Saving canvas images locally
Building on information presented in Enabling the back and forward buttons, we discuss here how to save a canvas image of the Mandelbrot set locally, using Internet Explorer 10.
Note As of this writing, the local file saving code presented here is only applicable to Internet Explorer 10.
The only difference between the last example (Mandelbrot 5) and Mandelbrot 6 is that the skeletal handleFormSubmit function is now fully fleshed out:
Active Directory Rights Management Services File API
Mandelbrot 6's handleFormSubmit
function handleFormSubmit(evt) {
/*
As of 8/2012, the following code only works starting with Internet Explorer 10.
*/
evt.preventDefault(); // Do not refresh the page when the Submit button is clicked.
window.BlobBuilder = window.BlobBuilder || window.MSBlobBuilder || window.WebKitBlobBuilder || window.MozBlobBuilder;
globals.canvas.toBlob = globals.canvas.toBlob || globals.canvas.msToBlob;
window.navigator.saveBlob = window.navigator.saveBlob || window.navigator.msSaveBlob;
if (window.BlobBuilder && globals.canvas.toBlob && window.navigator.saveBlob) {
var extensionlessFilename = evt.target[0].value;
var filename = extensionlessFilename + ".png";
var blobBuilderObject = new BlobBuilder(); // Create a blob builder object so that we can append content to it.
blobBuilderObject.append( globals.canvas.toBlob() ); // Append the user's drawing in PNG format to the builder object.
window.navigator.saveBlob(blobBuilderObject.getBlob(), filename); // Move the builder object content to a blob and save it to a file.
}
else {
document.getElementById('messageBox').innerHTML = "<strong style='color: red;'>CANNOT SAVE FILE</strong>";
} // if-else
document.getElementById('filenameForm').style.visibility = "hidden";
} // handleFormSubmit
Without the first line, evt.preventDefault(), the page would refresh when the Submit button is pressed, causing an unnecessary redrawing of the Mandelbrot image.
Next, we attempt to future proof the BlobBuilder constructor, and the toBlob and saveBlob methods, as follows:
window.BlobBuilder = window.BlobBuilder || window.MSBlobBuilder || window.WebKitBlobBuilder || window.MozBlobBuilder;
globals.canvas.toBlob = globals.canvas.toBlob || globals.canvas.msToBlob;
window.navigator.saveBlob = window.navigator.saveBlob || window.navigator.msSaveBlob;
Next, if the required features are supported, the current canvas image is saved to the Downloads folder. Otherwise, a (red) error message is displayed:
if (window.BlobBuilder && globals.canvas.toBlob && window.navigator.saveBlob) {
var extensionlessFilename = evt.target[0].value;
var filename = extensionlessFilename + ".png";
var blobBuilderObject = new BlobBuilder(); // Create a blob builder object so that we can append content to it.
blobBuilderObject.append( globals.canvas.toBlob() ); // Append the user's drawing in PNG format to the builder object.
window.navigator.saveBlob(blobBuilderObject.getBlob(), filename); // Move the builder object content to a blob and save it to a file.
}
else {
document.getElementById('messageBox').innerHTML = "<strong style='color: red;'>CANNOT SAVE FILE</strong>";
} // if-else
Finally, the filename input element is re-hidden:
document.getElementById('filenameForm').style.visibility = "hidden";
Mandelbrot 6 in its entirety is presented next:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="X-UA-Compatible" content="IE=10" />
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<title>Mandelbrot 6</title>
<style>
html, body {
margin: 0;
padding: 0;
text-align: center;
}
canvas {
border: 2px solid black;
}
table {
margin: 0 auto; /* Center the table. */
}
#messageBox {
text-align: left;
}
#elapsedTime {
width: 23em;
text-align: right;
}
button {
width: 5em;
}
#filenameForm {
visibility: hidden; /* As opposed to "display: none", keep room for this hidden element in the layout. */
}
}
</style>
</head>
<body>
<h1>Mandelbrot 6</h1>
<p>This example demonstrates how to locally save a canvas image.</p>
<table>
<tr>
<td id="messageBox"></td>
<td id="elapsedTime"></td>
</tr>
</table>
<canvas width="600" height="400" oncontextmenu="return false;"> <!-- Because the hold gesture event can fire more than once, the 'oncontextmenu="return false;"' attribute is used to stop the right-click context menu from appearing inappropriately. -->
Canvas not supported - upgrade your browser (after checking that your browser is in the correct mode).
