Vector Graphics in PDFKit

An introduction to vector graphics

Unlike images which are defined by pixels, vector graphics are defined through a series of drawing commands. This makes vector graphics scalable to any size without a reduction in quality (pixelization). The PDF format was designed with vector graphics in mind, so creating vector drawings is very easy. The PDFKit vector graphics APIs are very similar to that of the HTML5 canvas element, so if you are familiar at all with that API, you will find PDFKit easy to pick up.

Creating basic shapes

Shapes are defined by a series of lines and curves. lineTo, bezierCurveTo and quadraticCurveTo all draw from the current point (which you can set with moveTo) to the specified point (always the last two arguments). Bezier curves use two control points and quadratic curves use just one. Here is an example that illustrates defining a path.

doc.moveTo(0, 20)                               # set the current point
   .lineTo(100, 160)                            # draw a line
   .quadraticCurveTo(130, 200, 150, 120)        # draw a quadratic curve
   .bezierCurveTo(190, -40, 200, 200, 300, 150) # draw a bezier curve
   .lineTo(400, 90)                             # draw another line
   .stroke()                                    # stroke the path

The output of this example looks like this:

One thing to notice about this example is the use of method chaining. All methods in PDFKit are chainable, meaning that you can call one method right after the other without referencing the doc variable again. Of course, this is an option, so if you don't like how the code looks when chained, you don't have to write it that way.

SVG paths

PDFKit includes an SVG path parser, so you can include paths written in the SVG path syntax in your PDF documents. This makes it simple to include vector graphics elements produced in many popular editors such as Inkscape or Adobe Illustrator. The previous example could also be written using the SVG path syntax like this.

doc.path('M 0,20 L 100,160 Q 130,200 150,120 C 190,-40 200,200 300,150 L 400,90')
   .stroke()

The PDFKit SVG parser supports all of the command types supported by SVG, so any valid SVG path you throw at it should work as expected.

Shape helpers

PDFKit also includes some helpers that make defining common shapes much easier. Here is a list of the helpers.

The last one, polygon, allows you to pass in a list of points (arrays of x,y pairs), and it will create the shape by moving to the first point, and then drawing lines to each consecutive point. Here is how you'd draw a triangle with the polygon helper.

doc.polygon [100, 0], [50, 100], [150, 100]
doc.stroke()

The output of this example looks like this:

Fill and stroke styles

So far we have only been stroking our paths, but you can also fill them with the fill method, and both fill and stroke the same path with the fillAndStroke method. Note that calling fill and then stroke consecutively will not work because of a limitation in the PDF spec. Use the fillAndStroke method if you want to accomplish both operations on the same path.

In order to make our drawings interesting, we really need to give them some style. PDFKit has many methods designed to do just that.

Some of these are pretty self explanatory, but let's go through a few of them.

Line cap and line join

The lineCap and lineJoin properties accept constants describing what they should do. This is best illustrated by an example.

# these examples are easier to see with a large line width
doc.lineWidth(25)

# line cap settings
doc.lineCap('butt')
   .moveTo(50, 20)
   .lineTo(100, 20)
   .stroke()

doc.lineCap('round')
   .moveTo(150, 20)
   .lineTo(200, 20)
   .stroke()

# square line cap shown with a circle instead of a line so you can see it   
doc.lineCap('square')
   .moveTo(250, 20)
   .circle(275, 30, 15)
   .stroke()

# line join settings
doc.lineJoin('miter')
   .rect(50, 100, 50, 50)
   .stroke()

doc.lineJoin('round')
   .rect(150, 100, 50, 50)
   .stroke()

doc.lineJoin('bevel')
   .rect(250, 100, 50, 50)
   .stroke()

The output of this example looks like this.

Dashed lines

The dash method allows you to create non-continuous dashed lines. It takes a length specifying how long each dash should be, as well as an optional hash describing the additional properties space and phase.

The space option defines the length of the space between each dash, and the phase option defines the starting point of the sequence of dashes. By default the space attribute is equal to the length and the phase attribute is set to 0. You can use the undash method to make the line solid again.

The following example draws a circle with a dashed line where the space between the dashes is double the length of each dash.

doc.circle(100, 50, 50)
   .dash(5, space: 10)
   .stroke()

The output of this example looks like this:

Color

What is a drawing without color? PDFKit makes it simple to set the fill and stroke color and opacity. You can pass an array specifying an RGB or CMYK color, a hex color string, or use any of the named CSS colors.

