "source.stylus", "source.postcss", "text.xml"]

Matching is by scope prefix, so source.css also covers CSS embedded in HTML and text.xml covers SVG.

Post-install sample

Paste this into a CSS file and hover the values:

button {
    transition-timing-function: ease;
    transition-timing-function: ease-in;
    transition-timing-function: ease-out;
    transition-timing-function: ease-in-out;
    transition-timing-function: cubic-bezier(0.4, -0.2, 0.42, 1.2);
    transition-timing-function: steps(7);
    transition-timing-function: steps(5, jump-none);
    transition-timing-function: steps(8, jump-both);
    transition-timing-function: steps(4, jump-start);
    transition-timing-function: steps(2, jump-end);
    transition-timing-function: step-start;
    transition-timing-function: step-end;
    transition-timing-function: linear(0, 0.25 25% 75%, 1);
    transition-timing-function: linear(0, 0.063, 0.25, 0.563, 1 36.4%, 0.812, 0.75, 0.813, 1 72.7%, 0.953, 0.938, 0.953, 1 90.9%, 0.984, 1 100% 100%);
    transition-timing-function: ease, steps(3), cubic-bezier(1, 0, 0, 1), linear(0 0%, -0.25, 1.25, 1 100%);
}

How it works

Sublime Text's popup renderer, minihtml, supports neither SVG nor CSS animation, which is how the VS Code extension drew its preview. So VisuBezier rasterises the preview itself:

• core/easing.py parses and evaluates each easing — a Newton-Raphson solver for cubic-bezier(), jumpterm arithmetic for steps(), piecewise interpolation for linear().
• core/raster.py draws into an indexed-colour canvas whose palette is a single foreground-over-background ramp, giving anti-aliasing for free.
• core/png.py encodes that canvas.

minihtml also paints only the first frame of an animated GIF, so animation cannot be delegated to the image format either. An animated preview is instead a sequence of stills that the plugin cycles through with update_popup while the popup is open. core/gif.py survives for one job — generating the animated preview.gif above, via tools/make_preview.py.

All of it is pure Python with no third-party dependencies.

Differences from the VS Code extension

• **linear() animates.** VS Code's renderer had no linear() support, so the extension could draw the graph but fell back to ease for the animation. Evaluating the easing directly removes that limitation.
• Overshoot stays in frame. Squares are clamped to the animation track rather than escaping the preview area.
• Adjacent easings are both found. The upstream pattern consumed the delimiter after a match, so the second of ease,ease was missed.
• **linear() stop positions follow the spec.** They are forced to be non-decreasing, and a final stop keeps its own value rather than being snapped to 1.
• Settings use Sublime naming and scope selectors instead of VS Code language identifiers. See the changelog for the mapping.
• No inline icon. minihtml cannot place an image inside a line of text, so matches are marked with an underline only.

Known issues

• Easing functions containing anything other than numbers are ignored, including calc() and var().
• Rendering an animated preview takes roughly 60 ms the first time; results are cached per easing and settings combination. Set "animate": false for instant static previews.
• Animation runs on a timer driven by the plugin, because minihtml supports neither CSS animation nor animated GIFs. It stops as soon as the popup closes.

Tests

python3 tests/run.py

The suite runs outside Sublime Text against a stubbed API. Encoded images are verified against ImageMagick when it is installed, and those tests skip when it is not.

Credits

Original VS Code extension by Chris Kirk-Nielsen. Sublime Text port by Marc Görtz.

License

MIT