Crop Canvas / Export html5 canvas with certain wid

Question

There are hundreds of tutorials, how one can crop an image by drawImage() on a canvas.

context.drawImage(imageObj, sourceX, sourceY, sourceWidth, sourceHeight, destX, destY, destWidth, destHeight);

However, I have a canvas that fills the user's browser. By exporting the canvas as an image I would like to export only an area of 640px*480px from (0|0).

Problem: How can I tell javascript to use only 640*480 of the canvas for the toDataURL()?

Here is what I have so far:

$("#submitGraphic").click( function(){
    var canvas = document.getElementsByTagName("canvas");
    // canvas context
    var context = canvas[0].getContext("2d");
    // get the current ImageData for the canvas
    var data = context.getImageData(0, 0, canvas[0].width, canvas[0].height);
    // store the current globalCompositeOperation
    var compositeOperation = context.globalCompositeOperation;
    // set to draw behind current content
    context.globalCompositeOperation = "destination-over";
    //set background color
    context.fillStyle = "#FFFFFF";
    // draw background/rectangle on entire canvas
    context.fillRect(0,0,canvas[0].width,canvas[0].height);

    // not working, seems to clear the canvas? browser hangs?
    // seems that I can click a white image in the background
    /*canvas[0].width = 640;
    canvas[0].height = 480;*/

    // not working either
    /*canvas[0].style.width  = '640px';
    canvas[0].style.height = '480px';*/

    // not working at all
    /*context.canvas.width = 640;
    context.canvas.height = 480;*/

    // write on screen
    var img = canvas[0].toDataURL("image/png");
    document.write('<a href="'+img+'"><img src="'+img+'"/></a>');
})

PS: I do not want to resize or scale, just clipping/cropping to the fixed window. Here I read that you only specifiy canvas.width and canvas.height - but this clears the canvas.

Answer 1

I created a simple general purpose function that does the crop by returning a new canvas with the cropped area. While it doesn't do the crop "in place", it's simple. Remember to switch to the new context after the call.

const cropCanvas = (sourceCanvas,left,top,width,height) => {
    let destCanvas = document.createElement('canvas');
    destCanvas.width = width;
    destCanvas.height = height;
    destCanvas.getContext("2d").drawImage(
        sourceCanvas,
        left,top,width,height,  // source rect with content to crop
        0,0,width,height);      // newCanvas, same size as source rect
    return destCanvas;
}

For example...

    let myCanvas = document.createElement('canvas');
    myCanvas.width = 200;
    myCanvas.height = 200;
    let myContext = myCanvas.getContext("2d");
    // draw stuff...
    myCanvas = cropCanvas(myCanvas,50,50,100,100);
    myContext = myCanvas.getContext("2d"); 
    // now using the cropped 100x100 canvas

Answer 2

Pure html5 canvas crop:

$('document').ready( function(){
const divOffset = 1
var x1,x2,y1,y2, xDif, yDif = 0;
var isSelection, 
    isBottomRight, 
    isTopRight, 
    isTopLeft, 
    isBottomLeft = false

var r = document.getElementById('source').getBoundingClientRect();
var pos = [0, 0];
pos[0] = r.left; 
pos[1] = r.top; //got position coordinates of canvas

var sel = document.getElementById('sel')
var canvasSource = document.getElementById("source");
var ctxSource = canvasSource.getContext("2d"); 

var img = new Image()
img.src = "http://bohdaq.name/assets/localImage.jpg"
img.onload = function(){
  ctxSource.drawImage(img, 0, 0)
}

$( "#source" ).mousedown(function(event) {
   isSelection = true

   x1 = event.pageX - pos[0]
   y1 = event.pageY - pos[1]

   sel.style.setProperty('display', 'block')

   sel.style.setProperty('left', event.pageX + "px")
   sel.style.setProperty('top', event.pageY + "px")

   sel.style.setProperty('width', '0px')
   sel.style.setProperty('height', '0px')
});

$( "#source" ).mouseup(function(event) {
   isSelection = false
   if(isBottomRight){
     x2 = event.pageX - pos[0]
     y2 = event.pageY - pos[1]

     xDif = x2-x1
     yDif = y2-y1 
   } else if (isBottomLeft){
     y2 = event.pageY - pos[1]
     yDif = y2 - y1 

     xDif = x1 - x2
     x1 = x1 - xDif

   } else if(isTopRight){
     x2 = event.pageX - pos[0]
     xDif = x2 - x1 
     yDif = y1 - y2
     y1 = y1 - yDif         
   } else if (isTopLeft){
     xDif = x1 - x2
     x1 = x1 - xDif
     yDif = y1 - y2
     y1 = y1 - yDif         
   }
   sel.style.setProperty('display', 'none')
   crop(x1, y1, xDif, yDif)
});