</canvas><br>
<button type="button" id="resetButton">Reset</button>
<button type="button" id="lightenButton">Lighten</button>
<button type="button" id="saveButton">Save</button>
<form id="filenameForm">
Extensionless filename: <input id="filename" type="text"> <input type="submit" value="Submit">
</form>
<script>
var RE_MAX = 1.1; // This value will be adjusted as necessary to ensure that the rendered Mandelbrot set is never skewed (that is, true to it's actual shape).
var RE_MIN = -2.5;
var IM_MAX = 1.2;
var IM_MIN = -1.2;
var MAX_ITERATIONS = 1200; // Increase this value to improve detection of complex c values that belong to the Mandelbrot set.
var STATIC_ZOOM_BOX_FACTOR = 0.25; // Increase to make the double-click and hold gesture zoom box bigger.
var DEFAULT_MESSAGE = "Click or click-and-drag to zoom."
var globals = {}; // See the handleLoad function.
window.addEventListener('load', handleLoad, false);
/************************************************************************************************************************************************************/
Number.prototype.format = function() {
/*
Formats this integer so that it has commas in the expected places.
*/
var numberString = Math.round(this).toString(); // An integer value is assumed, so we ensure that it is indeed an integer.
var precompiledRegularExpression = /(\d+)(\d{3})/;
while ( precompiledRegularExpression.test(numberString) ) {
numberString = numberString.replace(precompiledRegularExpression, '$1' + ',' + '$2'); // For this integer, inject ","'s at the appropriate locations.
} // while
return numberString;
} // Number.prototype.format
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function handleLoad() {
var canvas = document.getElementsByTagName('canvas')[0];
var canvasWidth = canvas.width;
var canvasHeight = canvas.height;
var ctx = canvas.getContext('2d');
document.getElementsByTagName('table')[0].width = canvasWidth; // Make the table's width the same as the canvas's width.
document.getElementById('messageBox').innerHTML = DEFAULT_MESSAGE;
globals.canvas = canvas;
globals.canvas.context = ctx;
globals.canvas.context.imageDataObject = ctx.createImageData(canvasWidth, canvasHeight); // Create an appropriately sized but empty canvas image data object.
globals.staticZoomBoxWidth = STATIC_ZOOM_BOX_FACTOR * canvasWidth; // Maintains the original canvas width/height ratio.
globals.staticZoomBoxHeight = STATIC_ZOOM_BOX_FACTOR * canvasHeight; // Maintains the original canvas width/height ratio.
globals.pointer = {};
globals.pointer.down = false;
window.addEventListener('hashchange', handleHashChange, false); // This event handler executes whenever the URL hash string changes.
canvas.addEventListener('mousedown', handlePointer, false);
canvas.addEventListener('mousemove', handlePointer, false);
canvas.addEventListener('mouseup', handlePointer, false);
document.getElementById('resetButton').addEventListener('click', handleResetButton, false);
document.getElementById('lightenButton').addEventListener('click', handleLightenButton, false);
document.getElementById('saveButton').addEventListener('click', handleSaveButton, false);
document.getElementById('filenameForm').addEventListener('submit', handleFormSubmit, false);
ctx.fillStyle = "rgba(255, 0, 0, 0.3)"; // The color and opacity of the zoom box. This is what gets saved when calling ctx.save().
handleHashChange(); // On page load, simulate a page URL change to draw the initial Mandelbrot set.
