Cursor comet trail effect for Emacs. When you move the cursor, a comet of highlighted characters travels along the geometric line between the old and new positions, then fades away.

Works with both keyboard and mouse movement. Correctly handles visual line wrapping (visual-line-mode, word-wrap, toggle-truncate-lines).

Requires Emacs 29.1 or later.

How it works

Given two buffer positions, the package computes their visual (column, row) coordinates on screen, traces a line between them using Bresenham's algorithm, maps each cell back to a buffer position, and animates a comet sliding along that path with an ease-out brightness gradient.

Installation

use-package with :vc (Emacs 29+)

Recommended method for Emacs 29 and later:

(use-package comet
  :vc (:url "https://git.andros.dev/andros/comet.el"
       :rev :newest)
  :config
  (add-hook 'prog-mode-hook #'comet-trail-mode)
  (add-hook 'text-mode-hook #'comet-trail-mode))

use-package with :load-path

For manual installation:

(use-package comet
  :load-path "/path/to/comet.el"
  :config
  (add-hook 'prog-mode-hook #'comet-trail-mode)
  (add-hook 'text-mode-hook #'comet-trail-mode))

Manual

Clone the repository and place the file in a directory on your load-path:

git clone https://git.andros.dev/andros/comet.el.git

Then add to your user-init-file:

(add-to-list 'load-path "/path/to/comet.el")
(autoload 'comet-trail-mode "comet" nil t)

Or, to load it immediately at startup:

(add-to-list 'load-path "/path/to/comet.el")
(require 'comet)
(add-hook 'prog-mode-hook #'comet-trail-mode)

Usage

Run M-x comet-trail-mode RET to toggle the effect in the current buffer. The mode indicator Trail appears in the mode line when active.

Additional commands:

comet-draw: Draw a static highlighted line between two positions (works on the active region with M-x comet-draw).
comet-clear: Remove all comet overlays from the current buffer.

Customization

Run M-x customize-group RET comet RET to list all available options.

Key options:

comet-comet-length (default 4): Number of characters visible in the comet trail.
comet-comet-speed (default 0.3): Duration in seconds for the comet to traverse the full path.
comet-fade-exponent (default 2.0): Brightness falloff curve. 1.0 is linear, 2.0 is quadratic ease-out (recommended), 3.0 is cubic (sharper tail).
comet-minimum-distance (default 2): Minimum movement in cells to trigger an animation. Avoids visual noise on small cursor jumps.
comet-tick-interval (default 0.016): Seconds between animation frames (approximately 60fps).
comet-face (default comet-highlight): Face used for the comet color. Set to nil to inherit the cursor color automatically.

The default highlight color is #f0c674 (warm yellow) on dark themes and #c678dd (purple) on light themes.

Contributing

Contributions are welcome! Please see the contribution guidelines for instructions on how to submit issues or pull requests.

License

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.