Saturnboy
 6.10

The dream of SVG was probably born sometime in the late 90′s. Version 1.0 arrived in 2001, followed by the current version, SVG 1.1, in 2003. For anyone keeping score at home, that’s over 6 years ago, which is like 120 years ago in internet dog years. Also for those at home, SVG is a portable XML-based vector graphics format (it also does raster graphics), but you’d probably not be reading this if you didn’t already know.

Nowadays, I’m finally seeing some SVG in use. It works natively in all the real browsers (obviously not IE, which requires a plugin), and even on some mobile devices. But most importantly to me, I see it in my day-to-day work in Degrafa, and with the beta release of Flex 4, in Catalyst and FXG.

The vast majority of my experience in SVG is with paths (SVG Spec, § 8) and to a lesser extent Transforms (§ 7) and Filters (§ 15). Thankfully, these are some of the most useful and important pieces of SVG, and they all have nice one-to-one mappings to components in Degrafa and FXG.

SVG Path Primer

In SVG, a path is the outline of some object. It is described as a series of segments, where each segment can be different, either a line, curve, or arc. Path data is most often given in shorthand syntax as a series of commands followed by coordinates (we’ll ignore the long form for now). Let’s illuminate the discussion with some examples.

SVG:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
    "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg xmlns="http://www.w3.org/2000/svg" version="1.1"
    width="200" height="200">
  <path d="M 0,0 L 100,0 L 100,100 L 0,100 z"
      fill="#EECCEE"
      stroke="#FF00FF"
      stroke-width="3" />
</svg>

Degrafa in Flex 3:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application
        xmlns:mx="http://www.adobe.com/2006/mxml"
        xmlns:Degrafa="http://www.degrafa.com/2007"
        layout="absolute">
 
    <Degrafa:GeometryComposition graphicsTarget="{[cnv]}">
        <Degrafa:Path data="M 0,0 L 100,0 L 100,100 L 0,100 z">
	        <Degrafa:fill>
	            <Degrafa:SolidFill color="#EECCEE" />
	        </Degrafa:fill>
	        <Degrafa:stroke>
	            <Degrafa:SolidStroke color="#FF00FF" weight="3" />
	        </Degrafa:stroke>
        </Degrafa:Path>
    </Degrafa:GeometryComposition>
 
    <mx:Canvas id="cnv" />
</mx:Application>

MXML Graphics (aka FXG) in Flex 4:

<?xml version="1.0" encoding="utf-8"?>
<s:Application
        xmlns:fx="http://ns.adobe.com/mxml/2009"
        xmlns:s="library://ns.adobe.com/flex/spark"
        xmlns:mx="library://ns.adobe.com/flex/halo">
 
    <s:Graphic>
        <s:Path data="M 0,0 L 100,0 L 100,100 L 0,100 z">
            <s:fill>
                <mx:SolidColor color="#EECCEE" />
            </s:fill>
            <s:stroke>
                <mx:SolidColorStroke color="#FF00FF" weight="3" />
            </s:stroke>
        </s:Path>
    </s:Graphic>
</s:Application>

The examples above use the shorthand path syntax to draw a 100px square that starts at the coordinate origin (0,0), which is the upper left corner in SVG and Flex. Beware the coordinate origin when translating Inkscape SVG to Flex. The other interesting thing to note is the amazing similarity between Degrafa and FXG. Who knew all my time learning Degrafa will instantly translate to Flex 4 and FXG? Awesome!

SVG Path Shorthand

Here’s a quick overview of shorthand syntax for SVG path data:


Move
M <x,y>
Move the pen to the given point.

Line
L <x,y>+
Draw a line to given point. Multiple points may be specified to draw polyline.

Horizontal Line
H <x>
Draw a horizontal line to given coordinate.

Vertical Line
V <y>
Draw a vertical line to given coordinate.

Quadratic Bezier
Q <cx,cy x,y>+
Draw a quadratic Bezier curve to given coordinate using a control point. Multiple Beziers may be specified to draw polycurve.

Cubic Bezier
C <cx1,cy1 cx2,cy2 x,y>+
Draw a cubic Bezier curve to given coordinate using two control points. Multiple Beziers may be specified to draw a polycurve.

Arc
A <rx,ry rot,lrg,swp x,y>
Draw elliptical arc to the given point.

Close
Z
Close the path.