} // handleLoad
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function handleHashChange() {
var hashValues = getHashValues(); // This function examines window.location.hash but doesn't change it.
if (hashValues) {
globals.ReMax = hashValues.ReMax;
globals.ReMin = hashValues.ReMin;
globals.ImMax = hashValues.ImMax;
globals.ImMin = hashValues.ImMin;
globals.grayscaleFactor = hashValues.grayscaleFactor;
}
else {
globals.ReMax = adjusted_RE_MAX();
globals.ReMin = RE_MIN;
globals.ImMax = IM_MAX;
globals.ImMin = IM_MIN;
globals.grayscaleFactor = 1; // Multiplying any value by 1 has no effect.
} // if-else
drawMandelbrot(globals.ReMax, globals.ReMin, globals.ImMax, globals.ImMin, globals.grayscaleFactor);
} // handelHashChange
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function getHashValues() {
var dirtyComplexPlaneExtremaString = (window.location.hash).replace('#', ''); // Remove the leading "#" character from the string.
var complexPlaneExtremaString = dirtyComplexPlaneExtremaString.split(','); // Returns an array. Assumes the following string form: "ReMax,ReMin,ImMax,ImMin,grayscaleFactor" (note that if grayscaleFactor is 1, the image's grayscale is not affected).
var ReMax = parseFloat( complexPlaneExtremaString[0] );
var ReMin = parseFloat( complexPlaneExtremaString[1] );
var ImMax = parseFloat( complexPlaneExtremaString[2] );
var ImMin = parseFloat( complexPlaneExtremaString[3] );
var grayscaleFactor = parseFloat( complexPlaneExtremaString[4] );
if ( isNaN(ReMax) || isNaN(ReMin) || isNaN(ImMax) || isNaN(ImMin) || isNaN(grayscaleFactor) ) {
return null;
} // if
return {ReMax: ReMax, ReMin: ReMin, ImMax: ImMax, ImMin: ImMin, grayscaleFactor: grayscaleFactor};
} // getHashValues
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function adjusted_RE_MAX() {
var ReMax = globals.canvas.width * ( (IM_MAX - IM_MIN) / globals.canvas.height ) + RE_MIN;
if (RE_MAX != ReMax) {
alert("RE_MAX has been adjusted to: " + ReMax); // The user should never see this if RE_MAX is set correctly above.
} // if
return ReMax;
} // adjusted_RE_MAX
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function drawMandelbrot(ReMax, ReMin, ImMax, ImMin, grayscaleFactor) {
document.getElementById('messageBox').innerHTML = "Calculating..."; // This isn't displayed until the drawMandelbrot function block exits.
document.getElementById('elapsedTime').innerHTML = ""; // Erase the prior run's statistics.
if (window.setImmediate) {
window.setImmediate(calculateMandelbrot); // Allow the drawMandelbrot function to immediately terminate, thus printing "Calculating..." to the screen.
}
else {
window.setTimeout(calculateMandelbrot, 0); // Allow the drawMandelbrot function to immediately terminate, thus printing "Calculating..." to the screen.
}
function calculateMandelbrot() {
var startTime = new Date(); // Report how long it takes to render this particular region of the Mandelbrot set.
var canvas = globals.canvas;
var canvasWidth = canvas.width;
var canvasHeight = canvas.height;
var ctx = canvas.context;
var imageDataObjectData = ctx.imageDataObject.data; // imageDataObjectData is a reference to, not a copy of, ctx.imageDataObject.data
var maxPixelGrayscaleValue = 0; // This will contain the lightest shade of gray in the drawn Mandelbrot image.
var x_coefficient = (ReMax - ReMin) / canvasWidth; // Keep the below loops as computation-free as possible.
var y_coefficient = (ImMin - ImMax) / canvasHeight; // Keep the below loops as computation-free as possible.
var iterationSum = 0;
var currentPixel = 0;
for (var y = 0; y < canvasHeight; y++) {
var c_Im = (y * y_coefficient) + ImMax; // Note that c = c_Re + c_Im*i
for (var x = 0; x < canvasWidth; x++) {
var c_Re = (x * x_coefficient) + ReMin // Convert the canvas x-coordinate to a complex plane Re-coordinate. c_Re represents the real part of a c value.
var z_Re = 0; // The first z value (Zo) must be 0.
