Canvas is a common vector drawing target that can output SVG, PDF, EPS and raster images (which can be saved as PNG, JPG, ...). It has a wide range of path manipulation functionality such as flattening, stroking and dashing implemented. Additionally, it has a good text formatter and embeds fonts (TTF, OTF or WOFF) or converts them to outlines. It can be considered a Cairo or node-canvas alternative in Go. See the example below in Fig. 1 and Fig. 2 for an overview of the functionality.

Figure 1: top-left you can see text being fitted into a box and their bounding box (orange-red), the spaces between the words on the first row are being stretched to fill the whole width. You can see all the possible styles and text decorations applied. Also note the typographic substitutions (the quotes) and ligature support (fi, ffi, ffl, ...). Below the text box, the word "stroke" is being stroked and drawn as a path. Top-right we see a LaTeX formula that has been converted to a path. Left of that we see ellipse support showcasing precise dashing, notably the length of e.g. the short dash is equal wherever it is (approximated through arc length parametrization) on the curve. It also shows support for alternating dash lengths, in this case (2.0, 4.0, 2.0) for dashes and for spaces. Note that the dashes themselves are elliptical arcs as well (thus exactly precise even if magnified greatly). In the bottom-right we see a closed polygon of four points being smoothed by cubic Béziers that are smooth along the whole path, and next to it on the left an open path. In the middle you can see a rasterized image painted.

Figure 2abc: Three examples of what is possible with this library, for example the plotting of graphs, maps and documents.

Terminology: a path is a sequence of drawing commands (MoveTo, LineTo, QuadTo, CubeTo, ArcTo, Close) that completely describe a path. QuadTo and CubeTo are quadratic and cubic Béziers respectively, ArcTo is an elliptical arc, and Close is a LineTo to the last MoveTo command and closes the path (sometimes this has a special meaning such as when stroking). A path can consist of several path segments by having multiple MoveTos, Closes, or the pair of Close and MoveTo. Flattening is the action of converting the QuadTo, CubeTo and ArcTo commands into LineTos.

Articles

• Numerically stable quadratic formula
• Quadratic Bézier length
• Bézier spline through open path
• Bézier spline through closed path
• Point inclusion in polygon test

My own

• Arc length parametrization

Papers

• M. Walter, A. Fournier, Approximate Arc Length Parametrization, Anais do IX SIBGRAPHI (1996), p. 143--150
• T.F. Hain, A.L. Ahmad, S.V.R. Racherla, D.D. Langan, Fast, precise flattening of cubic Bézier path and offset curves, Computers & Graphics 29 (2005). p. 656--666
• M. Goldapp, Approximation of circular arcs by cubic polynomials, Computer Aided Geometric Design 8 (1991), p. 227--238
• Drawing and elliptical arc using polylines, quadratic or cubic Bézier curves (2003), L. Maisonobe

Status

Targets

Feature	Image	SVG	PDF	EPS
Draw path fill	yes	yes	yes	yes
Draw path stroke	yes	yes	yes	no
Draw path dash	yes	yes	yes	no
Embed fonts		yes	yes (TTF & OTF)	no
Draw text		yes	yes	as path
Draw image	yes	yes	yes	no

• EPS does not support transparency
• PDF and EPS do not support line joins for last and first dash for dashed and closed path

Path

Command	Flatten	Stroke	Length	SplitAt
LineTo	yes	yes	yes	yes
QuadTo	yes (CubeTo)	yes (CubeTo)	yes	yes (GL5 + Chebyshev10)
CubeTo	yes	yes	yes (GL5)	yes (GL5 + Chebyshev10)
ArcTo	yes	yes	yes (GL5)	yes (GL5 + Chebyshev10)

• Ellipse => Cubic Bézier: used by rasterizer and PDF targets (see Maisonobe paper)

NB: GL5 means a Gauss-Legendre n=5, which is an numerical approximation as there is no analytical solution. Chebyshev is a converging way to approximate a function by an n=10 degree polynomial. It uses the bisection method as well to determine the polynomial points.