Alas, the beta version of FXG does not support the Arc segment type, which I suspect is due to lack of support for arbitrary arcs in the underlying Flash Player rendering engine but I don’t know for sure. Thankfully, Degrafa offers full arc support (thanks Greg!). If you really need to draw arcs in FXG, for stuff like pie wedges, and are unafraid of getting into a cage match with your trigonometry textbook, you can do a good job approximating arcs with cubic Bezier curves. Alternately, you can just use Degrafa once it gets ported to Flex 4. Lastly, using uppercase for the segment type specifies absolute coordinates. This is the format commonly used by Illustrator and Inkscape when exporting drawings to SVG. One can easily switch to relative coordinates by just switching the commands to lowercase, but I would try to avoid it if at all possible as it tends to make one’s head hurt.

The Many Shapes of a Square

All of the squares above, use this shorthand data:

M 0,0 L 100,0 L 100,100 L 0,100 z

First, a Move to set the pen at the origin. Then, a Line right to (100,0), followed by a Line down to (100,100), followed by a Line left to (0,100). Then, a close (z) to return to the origin.

I can drop the commas if I want:

M 0 0 L 100 0 L 100 100 L 0 100 z

Or drop all but the first Line to make a polyline:

M 0,0 L 100,0 100,100 0,100 z

Or use Horizontal Line and Vertical Line:

M 0,0 H 100 V 100 H 0 z

Or even use relative coordinate (which makes my head hurt a little):

m 0,0 l 100,0 l 0,100 l -100,0 z
Curves

Straight lines are cool, but the real fun in life lies in the curves. Cubic Bezier curves should be very familiar to anyone who’s used a vector drawing program. Let’s replace the first segment in our square with a cubic Bezier segment. Now, the SVG shorthand becomes:

M 0,0 C 25,-25 50,25 100,0 L 100,100 L 0,100 z

When rendered, we get this:

square-funny

The shorthand command says curve to (100,0), but start out heading towards control point #1 at (25,-25) and end up coming in from control point #2 at (50,25).

Arcs

Again, let’s replace the first segment in our square with an arc segment. Now, the SVG shorthand becomes:

M 0,0 A 50,25 0 0,1 100,0 L 100,100 L 0,100 z

When rendered, we get this:

square-arc

The shorthand command says arc to (100,0), with an x-radius of 50 and a y-radius of 25, with a rotation of 0. The large-arc and sweep flags are a little confusing so you’ll want to review the SVG Spec, § 8.3.8 if you need to get down and dirty with arcs.

Conclusion

I’m a firm believer in “right tool for the job.” So, when in comes to getting SVG path data into Flex, I’m definitely going to use Illustrator or Inkscape as much as possible, and in the future I might just use Catalyst for everything. But there are a few important situations where the Flex developer absolutely must know SVG. First, if you want to do any kind of path morphing (like this), you’ll need precision control over your path segments. And second, if you want to do any dynamic path generation (like building a multi-level radial menu on the fly – which sounds like a good topic for a future post), you’ll need to manually construct your SVG paths.

Files

 6.3

Going from SVG data to a Degrafa Path couldn’t be easier: just copy & paste. You can watch this video tutorial or you can check out this demo.

But there is one trick for Inkscape: even though the coordinate origin on the Inkscape document is the normal cartesian origin in the bottom left and the y-axis points up, the SVG output always uses the upper left corner of the document as the origin and the y-axis points down (per the SVG Spec, § 7.3).

To demonstrate, I created a new document in Inkscape, set my dimensions to 500 x 500, and placed a simple path (which happens to be a square) in the upper left corner:

square

You can see by the rulers in Inkscape that the square’s origin is at (0,500).

If we save our square and examine the SVG output, we see:

<svg ...>
  <g ...>
    <path d="M 0,0 L 100,0 L 100,100 L 0,100 L 0,0 z" />
  </g>
</svg>

If we ignore everything in the file except the relevant path data, we can see the very first path command is M 0,0 which is path-speak for move to (0,0). This is exactly as expected from the SVG spec: upper left is the coordinate origin. The cartesian origin in Inkscape is bogus!

Next, we can just copy the path data from the SVG file and paste it into the data attribute of a Degrafa Path component.

<Degrafa:Path data="M 0,0 L 100,0 L 100,100 L 0,100 L 0,0 z">
    <Degrafa:fill>
        <Degrafa:SolidFill color="#EECCEE" />
    </Degrafa:fill>
    <Degrafa:stroke>
        <Degrafa:SolidStroke color="#FF00FF" weight="3" />
    </Degrafa:stroke>
</Degrafa:Path>

Give it a fill color and a stroke color, and we get a pretty purple square. Now I know my Degrafa Path component will have a square in the upper left, because I know my square was in the upper left in Inkscape. Nice and easy.

Files

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