var z_Im = 0; // The first z value (Zo) must be 0. Note that z = z_Re + z_Im*i
var c_belongsToMandelbrotSet = true; // Assume that the c associated with Zn belongs to the Mandelbrot set (i.e., Zn remains bounded under iteration of Zn+1 = (Zn)^2 + c).
var exponentialSmoothingSum = 0;
for (var iterationCount = 1; iterationCount <= MAX_ITERATIONS; iterationCount++) {
iterationSum++; // Keep track of how many iterations were performed in total so we can report this to the user.
var z_Re_squared = z_Re * z_Re; // A small speed optimization.
var z_Im_squared = z_Im * z_Im; // A small speed optimization.
exponentialSmoothingSum += Math.exp( -(z_Re_squared + z_Im_squared) ); // Technically, this should be e^(-|z|). However, avoiding the expensive square root operation doesn't really affect the resulting image.
if (exponentialSmoothingSum >= 255) { // Don't cycle through the gray colors.
exponentialSmoothingSum = 255;
}
if (z_Re_squared + z_Im_squared > 4) { // Checks if |z^2| is greater than 2. This approach avoids the expensive square root operation.
c_belongsToMandelbrotSet = false; // This complex c value is not part of the Mandelbrot set (because it will always tend towards infinity under iteration).
break; // Immediately check the next c value to see if it belongs to the Mandelbrot set or not.
} // if
// The next two lines perform Zn+1 = (Zn)^2 + c (recall that (x + yi)^2 = x^2 - y^2 + 2xyi, thus the real part is x^2 - y^2 and the imaginary part is 2xyi).
z_Im = (2 * z_Re * z_Im) + c_Im; // We must calculate the next value of z_Im first because it depends on the current value of z_Re (not the next value of z_Re).
z_Re = z_Re_squared - z_Im_squared + c_Re; // Calculate the next value of z_Re.
} // for
if (c_belongsToMandelbrotSet) { // This complex c value is probably part of the Mandelbrot set because Zn did not tend toward infinity within MAX_ITERATIONS iterations.
imageDataObjectData[currentPixel++] = 0; // Red. Note that there are 255 possible shades of red, green, blue, and alpha (i.e., opacity).
imageDataObjectData[currentPixel++] = 0; // Green.
imageDataObjectData[currentPixel++] = 0; // Blue.
imageDataObjectData[currentPixel++] = 255; // Alpha (i.e., 0% transparency).
}
else { // This complex c value is definitely not part of the Mandelbrot set because Zn tends toward infinity under iteration (i.e., |Zn| > 2).
var pixelGrayscaleValue = 255 - exponentialSmoothingSum % 256; // Force the value of exponentialSmoothingSum to be between 0 and 255 inclusively. Note that all values for red, green, and blue are identical when using a grayscale.
var adjustedPixelGrayscaleValue = pixelGrayscaleValue * grayscaleFactor;
imageDataObjectData[currentPixel++] = adjustedPixelGrayscaleValue; // Because we mod by 256, the value of exponentialSmoothingSum will always be between 0 and 255.
imageDataObjectData[currentPixel++] = adjustedPixelGrayscaleValue; // If exponentialSmoothingSum is 255 (its maximum possible value), then 255 % 256 = 255.
imageDataObjectData[currentPixel++] = adjustedPixelGrayscaleValue; // When exponentialSmoothingSum is 255, we have 255 - 255 = 0, so the shade values for RGB are all set to 0 (that is, the c-value pixel is rendered black - indicating that this particular c-value very slowly tends towards infinity).
imageDataObjectData[currentPixel++] = 255; // Always draw the c-value pixels with no transparency.
if (pixelGrayscaleValue > maxPixelGrayscaleValue) {
maxPixelGrayscaleValue = pixelGrayscaleValue; // Determine the lightest shade of gray in case the user clicks the Lighten button.
