javascript, dynamically loading canvases and drawing context in function; context disappears - javascript

To summarize, I am changing what canvases are shown via ajax calls and what gets drawn on each as well. The main problem is that my main drawing function fails on getContext and there is other weird behavior like missing canvases (but they are definitely there).
Here is the function I use to draw on my canvases:
function PlotSet(plot_properties){
var p = plot_properties;
var canvas = document.getElementById(p.panel_name);
var spinner_target = document.getElementById(p.spinner_name);
if (!canvas) {
alert('Error: Cannot find the canvas element!');
return;
} else if (!canvas.getContext) {
alert('Error: Canvas context does not exist!');
return;
} else {
var ctx = canvas.getContext('2d');
/*Do the drawing*/
}
}
I have 3 buttons in a section of my page that determine how many and the arrangement of the panels/canvas. When someone clicks one, another php page with that arrangement is loaded.
<div id="panel_single" name="panel_control" class="xs-btn sm-btn-h-grp sm-btn-one-panel"></div>
<div id="panel_split" name="panel_control" class="xs-btn sm-btn-h-grp sm-btn-four-panel"></div>
<div id="panel_rows" name="panel_control" class="xs-btn sm-btn-h-grp sm-btn-list-panel"></div>
<script>
$("#panel_single_settings").hide();
$("div[name=panel_control]").click(function(event) {
var nav_id = $(this).attr("id");
if(nav_id == "panel_single"){
$("#panel_single_settings").show();
}else{
$("#panel_single_settings").hide();
}
//$("span[name=vnav-text]").removeClass("vnav-active");
//$("#" + nav_id).addClass('vnav-active');
refresh_content($(".widget-panel"), nav_id, function(){
side_panel_hide($(".side_panel"), "600px");
});
});
</script>
Everything works when I click on these after loading page. But I have two issues when changing the drawing properties/redrawning. (1) the PlotSet function fails on getContext for everything except the first canvas, when multiple canvases are shown, when this function is used to redraw an already drawn canvas (using a pop-up form with ajax to set what is redrawn). (2) When only one panel is shown, the settings can be changed and the panel can be redrawn, but then when I then load a page with multiple panels, all panels but the first are blank (i.e. this page functions as expected, except when I redrawn a canvas on another page).
Single Panel example:
<script>
var plot_properties = {
//Settings to draw
}
PlotSet(plot_properties);
</script>
example of multi panel configuration:
<canvas class="analytics-graph-medium" id="graph_send"></canvas>
<canvas class="analytics-graph-medium" id="graph_recv"></canvas>
<canvas class="analytics-graph-medium" id="graph_cpu"></canvas>
<canvas class="analytics-graph-medium" id="graph_ram"></canvas>
<script>
var split_send_plot_properties = {
//Settings to draw
}
PlotSet(split_send_plot_properties);
var split_recv_plot_properties = {
//Settings to draw
}
PlotSet(split_recv_plot_properties);
.
.
.
//**************************************************************
//Panel Settings Events
//**************************************************************
$("#cpu_panel_settings").on("click", function(){
var panel_name = $(this).attr('id');
set_side_panel_html($(".side_panel"), "/gui/pages/status/dialogs/panel_settings.php", split_cpu_plot_properties, function(){
side_panel_show($(".side_panel"), "500px");
});
});
.
.
.
</script>
And this is the submit button action for changing the settings:
$("#mod_panel").click(function(){
plot_properties = {
//lots of properties to be drawn
}
PlotSet(plot_properties);
side_panel_hide($(".side_panel"), "600px");
});

Well, I seem to have solved the problem. I still do not understand the problem, but the solution is to add a new canvas with the same name, remove the old canvas and them to re-grab the new canvas with the same old name with getElementById. But only do that when the canvas can be found, but the context does not exist... otherwise use the original context.
function PlotSet(plot_properties){
var p = plot_properties;
var spinner_target = document.getElementById(p.spinner_name);
var canvas = document.getElementById(p.panel_name);
if (!canvas) {
alert('Error: Cannot find the canvas element!');
return;
} else if (!canvas.getContext) {
var new_canvas = document.createElement(p.panel_name);
canvas.parentNode.insertBefore(new_canvas, canvas.nextSibling);
canvas.parentNode.removeChild(canvas);
canvas = document.getElementById(p.panel_name);
}
var ctx = canvas.getContext('2d');
//Draw Something
}
This makes everything work, all the dynamic page loading with multiple canvases on them, plus changing settings/drawing of any canvas, and switching pages after a chane is made.
If anyone has any futher insight into what caused the problem or why this solution worked, I'm eager ears.

