Generating thumbnail of a pdf using PDF.js - javascript

I would like to generate a thumbnail from a pdf file using PDF.js, but it isn't like anothers js that have just one file and all needed to include the js in your project is to write:
<script src="any.js"></script>
How can I use PDF.js in my project? I'm using PHP in backend.

Based on helloworld example:
function makeThumb(page) {
// draw page to fit into 96x96 canvas
var vp = page.getViewport(1);
var canvas = document.createElement("canvas");
canvas.width = canvas.height = 96;
var scale = Math.min(canvas.width / vp.width, canvas.height / vp.height);
return page.render({canvasContext: canvas.getContext("2d"), viewport: page.getViewport(scale)}).promise.then(function () {
return canvas;
});
}
pdfjsLib.getDocument("https://raw.githubusercontent.com/mozilla/pdf.js/ba2edeae/web/compressed.tracemonkey-pldi-09.pdf").promise.then(function (doc) {
var pages = []; while (pages.length < doc.numPages) pages.push(pages.length + 1);
return Promise.all(pages.map(function (num) {
// create a div for each page and build a small canvas for it
var div = document.createElement("div");
document.body.appendChild(div);
return doc.getPage(num).then(makeThumb)
.then(function (canvas) {
div.appendChild(canvas);
});
}));
}).catch(console.error);
<script src="//npmcdn.com/pdfjs-dist/build/pdf.js"></script>

I figured it out, the scale is not a parameter. The parameters are an object with field of scale that needed to be set.
function makeThumb(page) {
// draw page to fit into 96x96 canvas
var vp = page.getViewport({ scale: 1, });
var canvas = document.createElement("canvas");
var scalesize = 1;
canvas.width = vp.width * scalesize;
canvas.height = vp.height * scalesize;
var scale = Math.min(canvas.width / vp.width, canvas.height / vp.height);
console.log(vp.width, vp.height, scale);
return page.render({ canvasContext: canvas.getContext("2d"), viewport: page.getViewport({ scale: scale }) }).promise.then(function () {
return canvas;
});
}

