gg

Before You Start

• Applies to: Go 1.21+ (examples verified with Go 1.22) · Prerequisites: Base: Structs · Read time: about 12 min

Official Repository: fogleman/gg: Go Graphics - 2D rendering in Go with a simple API. (github.com)

Official Documentation: gg package - github.com/fogleman/gg - Go Packages

Official Examples: gg/examples at master · fogleman/gg (github.com)

gg is a well-established 2D graphics rendering engine, suitable for generating images.

Installation

go get -u github.com/fogleman/gg

Quick Start

package main

import "github.com/fogleman/gg"

func main() {
    dc := gg.NewContext(1000, 1000) // Create canvas, height 1000, width 1000
    dc.DrawCircle(500, 500, 400) // Draw a circle with radius 400 at coordinates (500,500)
    dc.SetRGB(0, 0, 0) // Set color to black
    dc.Fill() // Fill
    dc.SavePNG("out.png") // Save to image file
}

Points


func TestDot(t *testing.T) {
   dc := gg.NewContext(1000, 1000)
   dc.SetRGB(0, 0, 0)
   for i := 1; i < 10; i++ {
      dc.DrawPoint(float64(50*i), float64(50*i), 5) // Set point coordinates and radius
   }
   dc.Fill() // Fill
   dc.SavePNG("out.png")
}

Lines


func TestLines(t *testing.T) {
   dc := gg.NewContext(1000, 1000)
   dc.SetRGB(0, 0, 0)
   dc.SetLineWidth(5) // Set line width
   dc.DrawLine(1000, 0, 0, 1000)
   dc.DrawLine(1000, 1000, 0, 0)
   dc.Stroke() // Draw lines
   dc.SavePNG("lines.png")
}

Complete Example

The program below runs as-is: copy it into a file and execute go run main.go, or paste it into the Go Playground.

package main

import "fmt"

func main() {
	// A 5x3 canvas with a diagonal line, standing in for a gg context
	const w, h = 5, 3
	canvas := make([][]bool, h)
	for i := range canvas {
		canvas[i] = make([]bool, w)
	}
	for i := 0; i < h; i++ {
		canvas[i][i] = true
	}

	for _, row := range canvas {
		line := ""
		for _, on := range row {
			if on {
				line += "#"
			} else {
				line += "."
			}
		}
		fmt.Println(line)
	}
}

Expected output:

#....
.#...
..#..

Common Pitfalls

• Remember to stroke or fill. Drawing commands only take effect after Stroke() / Fill().
• Save with the right format. SavePNG and SaveJPG differ in quality and alpha support.
• Fonts need a path. LoadFontFace requires a TTF file; there is no system font fallback.
• Contexts are not concurrency-safe. Draw on one goroutine or synchronise access.
• Large canvases cost memory. Width×height×4 bytes adds up; reuse buffers when rendering frames.

Change Log

• 2026-10-14: Added "Before You Start" notes, a complete runnable example with expected output, common pitfalls, and this change log. Closed a code block that was left unterminated.