Go Graphics

gg is a library for rendering 2D graphics in pure Go.

git.sr.ht/~sbinet/gg is a fork of fogleman/gg which doesn't seem to be maintained (as of January 2022).

Installation

go get -u git.sr.ht/~sbinet/gg

Documentation

• godoc: https://godoc.org/git.sr.ht/~sbinet/gg
• pkg.go.dev: https://pkg.go.dev/git.sr.ht/~sbinet/gg?tab=doc

Hello, Circle!

Look how easy!

func Example_circle() {
	dc := gg.NewContext(1000, 1000)
	dc.DrawCircle(500, 500, 400)
	dc.SetRGB(0, 0, 0)
	dc.Fill()

	err := dc.SavePNG("testdata/circle.png")
	if err != nil {
		log.Fatalf("could not save to file: %+v", err)
	}
}

Examples

There are lots of examples included. They're mostly for testing the code, but they're good for learning, too.

Creating Contexts

There are a few ways of creating a context.

NewContext(width, height int) *Context
NewContextForImage(im image.Image) *Context
NewContextForRGBA(im *image.RGBA) *Context

Drawing Functions

Ever used a graphics library that didn't have functions for drawing rectangles or circles? What a pain!

DrawPoint(x, y, r float64)
DrawLine(x1, y1, x2, y2 float64)
DrawRectangle(x, y, w, h float64)
DrawRoundedRectangle(x, y, w, h, r float64)
DrawCircle(x, y, r float64)
DrawArc(x, y, r, angle1, angle2 float64)
DrawEllipse(x, y, rx, ry float64)
DrawEllipticalArc(x, y, rx, ry, angle1, angle2 float64)
DrawRegularPolygon(n int, x, y, r, rotation float64)
DrawImage(im image.Image, x, y int)
DrawImageAnchored(im image.Image, x, y int, ax, ay float64)
SetPixel(x, y int)

MoveTo(x, y float64)
LineTo(x, y float64)
QuadraticTo(x1, y1, x2, y2 float64)
CubicTo(x1, y1, x2, y2, x3, y3 float64)
ClosePath()
ClearPath()
NewSubPath()

Clear()
Stroke()
Fill()
StrokePreserve()
FillPreserve()

It is often desired to center an image at a point. Use DrawImageAnchored with ax and ay set to 0.5 to do this. Use 0 to left or top align. Use 1 to right or bottom align. DrawStringAnchored does the same for text, so you don't need to call MeasureString yourself.

Text Functions

It will even do word wrap for you!

DrawString(s string, x, y float64)
DrawStringAnchored(s string, x, y, ax, ay float64)
DrawStringWrapped(s string, x, y, ax, ay, width, lineSpacing float64, align Align)
MeasureString(s string) (w, h float64)
MeasureMultilineString(s string, lineSpacing float64) (w, h float64)
WordWrap(s string, w float64) []string
SetFontFace(fontFace font.Face)
LoadFontFace(path string, points float64) error
LoadFontFaceFromBytes(raw []byte, points float64) error
LoadFontFaceFromFS(fsys fs.FS, path string, points float64) error

Color Functions

Colors can be set in several different ways for your convenience.

SetRGB(r, g, b float64)
SetRGBA(r, g, b, a float64)
SetRGB255(r, g, b int)
SetRGBA255(r, g, b, a int)
SetColor(c color.Color)
SetHexColor(x string)

Stroke & Fill Options

SetLineWidth(lineWidth float64)
SetLineCap(lineCap LineCap)
SetLineJoin(lineJoin LineJoin)
SetDash(dashes ...float64)
SetDashOffset(offset float64)
SetFillRule(fillRule FillRule)

Gradients & Patterns

gg supports linear, radial and conic gradients and surface patterns. You can also implement your own patterns.

SetFillStyle(pattern Pattern)
SetStrokeStyle(pattern Pattern)
NewSolidPattern(color color.Color)
NewLinearGradient(x0, y0, x1, y1 float64)
NewRadialGradient(x0, y0, r0, x1, y1, r1 float64)
NewConicGradient(cx, cy, deg float64)
NewSurfacePattern(im image.Image, op RepeatOp)

Transformation Functions

Identity()
Translate(x, y float64)
Scale(x, y float64)
Rotate(angle float64)
Shear(x, y float64)
ScaleAbout(sx, sy, x, y float64)
RotateAbout(angle, x, y float64)
ShearAbout(sx, sy, x, y float64)
TransformPoint(x, y float64) (tx, ty float64)
InvertY()

It is often desired to rotate or scale about a point that is not the origin. The functions RotateAbout, ScaleAbout, ShearAbout are provided as a convenience.

InvertY is provided in case Y should increase from bottom to top vs. the default top to bottom.

Stack Functions

Save and restore the state of the context. These can be nested.

Push()
Pop()

Clipping Functions

Use clipping regions to restrict drawing operations to an area that you defined using paths.

Clip()
ClipPreserve()
ResetClip()
AsMask() *image.Alpha
SetMask(mask *image.Alpha)
InvertMask()

Helper Functions

Sometimes you just don't want to write these yourself.

Radians(degrees float64) float64
Degrees(radians float64) float64
LoadImage(path string) (image.Image, error)
LoadPNG(path string) (image.Image, error)
SavePNG(path string, im image.Image) error

Another Example

See the output of this example below.

func Example_ellipse() {
	const S = 1024
	dc := gg.NewContext(S, S)
	dc.SetRGBA(0, 0, 0, 0.1)
	for i := 0; i < 360; i += 15 {
		dc.Push()
		dc.RotateAbout(gg.Radians(float64(i)), S/2, S/2)
		dc.DrawEllipse(S/2, S/2, S*7/16, S/8)
		dc.Fill()
		dc.Pop()
	}

	im, err := gg.LoadImage("testdata/gopher.png")
	if err != nil {
		panic(err)
	}
	dc.DrawImageAnchored(im, S/2, S/2, 0.5, 0.5)

	err = dc.SavePNG("testdata/ellipse.png")
	if err != nil {
		log.Fatalf("could not save to file: %+v", err)
	}
}