Related

How to create a draggable element on top of a PDF using PDF.js and jQuery

I am creating a service to stamp a pdf with an image using PDF.js and jQuery. I managed to create a draggable object using PDF.js but the the object leaves a trail of past objects when it is dragged.
I used context.clearRect(0, 0, canvas.width, canvas.height); to clear the past objects but it also clears the underlying PDF in the canvas.
How can I drag this object without affecting the underlying PDF?
Here is what I have done so far.
I am loading the PDF to the canvas using following code.
function loadPdfPreview(base64pdf){
pdfjsLib.GlobalWorkerOptions.workerSrc = 'https://cdnjs.cloudflare.com/ajax/libs/pdf.js/2.0.943/pdf.worker.min.js';
var loadingTask = pdfjsLib.getDocument({data: base64pdf});
loadingTask.promise.then(function (pdf) {
// Fetch the first page
pdf.getPage(1).then(function (page) {
var scale = 1.0;
var viewport = page.getViewport(scale);
// Prepare canvas using PDF page dimensions
canvas = document.getElementById('pdf-canvas');
context = canvas.getContext('2d');
canvas.height = viewport.height;
canvas.width = viewport.width;
canvasOffset = $("#pdf-canvas").offset();
offsetX = canvasOffset.left;
offsetY = canvasOffset.top;
// Render PDF page into canvas context
var renderContext = {
canvasContext: context,
viewport: viewport
};
page.render(renderContext).then(function () {
// creating the dummy object on success
drawObjectFromBlueprint(blueprint);
}, function (e) {
console.log(e);
});
});
});
}
I use following function to draw the object on top of the canvas after the pdf is loaded to the canvas.
function drawObjectFromBlueprint(blueprint) {
// drawing a draggable element inside the canvas
context.strokeStyle = "lightgray";
// Clearing the previous dummy objects
context.clearRect(0, 0, canvas.width, canvas.height);
// drawing the dummy object
context.beginPath();
context.moveTo(blueprint.x, blueprint.y);
context.lineTo(blueprint.right, blueprint.y);
context.lineTo(blueprint.right, blueprint.bottom);
context.lineTo(blueprint.x, blueprint.bottom);
context.closePath();
context.fillStyle = blueprint.fill;
context.fill();
context.stroke();
}
I handle the mouse move event using the following code.
function handleMouseMove(e) {
var mouseX = parseInt(e.clientX - offsetX1);
var mouseY = parseInt(e.clientY - offsetY1);
// always update the global blueprint
blueprint.x += (mouseX - lastX);
blueprint.y += (mouseY - lastY);
blueprint.right = blueprint.x + blueprint.width;
blueprint.bottom = blueprint.y + blueprint.height;
lastX = mouseX;
lastY = mouseY;
drawObjectFromBlueprint(blueprint);
}
And I listen to the mouse move event using following code.
$("#drawable-canvas").mousemove(function (e) {
handleMouseMove(e);
});
I want to re-draw the object without affecting the underlying PDF. Tried to load the PDF and re-draw the object by clearing the whole canvas, and it does not work as intended.
After two hours of trying figures out the way to do this. I used two canvases as #Nikolaus suggested on top of each other and used the bottom canvas to load the PDF and the top canvas for the drawing.
Following is my HTML:
<div class="col-md-12" id="pdfDisplay" style="display: none;">
<div id="pageContainer" class="pdfViewer nopadding" style="background-color:transparent">
<canvas id="pdf-canvas" style="border:1px solid black"></canvas>
<canvas id="drawable-canvas" style="border:1px solid black"></canvas>
</div>
</div>
Following is my CSS to place both canvases on top of each other.
#pageContainer { position: relative; }
#drawable-canvas { position: absolute; top: 0; left: 0; }
Following is my global Javascript variables:
var isUploading = false;
var base64pdf = "";
var initX = 50;
var initY = 50;
var initWidth = 200;
var initHeight = 150;
// blueprint options are in pixels
// this blueprint holds the latest values for the draggable object
// always update this global blueprint when making a change so it holds the latest values
var blueprint = {
x: initX,
y: initY,
width: initWidth,
height: initHeight,
right: (initX+initWidth), // x + width
bottom: (initY+initHeight), // y + height
fill: "skyblue"
};
var context = null;
var canvas = null;
var drawableContext = null;
var drawableCanvas = null;
var canvasOffset = null;
var offsetX = 0;