Related

Turning HTML content in to a canvas element [duplicate]

It would be incredibly useful to be able to temporarily convert a regular element into a canvas. For example, say I have a styled div that I want to flip. I want to dynamically create a canvas, "render" the HTMLElement into the canvas, hide the original element and animate the canvas.
Can it be done?
There is a library that try to do what you say.
See this examples and get the code
http://hertzen.com/experiments/jsfeedback/
http://html2canvas.hertzen.com/
Reads the DOM, from the html and render it to a canvas, fail on some, but in general works.
Take a look at this tutorial on MDN: https://developer.mozilla.org/en/HTML/Canvas/Drawing_DOM_objects_into_a_canvas (archived)
Its key trick was:
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var data = '<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">' +
'<foreignObject width="100%" height="100%">' +
'<div xmlns="http://www.w3.org/1999/xhtml" style="font-size:40px">' +
'<em>I</em> like ' +
'<span style="color:white; text-shadow:0 0 2px blue;">' +
'cheese</span>' +
'</div>' +
'</foreignObject>' +
'</svg>';
var DOMURL = window.URL || window.webkitURL || window;
var img = new Image();
var svg = new Blob([data], {type: 'image/svg+xml;charset=utf-8'});
var url = DOMURL.createObjectURL(svg);
img.onload = function () {
ctx.drawImage(img, 0, 0);
DOMURL.revokeObjectURL(url);
}
img.src = url;
That is, it used a temporary SVG image to include the HTML content as a "foreign element", then renders said SVG image into a canvas element. There are significant restrictions on what you can include in an SVG image in this way, however. (See the "Security" section for details — basically it's a lot more limited than an iframe or AJAX due to privacy and cross-domain concerns.)
Sorry, the browser won't render HTML into a canvas.
It would be a potential security risk if you could, as HTML can include content (in particular images and iframes) from third-party sites. If canvas could turn HTML content into an image and then you read the image data, you could potentially extract privileged content from other sites.
To get a canvas from HTML, you'd have to basically write your own HTML renderer from scratch using drawImage and fillText, which is a potentially huge task. There's one such attempt here but it's a bit dodgy and a long way from complete. (It even attempts to parse the HTML/CSS from scratch, which I think is crazy! It'd be easier to start from a real DOM node with styles applied, and read the styling using getComputedStyle and relative positions of parts of it using offsetTop et al.)
You can use dom-to-image library (I'm the maintainer).
Here's how you could approach your problem:
var parent = document.getElementById('my-node-parent');
var node = document.getElementById('my-node');
var canvas = document.createElement('canvas');
canvas.width = node.scrollWidth;
canvas.height = node.scrollHeight;
domtoimage.toPng(node).then(function (pngDataUrl) {
var img = new Image();
img.onload = function () {
var context = canvas.getContext('2d');
context.translate(canvas.width, 0);
context.scale(-1, 1);
context.drawImage(img, 0, 0);
parent.removeChild(node);
parent.appendChild(canvas);
};
img.src = pngDataUrl;
});
And here is jsfiddle
Building on top of the Mozdev post that natevw references I've started a small project to render HTML to canvas in Firefox, Chrome & Safari. So for example you can simply do:
rasterizeHTML.drawHTML('<span class="color: green">This is HTML</span>'
+ '<img src="local_img.png"/>', canvas);
Source code and a more extensive example is here.
No such thing, sorry.
Though the spec states:
A future version of the 2D context API may provide a way to render fragments of documents, rendered using CSS, straight to the canvas.
Which may be as close as you'll get.
A lot of people want a ctx.drawArbitraryHTML/Element kind of deal but there's nothing built in like that.
The only exception is Mozilla's exclusive drawWindow, which draws a snapshot of the contents of a DOM window into the canvas. This feature is only available for code running with Chrome ("local only") privileges. It is not allowed in normal HTML pages. So you can use it for writing FireFox extensions like this one does but that's it.
You could spare yourself the transformations, you could use CSS3 Transitions to flip <div>'s and <ol>'s and any HTML tag you want. Here are some demos with source code explain to see and learn: http://www.webdesignerwall.com/trends/47-amazing-css3-animation-demos/
the next code can be used in 2 modes, mode 1 save the html code to a image, mode 2 save the html code to a canvas.
this code work with the library: https://github.com/tsayen/dom-to-image
*the "id_div" is the id of the element html that you want to transform.
**the "canvas_out" is the id of the div that will contain the canvas
so try this code.
:
function Guardardiv(id_div){
var mode = 2 // default 1 (save to image), mode 2 = save to canvas
console.log("Process start");
var node = document.getElementById(id_div);
// get the div that will contain the canvas
var canvas_out = document.getElementById('canvas_out');
var canvas = document.createElement('canvas');
canvas.width = node.scrollWidth;
canvas.height = node.scrollHeight;
domtoimage.toPng(node).then(function (pngDataUrl) {
var img = new Image();
img.onload = function () {
var context = canvas.getContext('2d');
context.drawImage(img, 0, 0);
};
if (mode == 1){ // save to image
downloadURI(pngDataUrl, "salida.png");
}else if (mode == 2){ // save to canvas
img.src = pngDataUrl;
canvas_out.appendChild(img);
}
console.log("Process finish");
});
}
so, if you want to save to image just add this function:
function downloadURI(uri, name) {
var link = document.createElement("a");
link.download = name;
link.href = uri;
document.body.appendChild(link);
link.click();
}
Example of use:
<html>
<head>
</script src="/dom-to-image.js"></script>
</head>
<body>
<div id="container">
All content that want to transform
</div>
<button onclick="Guardardiv('container');">Convert<button>
<!-- if use mode 2 -->
<div id="canvas_out"></div>
</html>
Comment if that work.
Comenten si les sirvio :)
The easiest solution to animate the DOM elements is using CSS transitions/animations but I think you already know that and you try to use canvas to do stuff CSS doesn't let you to do. What about CSS custom filters? you can transform your elements in any imaginable way if you know how to write shaders. Some other link and don't forget to check the CSS filter lab.
Note: As you can probably imagine browser support is bad.
function convert() {
dom = document.getElementById('divname');
var script,
$this = this,
options = this.options,
runH2c = function(){
try {
var canvas = window.html2canvas([ document.getElementById('divname') ], {
onrendered: function( canvas ) {
window.open(canvas.toDataURL());
}
});
} catch( e ) {
$this.h2cDone = true;
log("Error in html2canvas: " + e.message);
}
};
if ( window.html2canvas === undefined && script === undefined ) {
} else {.
// html2canvas already loaded, just run it then
runH2c();
}
}

