Monday, December 13, 2021

Weird: Generative Art in Common Lisp

WEIRD-A Generative Art System

NOTE: you should not use this code for anything remotely important

About

weird is the next iteration of weir which was the next iteration of snek. The library is written to be useful for a broad range of ways in which I create art using various generative algorithms. Almost everything I have made over the past several years has been made using some version of this system.

Components

In addition to the 2d/3d vectors proided via cl-veq, the main components are:

A simple (undirected) graph data structure, weir. The structure can be manipulated directly, or via alterations. The latter is described in more detail below. Here is a simple example of how you can manipulate the structure directly:

(in-package :weir)
(let ((wer (make)))
  ; add three edges
  (loop repeat 3
        do (add-edge! wer
             (2add-vert! wer
               (rnd:2in-circ 200.0))
             (2add-vert! wer
               (veq:f2+ (veq:2rep 500.0)
                        (rnd:2in-circ 200.0))))
  ; iterate verts
  (itr-verts (wer v)
    ; prints vert coordinates
    (veq:vpr (2get-vert wer v)))

  ; iterate edges
  (itr-edges (wer vv)
    (veq:vpr (2get-verts wer vv)))

  ; move a vert relativ to current position:
  (2move-vert! wer 0 1.0 2.0)
  ; or to an absolute position
  (2move-vert! wer 1 1.0 2.0 :rel nil)

  ; edges are represented as lists of verts, and they are always
  ; sorted with the smallest vert index first, so both of these
  ; return t:
  (edge-exists wer '(0 1))
  (edge-exists wer '(1 0))

  ; get edges incident to vert 0
  (get-incident-edges wer 0))

See examples/draw.lisp and examples/draw.lisp for more.

Random numbers, some examples:

(in-package :rnd)
(rnd a) ; in range [0.0, a), defaults to a=1.0.
(rnd* a) ; in range [-a, a), defaults to a=1.0.
(rndrng a b) ; in range [a, b)
(rndi 10) ; random fixnum
(rndspace n a b) ; n numbers in [a, b)
(norm :mu 0.0 :sigma 1.0) ; normal distribution
(2in-circ a) ; in circle of radius a
(2in-rect w h) ; in a rectangle
(2nin-rect n w h) ; n in rectangle.
(2on-line ax ay bx by) ; point between points a and b

; do something with probability 0.1, second form is optional
(prob 0.1 (print "10% hi") (print "90% oh no"))

; perform either form 1 or (optionally) 2
(either (print "form 1") (print "form 2"))

See rnd.lisp, 2rnd.lisp and 3rnd.lisp, for all available functions.

A tool for drawing svg files: wsvg. See draw.lisp.

Weir Graphs and Alterations

The most interesting part of the weir graph structure is alterations. An alteration is a change that will be applied to the structure at the end of a given context, provided it is valid.

The main motivation behid this is that this makes it possible to "queue" up a number of changes that will be applied at a later time. This makes it possible to access the state in the weir instance while you are creating the alterations. Without there being any changes made to the state of the weir instance while the alterations are being created. Once all alterations are created, the valid ones will be applied.

Existing alterations in weir are postfixed with ?. it might look like this:

(weir:with (wer %)
  (% (add-vert? (veq:f2< 100.0 740.0))
  (% (add-edge? 1 4)))

(% ...) is used to collect alterations. They will be executed at the end of the with context. If an alteration evaluates to nil, nothing will happen.

Here is an example of how the forces are calcualted in my Tangle of Webs simulation:

(veq:vdef* reciprocal-edge-forces (wer &key (stp 0.1))
  (weir:with (wer %)
    ; state of wer is unaltered
    (weir:itr-edges (wer e) ; edge (v0 v1)
      ; vector from v0 to v1
      ; force is proportional to this "oriented distance"
      (veq:f2let ((force (veq:f2-
                           (veq:f2$ (weir:2get-verts wer e)
                                    1 0))))
        (loop for i in e and s in '(-1.0 1.0)
              ; alteration is created, but nothing happens
              do (% (weir:2move-vert? i
                      (veq:f2scale force (* s stp)))))))))
    ; alterations are applied at the end
    ; of the context

The important thing to note here is that for the forces to be calculated correctly, all edge lengths must be calculated before the forces are applied to the vertices.

Futures and Dependencies

You can assign a name to the result of an alteration using

(% (add-edge? 1 3) :res :some-name?)

This makes it possible to create alterations that depend on the result of other alterations:

(in-package :weir)
(with (wer %)
  (veq:f2let ((pt (veq:f2< 1f0 3f0)))
    (% (2add-vert? pt) :res :a?) ; alteration result is named :a?
    (% (2add-vert? (veq:f2< 1.0 2.0)) :res :b?) ; result named :b?
    (% (add-edge? :a? :b?)))) ; depends on :a? and :b?

; all alteration results:
(print (get-alteration-result-list wer))
; or as a `hash-map`:
(print (get-alteration-result-map wer))

alteration names must be keywords that end with ?. (There is an exception, see Looping below.) And using the same name for multiple alterations will result in undefined behaviour.

As you can see, a named alteration is akin to a future; a reference to a result that may or may not exist eventually. For this to work, any alteration that depends on a future that fails (or returns nil) will be skipped.

You can use (weir:with (wer % :bd t) ...) to see how an alteration is expanded. This might make it easier to see what is going on.

Looping

It is possible to use alterations inside loops as well. but it requires a bit more careful consideration. Here is an example:

(in-package :weir)
(with (wer % :db t)
  (loop for x in (math:linspace 20 -20.0 20.0) do
    (loop for z in (list 1.0 2.0) do
      (veq:f3let ((xy (veq:f3< x y z)))
        ; create a distinct name
        (let ((g? (gensym "g")))
          (% (add-grp? :name (gensym "line")) :res g?)
          (% (2add-path?
               (veq:f$_ (list (veq:f3-
                                xy (veq:f3< 1.0 8.0 (rnd:rnd)))
                              (veq:f3+
                                xy (veq:f3< 1.0 2.0 (rnd:rnd)))))
               :g g?)))))))

Writing

I have written about things related to this code at:

Note that these posts refer to older iterations of the code. So some of the things will be out of date.

On Use and Contributions

This code is written for my personal use, and parts of it is rather experimental. Also, it is likely to change at my whim. For this reason I don't recommend depending on this library for anything.

I release it publicly in case people find it useful or interesting. It is not, however, intended as a collaboration/Open Source project. As such I am unlikely to accept PRs, reply to issues, or take requests.

Installation and Dependencies

weird depends on cl-veq, and it requires Quicklisp to install dependencies (which are listed in weird.asd).

To install and load weird, do:

If this does not work, weird may not be in a place Quicklisp or ASDF can see them. To fix this, either:

For a long term solution, add the following to .sbclrc:

#+quicklisp
(push "/path/to/dir/containing/weird" ql:*local-project-directories*)

You will have to make sure cl-veq is also available in the same fashion for any of this to work.

Tests

Tests can be executed using: (asdf:test-system :weird).

Thanks

I would like to thank:

Who have provided me with useful hints and code feedback.

The ASDF config and test setup was kindly suggested and implemented by Robert Smith (https://twitter.com/stylewarning). Although I have made some changes since then.

Also, many thanks to https://twitter.com/xach for making Quicklisp.

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