var offsetY = 0;
var canvasOffset1 = null;
var offsetX1 = 0;
var offsetY1 = 0;
var lastX = 0;
var lastY = 0;
var mouseIsDown = false;
Javascript function to listen to mouse up, down and movement as I only need to track the mouse movements when the object is dragged using the mouse.
$("#drawable-canvas").mousedown(function (e) {
var mouseX = parseInt(e.clientX - offsetX);
var mouseY = parseInt(e.clientY - offsetY);
lastX = mouseX;
lastY = mouseY;
mouseIsDown = true;
});
$("#drawable-canvas").mousemove(function (e) {
handleMouseMove(e);
});
$("#drawable-canvas").mouseup(function (e) {
mouseIsDown = false;
});
Javascript function to load the PDF to the bottom canvas.
function loadPdfPreview(base64pdf){
pdfjsLib.GlobalWorkerOptions.workerSrc = 'https://cdnjs.cloudflare.com/ajax/libs/pdf.js/2.0.943/pdf.worker.min.js';
var loadingTask = pdfjsLib.getDocument({data: base64pdf});
loadingTask.promise.then(function (pdf) {
// Fetch the first page
pdf.getPage(1).then(function (page) {
var scale = 1.0;
var viewport = page.getViewport(scale);
// Prepare canvas using PDF page dimensions
canvas = document.getElementById('pdf-canvas');
context = canvas.getContext('2d');
canvas.height = viewport.height;
canvas.width = viewport.width;
canvasOffset = $("#pdf-canvas").offset();
offsetX = canvasOffset.left;
offsetY = canvasOffset.top;
canvasOffset1 = $("#drawable-canvas").offset();
offsetX1 = canvasOffset1.left;
offsetY1 = canvasOffset1.top;
// creating the drawable-canvas canvas for drawing purposes without affecting the pdf-canvas
// it has identical width and height and X and Y values
drawableCanvas = document.getElementById('drawable-canvas');;
drawableCanvas.height = viewport.height;
//bringing the drawable canvas up using z-index
drawableCanvas.style.zIndex = 1;
drawableCanvas.width = viewport.width;
drawableContext = drawableCanvas.getContext('2d');
// Render PDF page into canvas context
var renderContext = {
canvasContext: context,
viewport: viewport
};
page.render(renderContext).then(function () {
// creating the dummy object on success
drawObjectFromBlueprint(blueprint);
}, function (e) {
console.log(e);
});
});
});
}
Handling Mouse movement(dragging the object):
function handleMouseMove(e) {
if (!mouseIsDown) {
return;
}
var mouseX = parseInt(e.clientX - offsetX1);
var mouseY = parseInt(e.clientY - offsetY1);
// always update the global blueprint
blueprint.x += (mouseX - lastX);
blueprint.y += (mouseY - lastY);
blueprint.right = blueprint.x + blueprint.width;
blueprint.bottom = blueprint.y + blueprint.height;
lastX = mouseX;
lastY = mouseY;
drawObjectFromBlueprint(blueprint);
console.log(blueprint);
}
Javascript function to draw the object on the drawable canvas, totally independent from the pdf-canvas.
function drawObjectFromBlueprint(blueprint) {
// drawing a draggable element inside the canvas
drawableContext.strokeStyle = "lightgray";
// Clearing the previous dummy objects
drawableContext.clearRect(0, 0, drawableCanvas.width, drawableCanvas.height);
// drawing the dummy object
drawableContext.beginPath();
drawableContext.moveTo(blueprint.x, blueprint.y);
drawableContext.lineTo(blueprint.right, blueprint.y);
drawableContext.lineTo(blueprint.right, blueprint.bottom);
drawableContext.lineTo(blueprint.x, blueprint.bottom);
drawableContext.closePath();
drawableContext.fillStyle = blueprint.fill;
drawableContext.fill();
drawableContext.stroke();
}
In the callback of the page.render method, the pdf page will be drawn on the canvas. You have to save the drawn image separately so that the original image does not disappear by dragging.
// maybe globalScope...?
const tempCanvas = document.createElement('canvas');
const tempCtx = tempCanvas.getContext('2d');
// your code
page.render(renderContext).then(function () {
// Save the original page image.
tempCanvas.width = canvas.width;
tempCanvas.height = canvas.height;
tempCtx.drawImage(canvas, 0, 0);
// creating the dummy object on success
drawObjectFromBlueprint(blueprint);
}, ...
Next, please modify it to draw with the original page image in drawObjectFromBlueprint.
function drawObjectFromBlueprint(blueprint) {
// draw original page image
context.drawImage(tempCanvas, 0, 0);
// drawing a draggable element inside the canvas
context.strokeStyle = "lightgray";
...
}

