split base64 image into parts

Question

I am trying to split an image file with javascript or jquery into four equal parts. Is it possible to split image like 280px x 60px into four equal parts, and save them encoded with base64 into four variables?

Answer 1

Let's say we have a Base64 image like:

Let's go get those 4 images!

---

1. Create an in-memory canvas element and prepare it's context:

var canvas = document.createElement('canvas');
var ctx    = canvas.getContext("2d");

2. Than let's prepare an empty Array to later store our base64 strings:

var parts = [];

3. Now we need to create a new Image, assign an onload function and set it's src:

var img = new Image();
img.onload = split_4;
img.src = "data:image/png;base64,iVBORw0KGgoAAAA.......................etc"

4. The split_4 function:

If the image we need to slice is 260×80 means that we need to set our canvas element to 1/4 that size:

var w2 = img.width  / 2,  // 130
    h2 = img.height / 2;  // 40

canvas.width  = w2;
canvas.height = h2;   

5. Paint our canvas 4 times, each time moving our image to new XY positions:

//      0   0    1.iteration
//   -130   0    2.iteration
//      0 -40    3.iteration
//   -130 -40    4.iteration

and on each loop iteration we simply push the retrieved Canvas data into our parts Array:

for(var i=0; i<4; i++){

  var x = (-w2*i) % (w2*2),              // New X position
      y = (h2*i)<=h2? 0 : -h2 ;          // New Y position

  ctx.drawImage(this, x, y, w2*2, h2*2); // imgObject, X, Y, width, height
  parts.push( canvas.toDataURL() );      // ("image/jpeg") for jpeg

}

Now always inside the onload function you can retrieve all your image parts from array:

console.log( parts ); //  ["data:image/png;base64,iV...z9d/oBHAAAAAElFTkSuQmCC",
                      //   "data:image/png;base64,iV...yVhNNW1AAAAAElFTkSuQmCC", 
                      //   "data:image/png;base64,iV...Q2FoAAAAABJRU5ErkJggg==", 
                      //   "data:image/png;base64,iV...RQXgXQAAAAASUVORK5CYII="]

To get only (i.e.) the first image use: parts[0];

---

Example:

var canvas = document.createElement('canvas'), // In memory canvas
    ctx = canvas.getContext("2d"),
    parts = [], // to push into oud base64 strings
    img = new Image();

function split_4() {

  var w2 = img.width  / 2,
      h2 = img.height / 2;

  for (var i = 0; i < 4; i++) {

    var x = (-w2 * i) % (w2 * 2),
      y = (h2 * i) <= h2 ? 0 : -h2;

    canvas.width = w2;
    canvas.height = h2;

    ctx.drawImage(this, x, y, w2 * 2, h2 * 2); // img, x, y, w, h
    parts.push(canvas.toDataURL()); // ("image/jpeg") for jpeg

    //>> JUST FOR DEMO
    var slicedImage = document.createElement("img")
    slicedImage.src = parts[i];
    var div = document.getElementById("test");
    div.appendChild(slicedImage);
    //<< JUST FOR DEMO
  }
  console.log(parts);
};

img.onload = split_4;
img.src = "data:image/png;base64,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";

img {
  margin: 10px;
}

<div id="test"></div>