$('#source').mousemove(function(event){
  if(isSelection){
    x2 = event.pageX - pos[0]
    y2 = event.pageY - pos[1]
    if(x2>x1 && y2>y1){ //moving right bottom selection
      isBottomRight = true
      isBottomLeft = false
      isTopLeft = false
      isTopRight = false

      xDif = x2 - x1
      yDif = y2 - y1 

      sel.style.setProperty('width', xDif + 'px')
      sel.style.setProperty('height', yDif + 'px')
    } else if(x2<x1 && y2>y1){ //moving left bottom selection
      isBottomLeft = true
      isTopLeft = false
      isTopRight = false
      isBottomRight = false

      xDif = x1 - x2
      yDif = y2 - y1 

      sel.style.setProperty('left', x2 + 'px')
      sel.style.setProperty('width', xDif + 'px')
      sel.style.setProperty('height', yDif + 'px')

    } else if(x2>x1 && y2<y1){
      isTopRight = true
      isTopLeft = false
      isBottomLeft = false
      isBottomRight = false

      xDif = y1 - y2
      yDif = x2 - x1 

      sel.style.setProperty('top', y2 + 'px')
      sel.style.setProperty('width', yDif + 'px')
      sel.style.setProperty('height', xDif + 'px')
    } else if (x2<x1 && y2<y1){
      isTopLeft = true
      isTopRight = false
      isBottomLeft = false
      isBottomRight = false

      yDif = y1 - y2 
      xDif = x1 - x2

      sel.style.setProperty('left', x2 + pos[0] + divOffset + 'px')
      sel.style.setProperty('top', y2 + pos[1] + divOffset + 'px')
      sel.style.setProperty('width', xDif  + 'px')
      sel.style.setProperty('height', yDif  + 'px')
    }
 }
})

function crop(x, y, xDif, yDif){
    canvasSource.width = xDif
    canvasSource.height = yDif
    ctxSource.drawImage(img, x, y, xDif, yDif, 0, 0, xDif, yDif);
}

})

Answer 3

based on @GarySkiba answer:

// crop canvas function
const cropCanvas = (
  canvas: any,
  x: number,
  y: number,
  width: number,
  height: number
) => {
  // create a temp canvas
  const newCanvas = document.createElement('canvas');
  // set its dimensions
  newCanvas.width = width;
  newCanvas.height = height;
  // draw the canvas in the new resized temp canvas 
  newCanvas
    .getContext('2d')!
    .drawImage(canvas, x, y, width, height, 0, 0, width, height);
  return newCanvas
};

use it like:

// get canvas from somewhere
const canvas = ....;
// crop the left top 50x50 rect
const newCanvas = cropCanvas(canvas, 0, 0, 50, 50 );
// get the relative image
newCanvas.toDataURL('image/png')

Answer 4

You make a second off-screen canvas, you copy the image from the first canvas to the second (using the first as your image object), then you export the second canvas.

Answer 5

The best way is to just create a temporary canvas to draw onto from the current canvas. The user will never see this temp canvas. Then you just need use toDataUrl() on the temp canvas.

Live Demo

$("#submitGraphic").click( function(){
    var canvas = document.getElementsByTagName("canvas");
    // canvas context
    var context = canvas[0].getContext("2d");
    // get the current ImageData for the canvas
    var data = context.getImageData(0, 0, canvas[0].width, canvas[0].height);
    // store the current globalCompositeOperation
    var compositeOperation = context.globalCompositeOperation;
    // set to draw behind current content
    context.globalCompositeOperation = "destination-over";
    //set background color
    context.fillStyle = "#FFFFFF";
    // draw background/rectangle on entire canvas
    context.fillRect(0,0,canvas[0].width,canvas[0].height);

    var tempCanvas = document.createElement("canvas"),
        tCtx = tempCanvas.getContext("2d");

    tempCanvas.width = 640;
    tempCanvas.height = 480;

    tCtx.drawImage(canvas[0],0,0);

    // write on screen
    var img = tempCanvas.toDataURL("image/png");
    document.write('<a href="'+img+'"><img src="'+img+'"/></a>');
})​