downsize

Reduces an image to a specified file size in bytes. Also command line tool available.

Installation

$ go get -u github.com/lelenanam/downsize

Usage

import "github.com/lelenanam/downsize"

The downsize package provides a function downsize.Encode:

func Encode(w io.Writer, m image.Image, o *Options) error 

This function:

• takes any image type that implements image.Image interface as an input m
• reduces an image's dimensions to achieve a specified file size Options.Size in bytes
• writes result Image m to writer w with the given options
• default parameters are used if a nil *Options is passed

// Options are the encoding parameters.
type Options struct {
	// Size is desired output file size in bytes
	Size int
	// Format is image format to encode
	Format string
	// JpegOptions are the options for jpeg format
	JpegOptions *jpeg.Options
	// GifOptions are the options for gif format
	GifOptions *gif.Options
}

By default an image encodes with jpeg format and with the quality DefaultQuality = 80. All metadata is stripped after encoding.

const DefaultQuality = 80
var defaultFormat = "jpeg"
var defaultJpegOptions = &jpeg.Options{Quality: DefaultQuality}
var defaultOptions = &Options{Format: defaultFormat, JpegOptions: defaultJpegOptions}

Example

package main

import (
	"image"
	_ "image/gif"
	_ "image/jpeg"
	_ "image/png"
	"log"
	"os"

	"github.com/lelenanam/downsize"
)

func main() {
	file, err := os.Open("img.png")
	if err != nil {
		log.Fatal(err)
	}
	defer func() {
		if err := file.Close(); err != nil {
			log.Println("Cannot close input file: ", err)
		}
	}()

	img, format, err := image.Decode(file)
	if err != nil {
		log.Fatalf("Error: %v, cannot decode file %v", err, file.Name())
	}

	out, err := os.Create("resized.png")
	if err != nil {
		log.Fatal(err)
	}
	defer func() {
		if err := out.Close(); err != nil {
			log.Println("Cannot close output file: ", err)
		}
	}()

	opt := &downsize.Options{Size: 1048576, Format: format}
	if err = downsize.Encode(out, img, opt); err != nil {
		log.Fatalf("Error: %v, cannot downsize image to size: %v", err, opt.Size)
	}
}

Sample

The original jpeg image 2.4 MB:

Downsize to 200 KB, png format and default quality for result image:

opt := &downsize.Options{Size: 204800, Format: "png"}
err = downsize.Encode(out, img, opt)

Resized result 200 KB:

Downsize to 200 KB, jpeg format and default quality for result image:

opt := &downsize.Options{Size: 204800, Format: "jpeg"}
err = downsize.Encode(out, img, opt)

Resized result 200 KB:

Downsize to 200 KB, jpeg format and quality 50 for result image:

opt := &downsize.Options{Size: 204800, Format: "jpeg", JpegOptions: &jpeg.Options{Quality: 50}}
err = downsize.Encode(out, img, opt)

Resized result 200 KB, quality 50:

The original image 3.4 MB:

Downsize to 100 KB, auto determine format and default quality for result image:

opt := &downsize.Options{Size: 102400}
err = downsize.Encode(out, img, opt)

Resized result 100 KB:

Downsize to 100 KB, auto determine format and quality 50 for result image:

opt := &downsize.Options{Size: 102400, JpegOptions: &jpeg.Options{Quality: 50}}
err = downsize.Encode(out, img, opt)

Resized result 100 KB, quality 50:

Downsize to 50 KB, auto determine format and default duality for result image:

opt := &downsize.Options{Size: 51200}
err = downsize.Encode(out, img, opt)

Resized result 50 KB:

License

MIT License