How to cut an image / HTML canvas in half via javascript?

Question

I'm using html2canvas to turn a google map javascript API with custom features, into a canvas, and then an image.

Works fine on all browsers, except on IE 11 it generates an image with extra blank space to the right of the image, equal to the (width of browser window - map width). So the wider my window is, the more space to the right, and visa versa.

How can I slice this image (or HTMLcanvas) at exactly the edge of the actual image (768px wide)?

I found this code here but did not know how to modify it for this task:

var image = new Image();
image.onload = cutImageUp;
image.src = 'myimage.png';

function cutImageUp() {
    var imagePieces = [];
    for(var x = 0; x < numColsToCut; ++x) {
        for(var y = 0; y < numRowsToCut; ++y) {
            var canvas = document.createElement('canvas');
            canvas.width = widthOfOnePiece;
            canvas.height = heightOfOnePiece;
            var context = canvas.getContext('2d');
            context.drawImage(image, x * widthOfOnePiece, y * heightOfOnePiece, widthOfOnePiece, heightOfOnePiece, 0, 0, canvas.width, canvas.height);
            imagePieces.push(canvas.toDataURL());
        }
    }

    // imagePieces now contains data urls of all the pieces of the image

    // load one piece onto the page
    var anImageElement = document.getElementById('myImageElementInTheDom');
    anImageElement.src = imagePieces[0];
}

Answer 1

Here's a canvas cropper that creates an image. You'll need to adjust the cropping dimensions for your map.

// initialize the test canvas and wireup cut button.
(function() {
  var can = document.getElementById('test');
  var w = can.width = 400;
  var h = can.height = 200;
  var ctx = can.getContext('2d');

  ctx.fillStyle = "#336699";
  ctx.fillRect(0, 0, 200, 200);
  ctx.strokeStyle = "#000000";
  ctx.lineWidth = 20;
  ctx.strokeRect(0, 0, w, h);
  ctx.strokeRect(0, 0, w / 2, h);
  var btn = document.getElementById('cut');
  btn.addEventListener('click', function() {
     
    var croppedCan = crop(can, {x: 0, y: 0}, {x: 200, y: 200});
    
    // Create an image for the new canvas.
    var image = new Image();
    image.src = croppedCan.toDataURL();
  
    // Put the image where you need to.
    document.getElementsByTagName('body')[0].appendChild(image);
    return image;
    
  });
})();


// function crop
// Returns a cropped canvas given a cavnas and crop region.
//
// Inputs:
// can, canvas
// a, {x: number, y: number} - left top corner
// b, {x: number, y: number} - bottom right corner

function crop(can, a, b) {
    // get your canvas and a context for it
    var ctx = can.getContext('2d');
    
    // get the image data you want to keep.
    var imageData = ctx.getImageData(a.x, a.y, b.x, b.y);
  
    // create a new cavnas same as clipped size and a context
    var newCan = document.createElement('canvas');
    newCan.width = b.x - a.x;
    newCan.height = b.y - a.y;
    var newCtx = newCan.getContext('2d');
  
    // put the clipped image on the new canvas.
    newCtx.putImageData(imageData, 0, 0);
  
    return newCan;    
 }

<button id='cut'>Crop</button>
<hr/>
<canvas id='test'></canvas>
<hr/>

Answer 2

Here's the code I made which takes the google map, generates a canvas, slices it at the edge of the actual image to fix the IE 11 bug, then outputs that new image, and finally prints the container.

    // Insert map container for output to printer 
    var element = $("#map-container");
    var printContainer = $("#printContainer");

    html2canvas(element, {
        useCORS: true,
        onrendered: function (canvas) {

            // Must clear the printContainer before each session prints, or it will also print the previous info (if user presses the print results button twice)
            printContainer.empty();

            // Put the map into a canvas inside #printContainer
            printContainer.append(canvas);

            // Find the canvas we just made
            var myCanvas = printContainer.find("canvas")[0]; // add the [0] to get the native DOM element object
            myCanvas.id = 'generatedCanvas1';

            // Check if we're running IE 11 or earlier
            var ua = window.navigator.userAgent;
            var isIE = (ua.indexOf('MSIE') > 0 || ua.indexOf('Trident') > 0);

            if (isIE) {
                console.log("We're on IE");

