I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched c-beams glitter in the dark, near the Tannhäuser Gate. All those moments will be lost, in time, like tears in rain. Time to die.

Click the screenshot for an asciicast showing it in motion (Thanks asciinema!) It looks even better running in a large terminal locally, where the animation is smoother. You should download and run it!

$ pip install --user cbeams
$ cbeams

Downloading as a binary executable

Older releases contained a Linux binary executable, downloaded from:

https://github.com/tartley/cbeams/releases

But I'm not building those any more. Get it of PyPI using pip as above.

Downloading as source

Dependencies

Developed on on Ubuntu 14.04, likely works on other Linux. Does work on OSX. Does not work on Windows.

Tested on Python 3.4 & 3.5. Recent releases on 3.8. Probably also runs on other 3.x. Does not run on 2.x.

Python dependencies are specified in setup.py.

Usage

cbeams [-o]
cbeams [-h]

-o  Overwrites one screenful of the existing terminal contents
-h  Displays help.

Pressing ctrl-C exits cbeams, flipping back to the regular terminal buffer, so the animation doesn't overwrite any of your previous terminal contents.

For fun, there's also a '-o' arg, which overwrites the terminal text without flipping buffers. So you can see the expanding circles slowly eat away at your existing terminal text, but then when you ctrl-c, it's not possible to restore the terminal. So one screenful of your terminal text is overwritten and lost.

Why did I develop this?

The traditional way to do colors or animation in a terminal is to use the venerable UNIX library 'curses', or its open source clone 'ncurses'. There are many Python packages that expose ncurses for various uses. Anyone who has used these knows that curses is a definite contender for one of the worst APIs in existence. It systematically exposes callers to reams of the incidental complexity of the underlying implementation, accumulated by supporting decades of generations of different terminals and terminal emulators.

Fortunately, nowadays there is a better way. Erik Rose's 'Blessings' package layers a sane API on top of ncurses. The documentation page shows how 21 lines of incomprehensible code using curses is transformed into four straightforward lines of code using blessings.

I wanted an excuse to learn how blessings works, and cbeams is the result.

I tag it onto the end of long-running commands to use as a visual notification that the command has finished.

How it works

Aside from the use of Blessings, the other fun part of this project was in representing the circles.

Obviously the model represents circles as a center point, radius, and a color. To display these, we convert it into a representation that's useful for outputting to the terminal. Namely, each circle is converted into a sequence of horizontal slices. Each slice has a vertical position, a leftmost start position, and a rightmost end position. To display this to the terminal, we just print a line of colored space characters for each slice. Care was taken to ensure that the resulting printed shape ended up symmetrical about both the vertical and horizontal axes.

Interestingly, this representation as a series of horizontal slices lends itself to representing other arbitrary shapes as well as just circles.

With this in mind, obviously the animation doesn't actually display just circles. They are annuli or rings. So each ring is modelled as two circles, an outer with a color, and an inner with a lesser radius.

To display a ring, we don't just draw the outer circle then overwrite it by drawing the inner circle in black. This would display an annoying flicker (there's no double-buffer or vsync), and would also prevent us from being able to see through the holes in each ring. Instead, we perform the spatial subtraction of 'outer_circle - inner_circle' to construct a new shape - the ring as a series of horizontal slices.

It was tempting to write an arbitrary spatial operator to do this, capable of subtracting any arbitrary shape from another shape, each represented as a series of slices, resulting in a new series of slices. As satisfying as this might have been it's clearly out of scope for this project, and as such, test-driven development instead correctly steered me towards a simpler function, that used its knowledge of what the input shapes are to produce the required output in fewer lines of code.

Given this, drawing an annulus can use the exact same routine that we previously described to draw a circle. Just iterate over the slices, printing an appropriate length horizontal strip of colored spaces for each.

In each successive frame then, we asymptotically increase the inner and outer radii of each annulus towards the annulus' predetermined 'max radius'. When the inner radius gets close enough to that value, we remove that annulus from the world.

There's an extra layer of complexity though. What we've described above would work great if there was a double-buffering mechanism, whereby each frame erased the whole back buffer, draw the next frame in its entirety there, and then flipped buffers. This mechanism doesn't exist though. Hence, drawing consecutive frames as described above doesn't erase the color from a character square once an annulus' inner-radius passes over it.

To achieve that, instead of drawing the desired appearance of the current frame, we instead draw the delta between the current frame and the last one. Hence, we draw a thin colored annulus outside the outer radius, and a think black annulus inside the inner radius.

The width of these thin "delta" annuli are just the difference between the current radius value and the last frame. Hence, they are usually less than one character wide, taking the form of just a series of disconnected dots speckling the outer and inner edges of each visible circle. Over successive frames, the colored dots slowly expand the outer radius, while the black ones eat away at the inside radius, growing the black hole there.

Drawing the tiny deltas between successive frames like this prevents overlapping circles from flickering badly as they would if we continually overdrew each whole annulus on every frame.

Also, it ends up making the program run faster, and hence the animation look pleasantly smoother, because we have far fewer characters to draw to the terminal each frame.

We also have a random probability of adding new annuli into the world at each frame. This probability varies sinusoidally over time, so that there are quieter and noisier moments in the animation.

New annuli are assigned a randomly chosen color from a set of currently allowed colors. We add and remove colors from that set over time, so that sometimes all our rings are the same colors, sometimes two colors, and sometimes many colors. This helps to keep the animation evolving over time, instead of looking too 'samey' all the time.

Hacking

To populate a virtualenv, run tests, etc, see the commands in the Makefile. These can often work in Windows too, under Bash shells like Cygwin, Msys.

Populating the virtualenv in the manner shown in the Makefile will also add "-e ." to the virtualenv, which adds this project in 'develop mode', meaning both that source edits are immediately visible within the virtualenv, and that the application entry points listed in setup.py are converted into executable scripts on the PATH.

Thanks

To Erik Rose, for the fabulous Blessings package. https://pypi-hypernode.com/pypi/blessings

Links & Contact

Python package: http://pypi.python.org/pypi/cbeams/

Binaries, source, issues: https://github.com/tartley/cbeams/

Author: Jonathan Hartley, email: tartley at domain tartley.com, Twitter: @tartley.