Package vggio provides a vg.Canvas implementation backed by Gio, a toolkit that implements portable immediate GUI mode in Go.

More informations about Gio can be found at




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const DefaultDPI = 96

DefaultDPI is the default dot resolution for image drawing in dots per inch.


This section is empty.


func UseBackgroundColor

func UseBackgroundColor(c color.Color) option

UseBackgroundColor specifies the image background color. Without UseBackgroundColor, the default color is white.

func UseDPI

func UseDPI(dpi int) option

UseDPI sets the dots per inch of a canvas. It should only be used as an option argument when initializing a new canvas.


type Canvas

type Canvas struct {
	// contains filtered or unexported fields

Canvas implements the vg.Canvas interface, drawing to an image.Image using vgimg and painting that image into a Gioui context.

package main

import (


func main() {
	const (
		w   = 20 * vg.Centimeter
		h   = 15 * vg.Centimeter
		dpi = 96
	go func(w, h vg.Length) {
		win := app.NewWindow(
		defer win.Close()

		done := time.NewTimer(2 * time.Second)
		defer done.Stop()

		for e := range win.Events() {
			switch e := e.(type) {
			case system.FrameEvent:
				p := plot.New()
				p.Title.Text = "My title"
				p.X.Label.Text = "X"
				p.Y.Label.Text = "Y"

				quad := plotter.NewFunction(func(x float64) float64 {
					return x * x
				quad.Color = color.RGBA{B: 255, A: 255}

				exp := plotter.NewFunction(func(x float64) float64 {
					return math.Pow(2, x)
				exp.Dashes = []vg.Length{vg.Points(2), vg.Points(2)}
				exp.Width = vg.Points(2)
				exp.Color = color.RGBA{G: 255, A: 255}

				sin := plotter.NewFunction(func(x float64) float64 {
					return 10*math.Sin(x) + 50
				sin.Dashes = []vg.Length{vg.Points(4), vg.Points(5)}
				sin.Width = vg.Points(4)
				sin.Color = color.RGBA{R: 255, A: 255}

				p.Add(quad, exp, sin)
				p.Legend.Add("x^2", quad)
				p.Legend.Add("2^x", exp)
				p.Legend.Add("10*sin(x)+50", sin)
				p.Legend.ThumbnailWidth = 0.5 * vg.Inch

				p.X.Min = 0
				p.X.Max = 10
				p.Y.Min = 0
				p.Y.Max = 100


				gtx := layout.NewContext(new(op.Ops), e)
				cnv := vggio.New(gtx, w, h, vggio.UseDPI(dpi))


			case key.Event:
				switch e.Name {
				case "Q", key.NameEscape:

			case system.DestroyEvent:
	}(w, h)


func New

func New(gtx layout.Context, w, h vg.Length, opts ...option) *Canvas

New returns a new image canvas with the provided dimensions and options. The currently accepted options are UseDPI and UseBackgroundColor. If the resolution or background color are not specified, defaults are used.

func (*Canvas) DPI

func (c *Canvas) DPI() float64

DPI returns the resolution of the receiver in pixels per inch.

func (*Canvas) DrawImage

func (c *Canvas) DrawImage(rect vg.Rectangle, img image.Image)

DrawImage draws the image, scaled to fit the destination rectangle.

func (*Canvas) Fill

func (c *Canvas) Fill(p vg.Path)

Fill fills the given path.

func (*Canvas) FillString

func (c *Canvas) FillString(fnt font.Face, pt vg.Point, txt string)

FillString fills in text at the specified location using the given font. If the font size is zero, the text is not drawn.

func (*Canvas) Paint

func (c *Canvas) Paint() *op.Ops

Paint returns the painting operations.

func (*Canvas) Pop

func (c *Canvas) Pop()

Pop restores the context saved by the corresponding call to Push().

func (*Canvas) Push

func (c *Canvas) Push()

Push saves the current line width, the current dash pattern, the current transforms, and the current color onto a stack so that the state can later be restored by calling Pop().

func (*Canvas) Rotate

func (c *Canvas) Rotate(rad float64)

Rotate applies a rotation transform to the context. The parameter is specified in radians.

func (*Canvas) Scale

func (c *Canvas) Scale(x, y float64)

Scale applies a scaling transform to the context.

func (*Canvas) Screenshot

func (c *Canvas) Screenshot() (image.Image, error)

Screenshot returns a screenshot of the canvas as an image.

func (*Canvas) SetColor

func (c *Canvas) SetColor(clr color.Color)

SetColor sets the current drawing color. Note that fill color and stroke color are the same, so if you want different fill and stroke colors then you must set a color, draw fills, set a new color and then draw lines.

The initial color is black. If SetColor is called with a nil color then black is used.

func (*Canvas) SetLineDash

func (c *Canvas) SetLineDash(pattern []vg.Length, offset vg.Length)

SetLineDash sets the dash pattern for lines. The pattern slice specifies the lengths of alternating dashes and gaps, and the offset specifies the distance into the dash pattern to start the dash.

The initial dash pattern is a solid line.

func (*Canvas) SetLineWidth

func (c *Canvas) SetLineWidth(w vg.Length)

SetLineWidth sets the width of stroked paths. If the width is not positive then stroked lines are not drawn.

The initial line width is 1 point.

func (*Canvas) Size

func (c *Canvas) Size() (w, h vg.Length)

Size implement vg.CanvasSizer.

func (*Canvas) Stroke

func (c *Canvas) Stroke(p vg.Path)

Stroke strokes the given path.

func (*Canvas) Translate

func (c *Canvas) Translate(pt vg.Point)

Translate applies a translational transform to the context.