} // if
} // if-else
} // for
} // for
globals.maxPixelGrayscaleValue = maxPixelGrayscaleValue; // Store the lightest shade of gray in case the user clicks the Lighten button.
ctx.putImageData(ctx.imageDataObject, 0, 0); // Render our carefully constructed canvas image data array to the canvas.
var elapsedMilliseconds = (new Date()) - startTime;
document.getElementById('elapsedTime').innerHTML = iterationSum.format() + " iterations in " + (elapsedMilliseconds / 1000).toFixed(2) + " seconds"; // Note that the UI element is not updated until after this block terminates (which is the desired behavior).
document.getElementById('messageBox').innerHTML = DEFAULT_MESSAGE; // Erase the "Calculating..." message and replace it with the default message.
} // calculateMandelbrot
} // drawMandelbrot
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function xToRe(x) {
var x_coefficient = (globals.ReMax - globals.ReMin) / globals.canvas.width;
return (x * x_coefficient) + globals.ReMin; // Converts a canvas x-coordinate value to the associated complex plane Re-coordinate.
} // xToRe
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function yToIm(y) {
var y_coefficient = (globals.ImMin - globals.ImMax) / globals.canvas.height;
return (y * y_coefficient) + globals.ImMax; // Converts a canvas y-coordinate value to the associated complex plane Im-coordinate.
} // yToIm
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function handlePointer(evt) {
var canvasWidthHeightRatio = globals.canvas.width / globals.canvas.height;
var ctx = globals.canvas.context;
var canvasX;
var canvasY;
if (evt.offsetX && evt.offsetY) {
canvasX = evt.offsetX; // Not supported in Firefox.
canvasY = evt.offsetY; // Does not assume that the canvas element is a direct descendent of the body element.
} else {
canvasX = evt.clientX - evt.target.offsetLeft; // Supported in Firefox.
canvasY = evt.clientY - evt.target.offsetTop; // Assumes that the canvas element is a direct descendent of the body element.
} // if-else
var zoomBoxWidth;
var zoomBoxHeight;
var ReMax;
var ReMin;
var ImMax;
var ImMin;
var staticZoomBoxWidth = globals.staticZoomBoxWidth;
var staticZoomBoxHeight = globals.staticZoomBoxHeight;
var halfStaticZoomBoxWidth = staticZoomBoxWidth / 2;
var halfStaticZoomBoxHeight = staticZoomBoxHeight / 2;
switch (evt.type) {
case 'mousedown':
globals.pointer.down = true;
globals.pointer.x1 = canvasX;
globals.pointer.y1 = canvasY;
break;
case 'mousemove':
if (globals.pointer.down) {
zoomBoxHeight = Math.abs(canvasY - globals.pointer.y1);
zoomBoxWidth = zoomBoxHeight * canvasWidthHeightRatio; // We must keep the zoom box dimensions proportional to the canvas dimensions in order to ensure that the resulting zoomed Mandelbrot image does not become skewed.
ctx.putImageData(ctx.imageDataObject, 0, 0); // Assumes that an initial image of the Mandelbrot set is drawn before we get to this point in the code. The purpose of this line is to erase the prior zoom box rectangle before drawing the next zoom box rectangle.
ctx.fillRect(globals.pointer.x1, globals.pointer.y1, zoomBoxWidth, zoomBoxHeight); // With a freshly painted image of the current Mandelbrot set in place (see prior line), draw a new zoom box rectangle.
}
break;
case 'mouseup':
globals.pointer.down = false;
zoomBoxHeight = Math.abs(canvasY - globals.pointer.y1); // Only allow the zoom box to be drawn from an upper-left corner position down to a lower-right corner position.
zoomBoxWidth = zoomBoxHeight * canvasWidthHeightRatio; // Again, ensure that the width/height ratio of the zoom box is proportional to the canvas's (this simplifies the algorithm).
if (zoomBoxHeight == 0) { // No zoom box has been drawn, so honor the fixed sized zoom box.
ctx.fillRect(canvasX - halfStaticZoomBoxWidth, canvasY - halfStaticZoomBoxHeight, staticZoomBoxWidth, staticZoomBoxHeight); // Just leave this on the screen.
ReMin = xToRe(canvasX - halfStaticZoomBoxWidth); // Center the static zoom box about the point (evt.offsetX, evt.offsetY).
