19 Nov
2D Vector Graphics in Ruby / Rails
I've been working with Cairo the last day or two to build a sign generator. I chose Cairo because it seemed like a good fit for building a sign that had to be loss-less in detail and scalable, this rang vector graphics. I found a few good examples here and there, but they were very few and the documentation was not exactly ruby-friendly.
So I'm going to do a few posts about what I learned and I am going to start with the very basics, since all I could find were completed and elaborate images, way too complicated for a beginner.
Let's start with rotating a square box by 45 degrees. Very simple.
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| require 'rubygems' | |
| require 'cairo' | |
| class Box | |
| # height and width for the rectangle, angle for the rotation | |
| attr_accessor :height, :width, :angle | |
| def initialize(height=200, width=200, angle=45) | |
| @height, @width, @angle = [height, width, degrees_to_radians(angle)] | |
| # The first argument for a surface is the filename, the next two are our canvas/surface size. | |
| # I am making the surface twice as big as the object (WxH) we want to create, just to give it some padding. | |
| @surface = Cairo::SVGSurface.new("rotated_square.svg", @width * 2, @height * 2) | |
| # setup our context object to draw on | |
| @cr = Cairo::Context.new(@surface) | |
| # set it up with rotation | |
| setup_scene_with_rotation | |
| # draw the square | |
| return build_rotated_square | |
| end | |
| private | |
| def setup_scene_with_rotation | |
| # give it a white background | |
| @cr.set_source_color(:white) | |
| @cr.paint | |
| # rotate everything by our angle (in rads) from the middle | |
| # if you do not move the rotation point to the center, you will be rotating from 0,0 (bottom left of the canvas) | |
| # so first move to the middle of the surface (coords of W,H, since our canvas is 2W x 2H) | |
| # then do the rotation | |
| # then move back to 0,0 | |
| # we know that width, height is the center b/c we start at 0,0 | |
| # see @cr.current_point to be sure | |
| @cr.translate(@width, @height) | |
| @cr.rotate(@angle) | |
| @cr.translate(-@width, -@height) | |
| end | |
| def build_rotated_square | |
| # build a rectangle with height and width dimensions | |
| # x and y are the starting points from which the rectangle is drawn | |
| # x and y are the bottom left of the box | |
| x = @width / 2 | |
| y = @height / 2 | |
| @cr.rectangle(x, y, @width, @height) | |
| # the object has been created but is invisible, so let's draw a line around it | |
| # the line will be black | |
| @cr.set_source_color(:black) | |
| # apply the line | |
| @cr.stroke | |
| # create the SVG | |
| @cr.target.finish | |
| end | |
| # the angle comes in, in the form of the degrees and will need to be converted to radians | |
| def degrees_to_radians(angle_in_degrees) | |
| return angle_in_degrees / 180.0 * Math::PI | |
| end | |
| end | |
| # let's get our box ... I'm not using GTK, so it will just spit out an image in the local fs | |
| Box.new |
That's it, pretty simple, right?