How to store and restore canvas to use it again?

I am using PDF.js to show PDF in browser. PDF.js uses canvas to render PDF. I have js scripts that draws the lines on the canvas when user double clicks on the canvas. It also adds X check mark to remove the already drawn line.
based on my research i cannot simply just remove the line from the canvas because underneath pixels are gone when you draw something on it. To get it working i have to store lines and then clear canvas and re-load canvas and re-draw lines
Issue
I am not able to store canvas and restore canvas. When i click on X i was able to get lines re-drawn but canvas does not get restored. Canvas remains blank
Run the demo in full page
$(function () {
var $canvas = $("#myCanvas");
var canvasEl = $canvas.get(0);
var ctx = canvasEl.getContext("2d");
var lines = [];
var backupCanvas = document.createElement("canvas");
var loadingTask = pdfjsLib.getDocument('https://raw.githubusercontent.com/mozilla/pdf.js/ba2edeae/web/compressed.tracemonkey-pldi-09.pdf');
loadingTask.promise.then(function (doc) {
console.log("This file has " + doc._pdfInfo.numPages + " pages");
doc.getPage(1).then(page => {
var scale = 1;
var viewPort = page.getViewport(scale);
canvasEl.width = viewPort.width;
canvasEl.height = viewPort.height;
canvasEl.style.width = "100%";
canvasEl.style.height = "100%";
var wrapper = document.getElementById("wrapperDiv");
wrapper.style.width = Math.floor(viewPort.width / scale) + 'px';
wrapper.style.height = Math.floor(viewPort.height / scale) + 'px';
page.render({
canvasContext: ctx,
viewport: viewPort
});
storeCanvas();
});
});
function storeCanvas() {
backupCanvas.width = canvasEl.width;
backupCanvas.height = canvasEl.height;
backupCanvas.ctx = backupCanvas.getContext("2d");
backupCanvas.ctx.drawImage(canvasEl, 0, 0);
}
function restoreCanvas() {
ctx.drawImage(backupCanvas, 0, 0);
}
$canvas.dblclick(function (e) {
var mousePos = getMousePos(canvasEl, e);
var line = { startX: 0, startY: mousePos.Y, endX: canvasEl.width, endY: mousePos.Y, pageY: e.pageY };
lines.push(line);
drawLine(line, lines.length - 1);
});
function drawLine(line, index) {
// draw line
ctx.beginPath();
ctx.strokeStyle = '#df4b26';
ctx.moveTo(line.startX, line.startY);
ctx.lineTo(line.endX, line.endY);
ctx.closePath();
ctx.stroke();
// add remove mark
var top = line.pageY;
var left = canvasEl.width + 20;
var $a = $("<a href='#' class='w-remove-line'>")
.data("line-index", index)
.attr("style", "line-height:0")
.css({ top: top, left: left, position: 'absolute' })
.html("x")
.click(function () {
var index = $(this).data("line-index");
$(".w-remove-line").remove();
ctx.clearRect(0, 0, canvasEl.width, canvasEl.height);
// restore canvas
restoreCanvas();
lines.splice(index, 1);
for (var i = 0; i < lines.length; i++) {
drawLine(lines[i], i);
}
});
$("body").append($a);
}
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
X: Math.floor(evt.clientX - rect.left),
Y: Math.floor(evt.clientY - rect.top),
};
}
});
canvas {
border: 1px solid red;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/pdf.js/2.2.228/pdf.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<b> Double Click on PDF to draw line and then click on X to remove lines</b>
<div id="wrapperDiv">
<canvas id="myCanvas"></canvas>
</div>
The PDF.js render() function is async so you need to store the canvas after the render has finished. Your code is firing storeCanvas() too early and storing a blank canvas. Easy fix, render() returns a promise so ...
page.render({
canvasContext: ctx,
viewport: viewPort
}).then( () => {
storeCanvas();
});
https://jsfiddle.net/fyLant01/1/
Reference: from https://github.com/mozilla/pdf.js/blob/master/src/display/api.js#L998
/**
* Begins the process of rendering a page to the desired context.
* #param {RenderParameters} params Page render parameters.
* #return {RenderTask} An object that contains the promise, which
* is resolved when the page finishes rendering.
*/

Image gets distorted when using css to set canvas height/width 100%

I am currently trying to draw an image to canvas, I have this so far:
"use strict";
var debugging = true;
var canvas = document.getElementById('astoniaCanvas');
var ctx = canvas.getContext('2d');
function loadUI() {
var topOverlay = new Image();
topOverlay.src = "/images/00000999.png";
topOverlay.onload = function() {
ctx.drawImage(topOverlay, 0, 0, canvas.width, 10);
}
var bottomOverlay = new Image();
bottomOverlay.src = "/images/00000998.png";
if (debugging) {
console.log('Drawing');
}
}
loadUI();
That works fine, but the image loads and looks like this:
When it should look like this:
The dimensions of the good looking picture are 800x40.
If I remove the
canvas {
width: 100%;
height: 100%;
}
the image goes back to looking normal, how can I scale my canvas?
Any information would be great thanks.
You arent accounting for height. Canvas can be confusing when it comes to height/width vs clientHeight/clientWidth
When you create a canvas the css width and height has no bearing on the number of pixels the internal canvas contains. Unless specifically set a canvas comes with a width height of 300x150.
A trick I have used in the past is to use the clientWidth and a scale to set everything
"use strict";
var debugging = true;
var canvas = document.getElementById('astoniaCanvas');
var ctx = canvas.getContext('2d');
function loadUI() {
var topOverlay = new Image();
topOverlay.onload = function() {
// use a scale between the image width and the canvas clientWidth
var scale = topOverlay.width / canvas.clientWidth;
var newWidth = canvas.clientWidth;
var newHeight = topOverlay.height * scale;
// resize canvas based on clientWidth
canvas.width = newWidth;
canvas.height = newHeight;
ctx.drawImage(topOverlay, 0, 0, newWidth, newHeight);
}
topOverlay.src = "http://i.stack.imgur.com/AJnjh.png";
// var bottomOverlay = new Image();
// bottomOverlay.src = "/images/00000998.png";
if (debugging) {
console.log('Drawing');
}
}
loadUI()
<canvas id="astoniaCanvas" style="width: 100%"></canvas>