Planning

Features that are planned to be implemented in the future, with important issues in bold. Also see the TODOs in the code.

General

• Fix slowness in the rasterizer (text_example.go is slow! use rasterized cache for each glyph/path)
• Add targets such as OpenGL and HTML5 Canvas (consider WASM output)

Fonts

• Compressing fonts and embedding only used characters
• Use ligature tables
• Font embedding for EPSs
• Support WOFF2 font format
• Support Type1 font format?
• Support font hinting (for the rasterizer)?
• Support LaTeX and other paths as text spans?

Paths

• Intersection function between line, Bézier and ellipse and between themselves (for path merge, overlap/mask, clipping, etc.)
• Implement Bentley-Ottmann algorithm to find all line intersections (clipping)
• Get position and derivative/normal at length L along the path
• Simplify polygons using the Ramer-Douglas-Peucker algorithm

Optimization

• Avoid overlapping paths when offsetting in corners (needs path intersection code)
• Approximate Béziers by elliptic arcs instead of lines when stroking, if number of path elements is reduced by more than 2 times (unsure if worth it)

Far future

• Support fill gradients and patterns (hard)
• Load in PDFs, SVGs and EPSs and turn to paths/texts
• Load in Markdown/HTML formatting and turn into texts

Canvas

c := canvas.New(width, height float64)
c.PushState()          // save state set by function below on the stack
c.PopState()           // pop state from the stack
c.SetView(Matrix)      // set view transformation, all drawn elements are transformed by this matrix
c.ComposeView(Matrix)  // add transformation after the current view transformation
c.ResetView()          // use identity transformation matrix
c.SetFillColor(color.Color)
c.SetStrokeColor(color.Color)
c.SetStrokeCapper(Capper)
c.SetStrokeJoiner(Joiner)
c.SetStrokeWidth(width float64)
c.SetDashes(offset float64, lengths ...float64)

c.DrawPath(x, y float64, *Path)
c.DrawText(x, y float64, *Text)
c.DrawImage(x, y float64, image.Image, ImageEncoding, dpm float64)

c.Fit(margin float64)  // resize canvas to fit all elements with a given margin
c.WriteSVG(w io.Writer)
c.WriteEPS(w io.Writer)
c.WritePDF(w io.Writer)
c.WriteImage(dpm float64) *image.RGBA

Canvas allows to draw either paths, text or images. All positions and sizes are given in millimeters.

Text

dejaVuSerif := NewFontFamily("dejavu-serif")
err := dejaVuSerif.LoadFontFile("DejaVuSerif.ttf", canvas.FontRegular)  // TTF, OTF or WOFF
ff := dejaVuSerif.Face(size float64, color.Color, FontStyle, FontVariant, ...FontDecorator)

text = NewTextLine(ff, "string\nsecond line", halign) // simple text line
text = NewTextBox(ff, "string", width, height, halign, valign, indent, lineStretch)  // split on word boundaries and specify text alignment

// rich text allowing different styles of text in one box
richText := NewRichText()  // allow different FontFaces in the same text block
richText.Add(ff, "string")
text = richText.ToText(width, height, halign, valign, indent, lineStretch)

c.DrawText(0.0, 0.0, text)

Paths

A large deal of this library implements functionality for building paths. Any path can be constructed from a few basic operations, see below. The successive commands start from the current pen position (from the previous command's end point) and are drawn towards a new end point. A path can consist of multiple path segments (multiple MoveTos), but be aware that overlapping paths will cancel each other.

