SVG Output

SVG output

Code: src/SvgCreator/Svg/SvgWriter.cs

Structure

<svg width="W" height="H" viewBox="0 0 W H" xmlns="http://www.w3.org/2000/svg">
  <g fill="#000" fill-rule="evenodd" stroke="none">
    <path d="M…Z M…Z" />   <!-- one path per component: outer border + holes -->
  </g>
</svg>

• Shared attributes. Fill, fill rule and stroke are set once on the <g> rather than on every path.
• Safe XML. The document is written with XmlWriter, which escapes attribute values. An injected FillColor such as "><script> can't break out of the attribute; there's a test for this.
• No XML declaration. It's omitted because the output is a .NET string (UTF-16), and a declaration would state the wrong encoding once the file is saved as UTF-8.

Styles are layers

SvgConverter runs in two halves: LineArtTracer turns the image into a TracedDrawing (canvas size plus fitted outlines), and SvgWriter renders it. How it looks is an SvgStyle, built from two kinds of layer:

Layer	Interface	What it does	Example
Ink filter (0 or 1)	IInkFilter	A <filter> in <defs>, applied to the ink group and to every decoration, so everything looks printed the same way	StampFilter
Decorations (0 or more)	IDecoration	Generated (not traced) artwork in its own <g> after the ink. It can ask for a wider canvas	PostmarkCancellation

An OutputStyle is just a named combination of layers (SvgStyle.For): Stamp is the stamp filter alone, and Postmark is the stamp filter plus the cancellation decoration.

Where new artwork goes. Ask what the canvas is:

The new artwork…	Goes in	Example
is drawn over a traced drawing, on that drawing's canvas	A new IDecoration plus one line in SvgStyle.For. SvgWriter doesn't change	PostmarkCancellation; a straight-bar or slogan cancellation would go here too
defines its own canvas, or doesn't start from a traced drawing	Its own generator, writing through SvgWriter.WriteDocument with SvgStyle.Clean	The postage stamp: its canvas is the perforated paper, and its picture isn't traced. The envelope: no input image at all
combines several finished pieces	The composer (Composition/), which nests each generator's output as an inner <svg>	The mail piece: a stamp with a postmark over it, on an envelope

So a perforated edge is never a decoration. It belongs to the stamp's paper, and the stamp generator owns that.

Golden tests. SvgGoldenTests pins the exact SVG text of every style in tests/SvgCreator.Tests/Golden/, so a refactor can't change the output unnoticed. After an intentional output change, run UPDATE_GOLDEN=1 dotnet test and review the diff of those files.

Lesson: curves made the file bigger at first

After curve fitting, the segment count dropped by 45%, but the files roughly doubled in size. A cubic C command has 6 numbers to an L command's 2, and they were printed with 2 decimals each.

Version	image03 size
Polygon L commands, integer coordinates	13.3 KB
Absolute C commands, 2 decimals	28.7 KB
Relative c/l commands, 0.1 px precision	17.5 KB

Compact path encoding

1. Relative commands. Each subpath starts with an absolute M, then lowercase c/l with deltas from the previous point, which are small numbers with few digits.
2. Tenths precision. 0.1 px is invisible at any normal zoom. This is for traced geometry and path data, which are measured in image pixels. Values computed as fractions of a picture, such as everything the postage stamp writes (its paper path included), use up to four decimals instead (SvgFormat.Number), because they can be far below 0.1 px on a small picture.
3. No rounding drift. Coordinates are rounded to integer tenths first, and each delta is taken between rounded points. If you rounded the deltas themselves, the errors would pile up along a long path.
4. Shortest number form. 0.5 → .5, -0.5 → -.5. A space is only inserted where a digit would otherwise run into the next number; a minus sign separates numbers by itself.
5. Implicit closing line. If the last segment is a straight line back to the start, it's left out, because Z draws it.
6. Straight curves become lines. A cubic whose control points lie within 0.1 px of the chord is written as l.

Example: M2 1c1.3 4.7 10 5 10 .5c0-3.5-9.5-3.5-10-.5Z