Render .pdf to single Canvas using pdf.js and ImageData

I am trying to read an entire .pdf Document using PDF.js and then render all the pages on a single canvas.
My idea: render each page onto a canvas and get the ImageData (context.getImageData()), clear the canvas do the next page. I store all the ImageDatas in an array and once all pages are in there I want to put all the ImageDatas from the array onto a single canvas.
var pdf = null;
PDFJS.disableWorker = true;
var pages = new Array();
//Prepare some things
var canvas = document.getElementById('cv');
var context = canvas.getContext('2d');
var scale = 1.5;
PDFJS.getDocument(url).then(function getPdfHelloWorld(_pdf) {
pdf = _pdf;
//Render all the pages on a single canvas
for(var i = 1; i <= pdf.numPages; i ++){
pdf.getPage(i).then(function getPage(page){
var viewport = page.getViewport(scale);
canvas.width = viewport.width;
canvas.height = viewport.height;
page.render({canvasContext: context, viewport: viewport});
pages[i-1] = context.getImageData(0, 0, canvas.width, canvas.height);
context.clearRect(0, 0, canvas.width, canvas.height);
p.Out("pre-rendered page " + i);
});
}
//Now we have all 'dem Pages in "pages" and need to render 'em out
canvas.height = 0;
var start = 0;
for(var i = 0; i < pages.length; i++){
if(canvas.width < pages[i].width) canvas.width = pages[i].width;
canvas.height = canvas.height + pages[i].height;
context.putImageData(pages[i], 0, start);
start += pages[i].height;
}
});
So from the way I understnad thing this should work, right?
When I run this I end up with the canvas that is big enought to contain all the pages of the pdf but doesn't show the pdf...
Thank you for helping.
The PDF operations are asynchronous at all stages. This means you also need to catch the promise at the last render as well. If you not catch it you will only get a blank canvas as the rendering isn't finished before the loop continues to the next page.
Tip: I would also recommend that you use something else than getImageData as this will store uncompressed bitmap, for example the data-uri instead which is compressed data.
Here is a slightly different approach eliminating the for-loop and uses the promises better for this purpose:
LIVE FIDDLE
var canvas = document.createElement('canvas'), // single off-screen canvas
ctx = canvas.getContext('2d'), // to render to
pages = [],
currentPage = 1,
url = 'path/to/document.pdf'; // specify a valid url
PDFJS.getDocument(url).then(iterate); // load PDF document
/* To avoid too many levels, which easily happen when using chained promises,
the function is separated and just referenced in the first promise callback
*/
function iterate(pdf) {
// init parsing of first page
if (currentPage <= pdf.numPages) getPage();
// main entry point/function for loop
function getPage() {
// when promise is returned do as usual
pdf.getPage(currentPage).then(function(page) {
var scale = 1.5;
var viewport = page.getViewport(scale);
canvas.height = viewport.height;
canvas.width = viewport.width;
var renderContext = {
canvasContext: ctx,
viewport: viewport
};
// now, tap into the returned promise from render:
page.render(renderContext).then(function() {
// store compressed image data in array
pages.push(canvas.toDataURL());
if (currentPage < pdf.numPages) {
currentPage++;
getPage(); // get next page
}
else {
done(); // call done() when all pages are parsed
}
});
});
}
}
When you then need to retrieve a page you simply create an image element and set the data-uri as source:
function drawPage(index, callback) {
var img = new Image;
img.onload = function() {
/* this will draw the image loaded onto canvas at position 0,0
at the optional width and height of the canvas.
'this' is current image loaded
*/
ctx.drawImage(this, 0, 0, ctx.canvas.width, ctx.canvas.height);
callback(); // invoke callback when we're done
}
img.src = pages[index]; // start loading the data-uri as source
}
Due to the image loading it will be asynchronous in nature as well which is why we need the callback. If you don't want the asynchronous nature then you could also do this step (creating and setting the image element) in the render promise above storing image elements instead of data-uris.
Hope this helps!
I can’t speak to the part of your code that renders the pdf into a canvas, but I do see some problems.
Every resetting canvas.width or canvas.height automatically clears the canvas contents. So in the top section, your clearRect is not needed because the canvas is cleared by canvas.width prior to your every page.render.
More importantly, in the bottom section, all your previous pdf drawings are cleared by every canvas resizing (oops!).
getImageData() gets an array where each pixel is represented by 4 consecutive elements of that array (red then green then blue then alpha). Since getImageData() is an array, so it doesn’t have a pages[i].width or pages[i].height—it only has a pages[i].length. That array length cannot be used to determine widths or heights.
