How to Crop image in Golang?

To crop an image in Go, you can use the image.SubImage method in the image package to create a new image that is a sub-rectangle of the original image. This method takes one argument: reactangle coordinates of the new image. We can use image.Rectagle module to provide to coordinate of the new image we want to crop.

Interface

If you see when we create an instance of image which is can be png or jpg we can not find SubImage method. But it is available on the image package, to solve this we can use some custom interface to cast the SubImage method.

type SubImager interface {
	SubImage(r image.Rectangle) image.Image
}

Read file

Now let's open the image and parse it to jpg or png. In this article we will use png. So, let's import the package we need.

import (
	"image"
	"image/png"
	"os"
)

Replace "image/png" with "image/ppg" if you are working with jpg image or you can also import both package if you are working with both image format.

To read files we can use os package like this.

func main() {
	originalImageFile, err := os.Open("original.png")
	if err != nil {
		panic(err)
	}
	defer originalImageFile.Close()

	originalImage, err := png.Decode(originalImageFile)
	if err != nil {
		panic(err)
	}
   ...
}

Crop the Image

Here's an example image we will working with.

So here's the code to crop the images.

bounds := originalImage.Bounds()
width := bounds.Dx()
// height := bounds.Dy() you can use this to work with the height of the images
cropSize := image.Rect(0, 0, width/2+100, width/2+100)
cropSize = cropSize.Add(image.Point{100, 80})
croppedImage := originalImage.(SubImager).SubImage(cropSize)

It will be cropped to this.

Explanation:

• image.Rect: Since we need to pass image.Rectangle to method SubImage we can use this module to generate rectangle coordinate for us.
• image.Rect(0, 0, width/2+100, width/2+100): Is the width and height of the new cropped image size. In this case we are divide the current width image by half and add 100 more pixels. You can changes the value what ever you want.
• cropSize.Add(image.Point{100, 80}): This is the place of the left and top padding of image that you want to crop. In this case we add padding left to 100 pixels and padding top of 80 pixels.

Save image

After cropping the image the next step is to save it. And here's how you can save the image to a file.

croppedImageFile, err := os.Create("cropped.png")
if err != nil {
    panic(err)
}

defer croppedImageFile.Close()
if err := png.Encode(croppedImageFile, croppedImage); err != nil {
    panic(err)
}

You can change png.Encode to jpg.Encode if you work with jpg file.

Here is the complete code for this example.

package main

import (
	"image"
	"image/png"
	"os"
)

type SubImager interface {
	SubImage(r image.Rectangle) image.Image
}

func main() {
	originalImageFile, err := os.Open("original.png")
	if err != nil {
		panic(err)
	}
	defer originalImageFile.Close()

	originalImage, err := png.Decode(originalImageFile)
	if err != nil {
		panic(err)
	}

	bounds := originalImage.Bounds()
	width := bounds.Dx()
	cropSize := image.Rect(0, 0, width/2+100, width/2+100)
	cropSize = cropSize.Add(image.Point{100, 80})
	croppedImage := originalImage.(SubImager).SubImage(cropSize)

	croppedImageFile, err := os.Create("cropped.png")
	if err != nil {
		panic(err)
	}

	defer croppedImageFile.Close()
	if err := png.Encode(croppedImageFile, croppedImage); err != nil {
		panic(err)
	}
}

Bonus: I have created this simple cli to crop image with go check it out here: https://github.com/ahmadrosid/crop

I hope this tutorial was helpful and if you have any questions, shoot me a DM on Twitter.