Go Resizing Images

I am using the Go resize package here: https://github.com/nfnt/resize

1. I am pulling an Image from S3, as such:

    image_data, err := mybucket.Get(key)
    // this gives me data []byte

2. After that, I need to resize the image:

    new_image := resize.Resize(160, 0, original_image, resize.Lanczos3)
    // problem is that the original_image has to be of type image.Image

3. Upload the image to my S3 bucket

    err : = mybucket.Put('newpath', new_image, 'image/jpg', 'aclstring')
    // problem is that new image needs to be data []byte

How do I transform a data []byte to ---> image.Image and back to ----> data []byte?

Read http://golang.org/pkg/image

// you need the image package, and a format package for encoding/decoding
import (
	"bytes"
	"image"
	"image/jpeg" // if you don't need to use jpeg.Encode, use this line instead 
	// _ "image/jpeg"
    "github.com/nfnt/resize"
    
)
// Decoding gives you an Image.
// If you have an io.Reader already, you can give that to Decode 
// without reading it into a []byte.
image, _, err := image.Decode(bytes.NewReader(data))
// check err
newImage := resize.Resize(160, 0, original_image, resize.Lanczos3)
// Encode uses a Writer, use a Buffer if you need the raw []byte
err = jpeg.Encode(someWriter, newImage, nil)
// check err

The OP is using a specific library/package, but I think that the issue of "Go Resizing Images" can be solved without that package.

You can resize de image using golang.org/x/image/draw:

input, _ := os.Open("your_image.png")
defer input.Close()
output, _ := os.Create("your_image_resized.png")
defer output.Close()
// Decode the image (from PNG to image.Image):
src, _ := png.Decode(input)
// Set the expected size that you want:
dst := image.NewRGBA(image.Rect(0, 0, src.Bounds().Max.X/2, src.Bounds().Max.Y/2))
// Resize:
draw.NearestNeighbor.Scale(dst, dst.Rect, src, src.Bounds(), draw.Over, nil)
// Encode to `output`:		
png.Encode(output, dst)

In that case I choose draw.NearestNeighbor, because it's faster, but looks worse. but there's other methods, you can see on https://pkg.go.dev/golang.org/x/image/draw#pkg-variables:

• draw.NearestNeighbor
  NearestNeighbor is the nearest neighbor interpolator. It is very fast, but usually gives very low quality results. When scaling up, the result will look 'blocky'.

• draw.ApproxBiLinear
  ApproxBiLinear is a mixture of the nearest neighbor and bi-linear interpolators. It is fast, but usually gives medium quality results.

• draw.BiLinear
  BiLinear is the tent kernel. It is slow, but usually gives high quality results.

• draw.CatmullRom
  CatmullRom is the Catmull-Rom kernel. It is very slow, but usually gives very high quality results.

Want to do it 29 times faster? Try amazing vipsthumbnail instead:

sudo apt-get install libvips-tools
vipsthumbnail --help-all

This will resize and nicely crop saving result to a file:

vipsthumbnail original.jpg -s 700x200 -o 700x200.jpg -c

Calling from Go:

func resizeExternally(from string, to string, width uint, height uint) error {
	var args = []string{
		"--size", strconv.FormatUint(uint64(width), 10) + "x" +
			strconv.FormatUint(uint64(height), 10),
		"--output", to,
		"--crop",
		from,
	}
	path, err := exec.LookPath("vipsthumbnail")
	if err != nil {
		return err
	}
	cmd := exec.Command(path, args...)
	return cmd.Run()
}

You could use bimg, which is powered by libvips (a fast image processing library written in C).

If you are looking for a image resizing solution as a service, take a look to imaginary