So to get you started, I would start by changing your code to this (very, very untested!):
var pdf = null;
PDFJS.disableWorker = true;
var pages = new Array();
//Prepare some things
var canvas = document.getElementById('cv');
var context = canvas.getContext('2d');
var scale = 1.5;
var canvasWidth=0;
var canvasHeight=0;
var pageStarts=new Array();
pageStarts[0]=0;
PDFJS.getDocument(url).then(function getPdfHelloWorld(_pdf) {
pdf = _pdf;
//Render all the pages on a single canvas
for(var i = 1; i <= pdf.numPages; i ++){
pdf.getPage(i).then(function getPage(page){
var viewport = page.getViewport(scale);
// changing canvas.width and/or canvas.height auto-clears the canvas
canvas.width = viewport.width;
canvas.height = viewport.height;
page.render({canvasContext: context, viewport: viewport});
pages[i-1] = context.getImageData(0, 0, canvas.width, canvas.height);
// calculate the width of the final display canvas
if(canvas.width>maxCanvasWidth){
maxCanvasWidth=canvas.width;
}
// calculate the accumulated with of the final display canvas
canvasHeight+=canvas.height;
// save the "Y" starting position of this pages[i]
pageStarts[i]=pageStarts[i-1]+canvas.height;
p.Out("pre-rendered page " + i);
});
}
canvas.width=canvasWidth;
canvas.height = canvasHeight; // this auto-clears all canvas contents
for(var i = 0; i < pages.length; i++){
context.putImageData(pages[i], 0, pageStarts[i]);
}
});
Alternatively, here’s a more traditional way of accomplishing your task:
Use a single “display” canvas and allow the user to “page through” each desired page.
Since you already start by drawing each page into a canvas, why not keep a separate, hidden canvas for each page. Then when the user wants to see page#6, you just copy the hidden canvas#6 onto your display canvas.
The Mozilla devs use this approach in their pdfJS demo here: http://mozilla.github.com/pdf.js/web/viewer.html
You can check out the code for the viewer here: http://mozilla.github.com/pdf.js/web/viewer.js
You can pass the number page to the promises , get that page canvas data and render in the right order on canvas
var renderPageFactory = function (pdfDoc, num) {
return function () {
var localCanvas = document.createElement('canvas');
///return pdfDoc.getPage(num).then(renderPage);
return pdfDoc.getPage(num).then((page) => {
renderPage(page, localCanvas, num);
});
};
};
var renderPages = function (pdfDoc) {
var renderedPage = $q.resolve();
for (var num = 1; num <= pdfDoc.numPages; num++) {
// Wait for the last page t render, then render the next
renderedPage = renderedPage.then(renderPageFactory(pdfDoc, num));
}
};
renderPages(pdf);
Complete example
function renderPDF(url, canvas) {
var pdf = null;
PDFJS.disableWorker = true;
var pages = new Array();
var context = canvas.getContext('2d');
var scale = 1;
var canvasWidth = 256;
var canvasHeight = 0;
var pageStarts = new Array();
pageStarts[0] = 0;
var k = 0;
function finishPage(localCanvas, num) {
var ctx = localCanvas.getContext('2d');
pages[num] = ctx.getImageData(0, 0, localCanvas.width, localCanvas.height);
// calculate the accumulated with of the final display canvas
canvasHeight += localCanvas.height;
// save the "Y" starting position of this pages[i]
pageStarts[num] = pageStarts[num -1] + localCanvas.height;
if (k + 1 >= pdf.numPages) {
canvas.width = canvasWidth;
canvas.height = canvasHeight; // this auto-clears all canvas contents
for (var i = 0; i < pages.length; i++) {
context.putImageData(pages[i+1], 0, pageStarts[i]);
}
var img = canvas.toDataURL("image/png");
$scope.printPOS(img);
}
k++;
}
function renderPage(page, localCanvas, num) {
var ctx = localCanvas.getContext('2d');
var viewport = page.getViewport(scale);
// var viewport = page.getViewport(canvas.width / page.getViewport(1.0).width);
// changing canvas.width and/or canvas.height auto-clears the canvas
localCanvas.width = viewport.width;
/// viewport.width = canvas.width;
localCanvas.height = viewport.height;
var renderTask = page.render({canvasContext: ctx, viewport: viewport});
renderTask.then(() => {
finishPage(localCanvas, num);
});
}
PDFJS.getDocument(url).then(function getPdfHelloWorld(_pdf) {
pdf = _pdf;
var renderPageFactory = function (pdfDoc, num) {
return function () {
var localCanvas = document.createElement('canvas');
///return pdfDoc.getPage(num).then(renderPage);
return pdfDoc.getPage(num).then((page) => {
renderPage(page, localCanvas, num);
});
};
};
var renderPages = function (pdfDoc) {
var renderedPage = $q.resolve();
for (var num = 1; num <= pdfDoc.numPages; num++) {
// Wait for the last page t render, then render the next
renderedPage = renderedPage.then(renderPageFactory(pdfDoc, num));
}
};
renderPages(pdf);
});
}