p := &Path{}
p.MoveTo(x, y float64)                                            // new path segment starting at (x,y)
p.LineTo(x, y float64)                                            // straight line to (x,y)
p.QuadTo(cpx, cpy, x, y float64)                                  // a quadratic Bézier with control point (cpx,cpy) and end point (x,y)
p.CubeTo(cp1x, cp1y, cp2x, cp2y, x, y float64)                    // a cubic Bézier with control points (cp1x,cp1y), (cp2x,cp2y) and end point (x,y)
p.ArcTo(rx, ry, rot float64, largeArc, sweep bool, x, y float64)  // an arc of an ellipse with radii (rx,ry), rotated by rot (in degrees CCW), with flags largeArc and sweep (booleans, see https://www.w3.org/TR/SVG/paths.html#PathDataEllipticalArcCommands)
p.Arc(rx, ry, rot float64, theta0, theta1 float64)                // an arc of an ellipse with radii (rx,ry), rotated by rot (in degrees CCW), beginning at angle theta0 and ending at angle theta1
p.Close()                                                         // close the path, essentially a LineTo to the last MoveTo location

p = Rectangle(w, h float64)
p = RoundedRectangle(w, h, r float64)
p = BeveledRectangle(w, h, r float64)
p = Circle(r float64)
p = Ellipse(rx, ry float64)
p = RegularPolygon(n int, r float64, up bool)
p = RegularStarPolygon(n, d int, r float64, up bool)
p = StarPolygon(n int, R, r float64, up bool)

We can extract information from these paths using:

p.Empty() bool                 // true if path contains no commands other than MoveTo or Close
p.Pos() (x, y float64)         // current pen position
p.StartPos() (x, y float64)    // position of last MoveTo
p.Coords() []Point             // (start) positions of all commands
p.CCW() bool                   // true if the path is (mostly) counter clockwise
p.Interior(x, y float64) bool  // true if (x,y) is in the interior of the path, ie. gets filled (depends on FillRule)
p.Filling() []bool             // for all path segments, true if the path segment is filling (depends on FillRule)
p.Bounds() Rect                // bounding box of path
p.Length() float64             // length of path in millimeters

These paths can be manipulated and transformed with the following commands. Each will return a pointer to the path.

p = p.Copy()
p = p.Append(q *Path)                 // append path q to p and return a new path
p = p.Join(q *Path)                   // join path q to p and return a new path
p = p.Reverse()                       // reverse the direction of the path
ps = p.Split() []*Path                // split the path segments, ie. at Close/MoveTo
ps = p.SplitAt(d ...float64) []*Path  // split the path at certain lengths d

p = p.Transform(Matrix)               // apply multiple transformations at once and return a new path
p = p.Translate(x, y float64)

p = p.Flatten()                                            // flatten Bézier and arc commands to straight lines
p = p.Offset(width float64)                                // offset the path outwards (width > 0) or inwards (width < 0), depends on FillRule
p = p.Stroke(width float64, capper Capper, joiner Joiner)  // create a stroke from a path of certain width, using capper and joiner for caps and joins
p = p.Dash(offset float64, d ...float64)                   // create dashed path with lengths d which are alternating the dash and the space, start at an offset into the given pattern (can be negative)

p = p.Optimize()  // optimize and shorten path

Polylines

Some operations on paths only work when it consists of line elements only. We can either flatten an existing path or use the command coordinates to create a polyline.

polyline := PolylineFromPath(p)       // create by flattening p
polyline = PolylineFromPathCoords(p)  // create from the command coordinates of p

polyline.Smoothen()              // smoothen it by cubic Béziers
polyline.FillCount() int         // returns the fill count as dictated by the FillRule
polyline.Interior(x, y float64)  // returns true if (x,y) is in the interior of the polyline

Path stroke

Below is an illustration of the different types of Cappers and Joiners you can use when creating a stroke of a path:

LaTeX

To generate outlines generated by LaTeX, you need latex and dvisvgm installed on your system.

p, err := ParseLaTeX(`$y=\sin\(\frac{x}{180}\pi\)$`)
if err != nil {
    panic(err)
}

Where the provided string gets inserted into the following document template:

\documentclass{article}
\begin{document}
\thispagestyle{empty}
{{input}}
\end{document}

Example

See https://github.com/tdewolff/canvas/tree/master/example for a working examples.

License

Released under the MIT license.