The fillColor and strokeColor methods accept an optional second argument as a shortcut for setting the fillOpacity and strokeOpacity. Finally, the opacity method is a convenience method that sets both the fill and stroke opacity to the same value.

The fill and stroke methods also accept a color as an argument so that you don't have to call fillColor or strokeColor beforehand. The fillAndStroke method accepts both fill and stroke colors as arguments.

doc.circle(100, 50, 50)
   .lineWidth(3)
   .fillOpacity(0.8)
   .fillAndStroke("red", "#900")

This example produces the following output:

Gradients

PDFKit also supports gradient fills. Gradients can be used just like color fills, and are applied with the same methods (e.g. fillColor, or just fill). Before you can apply a gradient with these methods, however, you must create a gradient object.

There are two types of gradients: linear and radial. They are created by the linearGradient and radialGradient methods. Their function signatures are listed below:

Once you have a gradient object, you need to create color stops at points along that gradient. Stops are defined at percentage values (0 to 1), and take a color value (any usable by the fillColor method), and an optional opacity.

You can see both linear and radial gradients in the following example:

# Create a linear gradient
grad = doc.linearGradient(50, 0, 150, 100)
grad.stop(0, 'green')
    .stop(1, 'red')

doc.rect 50, 0, 100, 100
doc.fill grad

# Create a radial gradient
grad = doc.radialGradient(300, 50, 0, 300, 50, 50)
grad.stop(0, 'orange', 0)
    .stop(1, 'orange', 1)

doc.circle 300, 50, 50    
doc.fill grad

Here is the output from the this example:

Winding rules

Winding rules define how a path is filled and are best illustrated by an example. The winding rule is an optional attribute to the fill and fillAndStroke methods, and there are two values to choose from: non-zero and even-odd.

# Initial setup
doc.fillColor('red')
   .translate(-100, -50)
   .scale(0.8)

# Draw the path with the non-zero winding rule
doc.path('M 250,75 L 323,301 131,161 369,161 177,301 z')
   .fill('non-zero')

# Draw the path with the even-odd winding rule   
doc.translate(280, 0)
   .path('M 250,75 L 323,301 131,161 369,161 177,301 z')
   .fill('even-odd')

You'll notice that I used the scale and translate transformations in this example. We'll cover those in a minute. The output of this example, with some added labels, is below.

Saving and restoring the graphics stack

Once you start producing more complex vector drawings, you will want to be able to save and restore the state of the graphics context. The graphics state is basically a snapshot of all the styles and transformations (see below) that have been applied, and many states can be created and stored on a stack. Every time the save method is called, the current graphics state is pushed onto the stack, and when you call restore, the last state on the stack is applied to the context again. This way, you can save the state, change some styles, and then restore it to how it was before you made those changes.

Transformations

Transformations allow you to modify the look of a drawing without modifying the drawing itself. There are three types of transformations available, as well as a method for setting the transformation matrix yourself. They are translate, rotate and scale.

The translate transformation takes two arguments, x and y, and effectively moves the origin of the document which is (0, 0) by default, to the left and right x and y units.

The rotate transformation takes an angle and optionally, an object with an origin property. It rotates the document angle degrees around the passed origin or by default, the center of the page.

The scale transformation takes a scale factor and an optional origin passed in an options hash as with the rotate transformation. It is used to increase or decrease the size of the units in the drawing, or change it's size. For example, applying a scale of 0.5 would make the drawing appear at half size, and a scale of 2 would make it appear twice as large.

If you are feeling particularly smart, you can modify the transformation matrix yourself using the transform method.

We used the scale and translate transformations above, so here is an example of using the rotate transformation. We'll set the origin of the rotation to the center of the rectangle.

doc.rotate(20, origin: [150, 70])
   .rect(100, 20, 100, 100)
   .fill('gray')

This example produces the following effect.

Clipping

A clipping path is a path defined using the normal path creation methods, but instead of being filled or stroked, it becomes a mask that hides unwanted parts of the drawing. Everything falling inside the clipping path after it is created is visible, and everything outside the path is invisible. Here is an example that clips a checkerboard pattern to the shape of a circle.

# Create a clipping path   
doc.circle(100, 100, 100)
   .clip()

# Draw a checkerboard pattern
for row in [0...10]
  for col in [0...10]
    color = if (col % 2) - (row % 2) then '#eee' else '#4183C4'
    doc.rect(row * 20, col * 20, 20, 20)
       .fill(color)

The result of this example is the following:

That's it for vector graphics in PDFKit. Now let's move on to learning about PDFKit's text support!