Higher DPI graphics with HTML5 canvas

Is there a way to set a custom DPI/PPI when creating an image using the HTML5 canvas? I know how can I draw on the canvas and export it as an image, but how can I make sure the output image is of certain DPI/PPI. I guess using SVG elemnts to draw on the canvas is a way, but wouldn't that be flattened out when I export the whole canvas as an image? Or calculating the device DPI and then scaling the image to meet my DPI requirement, but that doesn't seem like the correct solution.
Canvases have two different 'sizes': their DOM width/height and their CSS width/height. You can increase a canvas' resolution by increasing the DOM size while keeping the CSS size fixed, and then using the .scale() method to scale all of your future draws to the new bigger size. Here's an example:
function changeResolution(canvas, scaleFactor) {
// Set up CSS size.
canvas.style.width = canvas.style.width || canvas.width + 'px';
canvas.style.height = canvas.style.height || canvas.height + 'px';
// Resize canvas and scale future draws.
canvas.width = Math.ceil(canvas.width * scaleFactor);
canvas.height = Math.ceil(canvas.height * scaleFactor);
var ctx = canvas.getContext('2d');
ctx.scale(scaleFactor, scaleFactor);
}
The canvas default resolution is 96dpi (CSS inches, not based on the actual screen). So a scaleFactor of 2 gives 192dpi, 3 is 288dpi, etc. In fact, here's a version that should give your desired DPI:
function setDPI(canvas, dpi) {
// Set up CSS size.
canvas.style.width = canvas.style.width || canvas.width + 'px';
canvas.style.height = canvas.style.height || canvas.height + 'px';
// Resize canvas and scale future draws.
var scaleFactor = dpi / 96;
canvas.width = Math.ceil(canvas.width * scaleFactor);
canvas.height = Math.ceil(canvas.height * scaleFactor);
var ctx = canvas.getContext('2d');
ctx.scale(scaleFactor, scaleFactor);
}
Have fun! Note that both these code samples can only be used once per canvas, they assume the current DOM size is the original (they could be tweaked to change that). Also the rescaling needs to happen before you do any drawing on the canvas. Thanks to this post for the method and information!
Edit: Here is a more robust function that will scale future draws and maintain existing canvas contents. This can be called to rescale multiple times.
function setDPI(canvas, dpi) {
// Set up CSS size.
canvas.style.width = canvas.style.width || canvas.width + 'px';
canvas.style.height = canvas.style.height || canvas.height + 'px';
// Get size information.
var scaleFactor = dpi / 96;
var width = parseFloat(canvas.style.width);
var height = parseFloat(canvas.style.height);
// Backup the canvas contents.
var oldScale = canvas.width / width;
var backupScale = scaleFactor / oldScale;
var backup = canvas.cloneNode(false);
backup.getContext('2d').drawImage(canvas, 0, 0);
// Resize the canvas.
var ctx = canvas.getContext('2d');
canvas.width = Math.ceil(width * scaleFactor);
canvas.height = Math.ceil(height * scaleFactor);
// Redraw the canvas image and scale future draws.
ctx.setTransform(backupScale, 0, 0, backupScale, 0, 0);
ctx.drawImage(backup, 0, 0);
