issa is an image summary string module that produces a small and very compact representation of a source image. Image summaries produced by issa are expected to be a tiny fraction of the source image size at default downsampling level (16).

This makes issa image summaries suitable for using as a rich image previews that can be embedded with JSON/HTML payload before actual images are requested. Initial analysis suggested that for a 400Kb image, issa would produce a 3Kb base64 encoded summary, that would take about 2Kb storage space dehydrated. Hydration doesn't take a lot of computational resources and can be done on the critical path.

Space savings are achieved by two factors:

• issa uses a standard 255 palette and maps all source image colors to that palette. Combined with sampling this is (obviously) the main source of the reduction
• Additionally, having a standard palette and using GIF image allows issa to remove a reproducible header of the resulting base64 string, that is about 20% of the typical GIF image

Typical workflow of an app using issa has the following steps:

• Summarize, dehydrate and store source images into an image summary
• Dehydrate images as needed and embed into HTML/JSON payload

• Load source image
• Convert source image into paletted GIF
• Dehydrate paletted GIF

NOTE: example below oversimplifies error handling for brevity

package main

import (
	"github.com/boggydigital/issa"
	"fmt"
	"image"
	_ "image/jpeg"
	"os"
)

func main() {
	fi, err := os.Open("image.jpeg")
	if err != nil {panic(err)}
	defer fi.Close()

	jpegImage, _, err := image.Decode(fi)
	if err != nil {panic(err)}

	gifImage := issa.GIFImage(
		jpegImage,
		issa.StdPalette(),
		issa.DefaultDownSampling)

	dhi, err := issa.Dehydrate(gifImage)
	if err != nil {panic(err)}

	fmt.Println(dhi)
}

• Load dehydrated string
• Hydrate it

package main

import (
	"github.com/boggydigital/issa"
	"fmt"
)

func main() {

	dhi := ""
	// load dehydrated string into dhi from the store

	hi := issa.Hydrate(dhi)

	fmt.Printf(hi)
}

Source image along with an issa (from https://en.wikipedia.org/wiki/Semaeopus):

Dehydrated image string at the default downsampling level:

25=19=ItACtCRxIsKDBgwgTKly4ENATQM8YQsuXLl/BJwBIqCBlbRU0gtBWWcvnhASAJxdJqESSZ5VLgS6f5UGiEkCVgoAykkBCs96qfC5JOeGpkgSggs+KEiWRp0qePCtpqoxYsJ7SlRmlSiVR7yA0QFGdlATAcykgaB8LQnuiFInYt0uroD3ItijXplDtokxb0J5OEk5kplu1FcBRvgXzch1pUbHchfmcIn725AkpxAwza97MufPmgAA7

Hydrated image string at the default downsampling level (copy and paste as an URL to check out):

data:image/gif;base64,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