flagicon.go

/*
Package flagicon provides a function to generate flag icons from image files

By default the flag icon will be of size 14x14 pixels; the original image will
be resized to this size, with a 2px border with color #808080.

If the image is not square, flagicon first crops the image (centered at the center
of the image) such that it is square before it resizes the image.

*/

package main

import (
	"github.com/alexflint/go-arg"
	"github.com/fogleman/gg"
	"github.com/nfnt/resize"
	"github.com/oliamb/cutter"
	"image"
	"log"
	"math"
)

// resizeImage resizes the base image to a 14x14 square used for the flagicon
// If the base image is not square, resizeImage first crops the image into a
// square shape, centered on the center of the image.
func resizeImage(base image.Image, left bool) image.Image {
	if base.Bounds().Dx() != base.Bounds().Dy() {
		newHeight := int(math.Min(float64(base.Bounds().Dx()), float64(base.Bounds().Dy())))
		if left {
			cropped, err := cutter.Crop(base, cutter.Config{
				Width:  newHeight,
				Height: newHeight,
			})
			if err != nil {
				log.Fatal(err)
			}
			return resizeToThumbnail(cropped)

		} else {
			cropped, err := cutter.Crop(base, cutter.Config{
				Width:  newHeight,
				Height: newHeight,
				Mode:   cutter.Centered,
			})
			if err != nil {
				log.Fatal(err)
			}
			return resizeToThumbnail(cropped)
		}
	}
	return resizeToThumbnail(base)
}

// resizeToThumbnail resizes the base image to a 14x14 square
func resizeToThumbnail(base image.Image) image.Image {
	return resize.Thumbnail(uint(14), uint(14), base, resize.Bilinear)
}

func main() {
	var args struct {
		Input  string `arg:"positional" help:"Path to input image file"`
		Output string `arg:"-o" help:"Output file name"`
		Left   bool   `arg:"-l" help:"Generate flagicon from left-side of image, rather than the center"`
	}
	args.Output = "out.png"
	args.Left = false
	arg.MustParse(&args)

	im, err := gg.LoadImage(args.Input)
	if err != nil {
		log.Fatal(err)
	}
	image := resizeImage(im, args.Left)

	dc := gg.NewContext(14, 14)

	// Clip and draw circle image
	dc.DrawCircle(7, 7, 7)
	dc.Clip()
	dc.DrawImageAnchored(image, 7, 7, 0.5, 0.5)

	// Draw image border
	dc.DrawCircle(7, 7, 7)
	dc.SetHexColor("#808080")
	dc.SetLineWidth(2)
	dc.Stroke()

	dc.SavePNG(args.Output)
}