ctx.setTransform(scaleFactor, 0, 0, scaleFactor, 0, 0);
}
You cannot (ugh) access the DPI of a display of the current web page in any browser:
Detecting the system DPI/PPI from JS/CSS?
For printing: You most likely cannot set the DPI of exported <canvas> image (PNG, JPEG) using browser standard functions. However, if you use a pure Javascript encoder image encoder you are free to create any sort of binary file you wish and manually adjust the DPI value embedded int he binary.
https://gist.github.com/1245476
If you just want to set the dpi of the PNG (ie not increase the number of pixels) then this library lets you set the pHYs chunk (amongst other things):
https://github.com/imaya/CanvasTool.PngEncoder
Minimal example to export an HTML5 canvas to base64-encoded PNG:
// convert dots per inch into dots per metre
var pixelsPerM = dpi * 100 / 2.54;
var param = {
bitDepth : 8,
colourType : 2,
filterType : 0,
height : canvas.height,
interlaceMethod : 0,
phys : {
unit : 1,
x : pixelsPerM,
y : pixelsPerM
},
width : canvas.width
};
var array = canvas.getContext('2d').getImageData(0, 0, canvas.width,
canvas.height).data;
var png = new window.CanvasTool.PngEncoder(array, param).convert();
var base64 = 'data:image/png;base64,' + btoa(png);
Use the library changedpi:
npm install changedpi --save
Also see
https://github.com/shutterstock/changeDPI
https://github.com/hongru/canvas2image
Example code that also allows to adapt the px size and resolution for png or jpg export:
Canvas2Image.saveAsImage('fileName.png', canvas, 2000, 3000, 300, 'png');
-
import Url from './url';
import * as ChangeDpi from 'changeDPI';
export default class Canvas2Image {
static saveAsImage(fileName, canvas, width, height, dpi, type) {
type = this._fixType(type);
canvas = this._scaleCanvas(canvas, width, height);
let dataUrl = canvas.toDataURL(type);
let dataUrlWithDpi = ChangeDpi.changeDpiDataUrl(dataUrl, dpi)
dataUrlWithDpi = dataUrlWithDpi.replace(type, 'image/octet-stream');
Url.download(fileName, dataUrlWithDpi);
}
static _fixType(type) {
type = type.toLowerCase().replace(/jpg/i, 'jpeg');
const r = type.match(/png|jpeg|bmp|gif/)[0];
return `image/${r}`;
}
static _scaleCanvas(canvas, width, height) {
const w = canvas.width;
const h = canvas.height;
if (width === undefined) {
width = w;
}
if (height === undefined) {
height = h;
}
const retCanvas = document.createElement('canvas');
const retCtx = retCanvas.getContext('2d');
retCanvas.width = width;
retCanvas.height = height;
retCtx.drawImage(canvas, 0, 0, w, h, 0, 0, width, height);
return retCanvas;
}
}
-
export default class Url {
static download(fileName, url) {
const element = document.createElement('a');
element.setAttribute('href', url);
element.setAttribute('download', fileName);
element.style.display = 'none';
document.body.appendChild(element);
element.click();
document.body.removeChild(element);
}
static createUrlForBlob(blob) {
return this._URL.createObjectURL(blob);
}
static clearBlobUrl(blobUrl) {
this._URL.revokeObjectURL(blobUrl);
}
static get _URL() {
return window.URL || window.webkitURL || window;
}
}

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