                // ==========================================================================================
                // ======= IE Fix for canvas / image generation - slice the canvas ==========================
                // ==========================================================================================


                // function crop
                // Returns a cropped canvas given a cavnas and crop region.
                //
                // Inputs:
                // can, canvas
                // a, {x: number, y: number} - left top corner
                // b, {x: number, y: number} - bottom right corner


                (function() {
                    var croppedCan = crop(myCanvas, { x: 0, y: 0 }, { x: 800, y: 768 });

                    // Create an image for the new canvas.
                    var image = new Image();
                    image.src = croppedCan.toDataURL();


                    // Should we print the map image? Only if this is true...
                    if ($('*').hasClass('map-invisible posrel map-show')) {

                        //var dataUrl = canvas.toDataURL("image/png");

                        imageMap = '<p style="text-align:center;"><img id="canvasImage" src="' + image.src + '"  height="800" width="768" /></p>';
                        div.append('<p>&nbsp;</p>').html();
                        div.append(imageMap);
                    }

                    // Put the image where you need to.
                    //document.getElementById('printContainer').appendChild(image);
                    return image;

                    //});
                })();


                function crop(can, a, b) {
                    // get your canvas and a context for it
                    var ctx = can.getContext('2d');

                    // get the image data you want to keep.
                    var imageData = ctx.getImageData(a.x, a.y, b.x, b.y);

                    // create a new cavnas same as clipped size and a context
                    var newCan = document.createElement('canvas');
                    newCan.width = b.x - a.x;
                    newCan.height = b.y - a.y;
                    var newCtx = newCan.getContext('2d');

                    // put the clipped image on the new canvas.
                    newCtx.putImageData(imageData, 0, 0);

                    return newCan;
                }


                // ==========================================================================================
                // ======= END IE Fix for canvas / image generation - slice the canvas ======================
                // ==========================================================================================
            } else {
               console.log("We're not on IE");
                // For all other browsers except IE

                // Should we print the map image? Only if this is true...
                if ($('*').hasClass('map-invisible posrel map-show')) {

                    var image = new Image();
                    image.src = canvas.toDataURL("image/png");

                    imageMap = '<p style="text-align:center;"><img id="canvasImage" src="' + image.src + '"  height="800" width="768" /></p>';
                    div.append('<p>&nbsp;</p>').html();
                    div.append(imageMap);
                }
            }


            // Build the data set
            div.append(criteriaDiv);
            div.append(pageTable).html();


            // Add the new data into the hidden printContainer
            printContainer.append(div);

            // Remove the original canvas which was cropped so it doesnt print with the new canvas image
            $("#generatedCanvas1").remove();

            // Fire the print command
            printContainer.printThis({
                //debug: true
                printDelay: 1500            // variable print delay needed so that css has time to load for the printout

            });


            // For Debugging with the "#printMe" button
            $(function () {
                $("#printMe").click(function () {
                    //$printIframe.printThis({
                    //    debug: true,
                    printDelay: 1500            // variable print delay

                    //});
                    var $iframe = $("iframe[name='printIframe']");


                    setTimeout(function () {
                        if ($iframe.hasClass("MSIE")) {
                            // check if the iframe was created with the ugly hack
                            // and perform another ugly hack out of neccessity
                            window.frames["printIframe"].focus();
                            $head.append("<script>  window.print(); </script>");
                        } else {
                            // proper method
                            if (document.queryCommandSupported("print")) {
                                $iframe[0].contentWindow.document.execCommand("print", false, null);
                            } else {
                                $iframe[0].contentWindow.focus();
                                $iframe[0].contentWindow.print();
                            }
                        }


                    }, 333);
                });
            });

            // PrintThis usage and options
            //*  $("#mySelector").printThis({
            //*      debug: false,               * show the iframe for debugging
            //*      importCSS: true,            * import page CSS
            //*      importStyle: false,         * import style tags
            //*      printContainer: true,       * grab outer container as well as the contents of the selector
            //*      loadCSS: "path/to/my.css",  * path to additional css file - us an array [] for multiple
            //*      pageTitle: "",              * add title to print page
            //*      removeInline: false,        * remove all inline styles from print elements
            //*      printDelay: 333,            * variable print delay
            //*      header: null,               * prefix to html
            //*      formValues: true            * preserve input/form values
            //*  });
        }
    });