Incanter now includes the incanter.processing library, a fork of Roland Sadowski‘s clj-processing library, making it possible to create Processing visualizations with Clojure and Incanter. Incanter.processing provides much more flexibility when creating customized data visualizations than incanter.charts — of course, it is also more complex to use.
Several nice examples of the kinds of visualizations that can be created in Processing can be found on Ben Fry’s website and blog, including the cool zipcode, human vs. chimps, baseball salary vs. performance examples.
- Processing: A Programming Handbook for Visual Designers and Artists by Ben Fry
- Visualizing Data: Exploring and Explaining Data with the Processing Environment by Casey Reas and Ben Fry
Learning Processing: A Beginner’s Guide to Programming Images, Animation, and Interaction (Morgan Kaufmann Series in Computer Graphics) by Daniel Shiffman
Incanter.processing was forked from of Roland Sadowski’s clj-processing library in order to provide cleaner integration with Incanter. I have added a few functions, eliminated some, and changed the names of others. There were a few instances where I merged multiple functions (e.g. *-float, *-int) into a single one to more closely mimic the original Processing API; I incorporated a couple functions into Incanter’s existing multi-methods (e.g. save and view); I eliminated a few functions that duplicated existing Incanter functions and caused naming conflicts (e.g. sin, cos, tan, asin, acos, atan, etc); and I changed the function signatures of pretty much every function in clj-processing to require the explicit passing of the ‘sketch’ (PApplet) object, whereas the original library passes it implicitly by requiring that it is bound to a variable called *applet* in each method of the PApplet proxy.
These changes make it easier to use Processing within Incanter, but if you just want to write Processing applications in Clojure without all the dependencies of Incanter, then the original clj-processing library is the best choice.
A Simple Example
The following is sort of a “hello world” example that demonstrates the basics of creating an interactive Processing visualization (a.k.a sketch), including defining the sketch’s setup, draw, and mouseMoved methods and representing state in the sketch using closures and refs. This example is based on this one, found at John Resig‘s Processing-js website.
(use '(incanter core processing))
Now define some refs that will represent the state of the sketch object,
(let [radius (ref 50.0) X (ref nil) Y (ref nil) nX (ref nil) nY (ref nil) delay 16
The variable radius will provide the value of the circle's radius; X and Y will indicate the location of the circle; and nX and nY will indicate the location of the mouse. We use refs for these values because their values are mutable and need to be available across multiple functions in the sketch object.
Now define a sketch object, which is just a proxied processing.core.PApplet, and its required setup method,
sktch (sketch (setup  (doto this (size 200 200) (stroke-weight 10) (framerate 15) smooth) (dosync (ref-set X (/ (width this) 2)) (ref-set Y (/ (width this) 2)) (ref-set nX @X) (ref-set nY @Y)))
The first part of the setup method sets the size of the sketch, the stroke weight to be used when drawing, the framerate of the animation, and indicates that anti-aliasing should be used. The next part of the method uses a dosync block and ref-set to set initial values for the refs. Note the @ syntax to dereference (access the values of) the refs X and Y.
Processing sketches that use animation require the definition of a draw method, which in this case will be invoked 15 times per second as specified by the framerate.
(draw  (dosync (ref-set radius (+ @radius (sin (/ (frame-count this) 4)))) (ref-set X (+ @X (/ (- @nX @X) delay))) (ref-set Y (+ @Y (/ (- @nY @Y) delay)))) (doto this (background 125) ;; gray (fill 0 121 184) (stroke 255) (ellipse @X @Y @radius @radius)))
The first part of the draw method uses dosync and ref-set to set new values for the radius, X, and Y refs for each frame of the animation. The sin function is used to grow and shrink the radius over time. The location of the circle, as indicated by X and Y, is determined by the mouse location (nX and nY) with a delay factor.
The next part of the draw method draws the background (i.e. gray background) and the circle with the ellipse function.
Finally, we need to define the mouseMoved method in order to track the mouse location, using the mouse-x and mouse-y functions, and set the values of the nX and nY refs. All event functions in incanter.processing, including mouseMoved, require an event argument; this is due to limitations of Clojure’s proxy macro, and isn’t required when using the Processing’s Java API directly.
(mouseMoved [mouse-event] (dosync (ref-set nX (mouse-x mouse-event)) (ref-set nY (mouse-y mouse-event)))))]
Now that the sketch is fully defined, use the view function to display it,
(view sktch :size [200 200]))
The complete code for this example can be found here.
In future posts I will walk through other examples of Processing visualizations, some of which can be found in the Incanter distribution under incanter/examples/processing.