Crop Images In Javascript (Simple Examples)

Welcome to a tutorial on how to crop images in Javascript. Want to lessen the server load by cropping the image on the client side? Or just curious about how those “online image crop tools” work?

That covers the basics, read on for an actual example!

 

 

TLDR – QUICK SLIDES

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Fullscreen Mode – Click Here

 

TABLE OF CONTENTS

 

 

 

EXAMPLE 1) SIMPLE IMAGE CROP

<!-- (A) HTML CANVAS -->
<canvas id="demo" width="300" height="300"></canvas>
 
 <script>
// (B) SOURCE IMAGE OBJECT
var img = new Image();
 
// (C) CROP ON LOAD
img.onload = () => {
  // (C1) GET CANVAS + 2D CONTEXT
  let canvas = document.getElementById("demo"),
  ctx = canvas.getContext("2d");
 
  // (C2) CROP BY COPYING PART OF SOURCE IMAGE TO CANVAS
  ctx.drawImage(img,
    // SOURCE X, Y, WIDTH, HEIGHT
    50, 100, 300, 300,
    // DESTINATION X, Y, WIDTH, HEIGHT
    0, 0, 300, 300
  );
};
 
// (D) GO!
img.src = "easter.png";

This is the “full version” of the introduction snippet, I think the confusion mostly lies with the “crop” – We are not actually doing a crop, but copying a part of the source image onto the canvas. Just what are those SX, SY, SWIDTH, SHEIGHT, DX, DY, DWIDTH, DHEIGHT? I will let this explain itself:

 

 

EXAMPLE 2) PICK IMAGE & CROP

<!-- (A) FILE PICKER -->
<input type="file" id="picker" accept="image/jpeg,image/png,image/webp"/>
 
 <script>
// (B) AUTO START ON SELECT FILE
let picker = document.getElementById("picker");
picker.onchange = () => {
  // (C) CREATE CANVAS
  let canvas = document.createElement("canvas"),
      ctx = canvas.getContext("2d");
  canvas.width = 300;
  canvas.height = 300;

  // (D) CREATE ANCHOR + SELECTED IMAGE URL
  let anchor = document.createElement("a"),
      surl = URL.createObjectURL(picker.files[0]);
 
  // (E) CREATE IMAGE + CROP
  let img = new Image();
  img.onload = () => {
    // (E1) DO YOUR OWN CROP CALCULATIONS...
    ctx.drawImage(img, 50, 100, 300, 300, 0, 0, 300, 300);
 
    // (E2) "FORCE DOWNLOAD"
    anchor.href = canvas.toDataURL("image/png");
    anchor.download = "cropped.png";
    anchor.click(); // MAY NOT WORK - BETTER TO LET USER CLICK
 
    // (E3) CLEAN UP
    anchor.remove();
    canvas.remove();
    img.remove();
    URL.revokeObjectURL(surl);
  };
 
  // (F) GO!
  img.src = surl;
};

This is for you guys who are looking to do the “conventional pick file, crop, and download”. Not going to explain this line-by-line, but it essentially does the same “simple crop” – Except that we have to jump through a lot of hoops to read the selected image file, and to “force a download”.

 

 

 

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EXAMPLE CODE DOWNLOAD

Click here for the source code on GitHub gist, just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

COMPATIBILITY CHECKS

• Canvas – CanIUse
• Draw Image – CanIUse
• Arrow Function – CanIUse
• To Data URL – CanIUse

The examples should work on all modern browsers.

 

LINKS & REFERENCES

• Canvas API – MDN
• Canvas Element – MDN
• Draw Image – MDN
• Download Canvas As Image – Code Boxx

 

INFOGRAPHIC CHEAT SHEET