ImMax = yToIm(canvasY - halfStaticZoomBoxHeight);
ReMax = xToRe(canvasX + halfStaticZoomBoxWidth);
ImMin = yToIm(canvasY + halfStaticZoomBoxHeight);
}
else { // A (possibly tiny) zoom box has been drawn, so honor it.
ReMin = xToRe(globals.pointer.x1); // Convert the mouse's x-coordinate value (on the canvas) to the associated Re-coordinate value in the complex plane.
ImMax = yToIm(globals.pointer.y1); // Convert the mouse's y-coordinate value (on the canvas) to the associated Im-coordinate value in the complex plane.
ReMax = xToRe(zoomBoxWidth + globals.pointer.x1); // Convert the zoom box's final x-coordinate value to the associated Re-coordinate value in the complex plane.
ImMin = yToIm(zoomBoxHeight + globals.pointer.y1); // Convert the zoom box's final y-coordinate value to the associated Re-coordinate value in the complex plane.
} // if-else
window.location.hash = ReMax + "," + ReMin + "," + ImMax + "," + ImMin + "," + globals.grayscaleFactor; // This triggers the handleHashChange event handler which, among other things, is responsible for drawing the Mandelbrot set.
break;
default:
alert("Error in switch statement."); // Although unnecessary, defensive programming techniques such as this are highly recommended.
} // switch
} // handlePointer
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function handleResetButton() {
window.location.hash = adjusted_RE_MAX() + "," + RE_MIN + "," + IM_MAX + "," + IM_MIN + "," + 1; // // This triggers the handleHashChange event handler which, among other things, is responsible for drawing the Mandelbrot set.
} // handleResetButton
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function handleLightenButton() {
/*
This creates a value (factor) such that black (0) stays black and the lightest gray value in the image becomes white (255). Thus, clicking the
Lighten button can remove mathematical meaning of the (proper) grayscale but can make dark images more visible.
*/
var grayscaleFactor = 255 / globals.maxPixelGrayscaleValue; // For the canvas element, 255 is white, 0 is black.
window.location.hash = globals.ReMax + "," + globals.ReMin + "," + globals.ImMax + "," + globals.ImMin + "," + grayscaleFactor; // This invokes handleHashChange which, among other things, is responsible for drawing the Mandelbrot set.
} // handleResetButton
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function handleSaveButton() {
document.getElementById('filenameForm').style.visibility = "visible";
document.getElementById('filename').focus(); // Place the cursor in the filename text input box.
} // handleResetButton
/*----------------------------------------------------------------------------------------------------------------------------------------------------------*/
function handleFormSubmit(evt) {
/*
As of 8/2012, the following code only works starting with Internet Explorer 10.
*/
evt.preventDefault(); // Do not refresh the page when the Submit button is clicked.
window.BlobBuilder = window.BlobBuilder || window.MSBlobBuilder || window.WebKitBlobBuilder || window.MozBlobBuilder;
globals.canvas.toBlob = globals.canvas.toBlob || globals.canvas.msToBlob;
window.navigator.saveBlob = window.navigator.saveBlob || window.navigator.msSaveBlob;
if (window.BlobBuilder && globals.canvas.toBlob && window.navigator.saveBlob) {
var extensionlessFilename = evt.target[0].value;
var filename = extensionlessFilename + ".png";
var blobBuilderObject = new BlobBuilder(); // Create a blob builder object so that we can append content to it.
blobBuilderObject.append( globals.canvas.toBlob() ); // Append the user's drawing in PNG format to the builder object.
window.navigator.saveBlob(blobBuilderObject.getBlob(), filename); // Move the builder object content to a blob and save it to a file.
}
else {
document.getElementById('messageBox').innerHTML = "<strong style='color: red;'>CANNOT SAVE FILE</strong>";
} // if-else
document.getElementById('filenameForm').style.visibility = "hidden";
} // handleFormSubmit
</script>
</body>
</html>
To support touch-enabled devices, the Enabling touch topic describes how to implement touch support (plus one gesture).