draggable objects and canvas in loop

I have three questions. The first question is the most important but I appreciate to get answers to the others.
First question, see this jsfiddle:
https://jsfiddle.net/owLdgrdq/10
//copia nariz
$(document).on('click', '#add-nariz', function(){
var a = $('#add-nariz');
var src = a.attr('src');
var elem = $('<img class="objetos" src="' + src + '" width="30px" height="30px" style="positon: relative;" />');
$('#fotoAlterada').append(elem);
elem.draggable();
});
//copia bolinha azul
$(document).on('click', '#add-bb', function(){
var a = $('#add-bb');
var src = a.find('img:first').attr('src');
var elem = $('<img class="objetos" src="' + src + '" width="30px" height="30px" style="positon: relative;" />');
$('#fotoAlterada').append(elem);
elem.draggable();
});
$(document).on('click', '#salvaImagem', function(){
var m = $('#foto');
var totX = parseInt(m.css('width'));
var totY = parseInt(m.css('height'));
var c = document.getElementById('myCanvas');
c.width = totX;
c.height = totY;
var ctx = c.getContext('2d');
alert(totX + '\n' + totY);
var base = document.getElementById('foto');
ctx.drawImage(base,0,0,totX,totY,0,0,totX,totY);
var posicoes = [];
$(".objetos").each(function(){
var img = $(this);
x = parseInt(img.css("left"))+totX;
y = parseInt(img.css("top"))+totY;
altura = parseInt(img.css("width"));
largura = parseInt(img.css("height"));
posicoes.push([
x,
y,
largura,
altura
]);
});
alert( JSON.stringify(posicoes));
var j;
var numAderecos = posicoes.length;
for(j = 0; j < numAderecos; j++){
ctx.drawImage(base,posicoes[j][0],posicoes[j][1],posicoes[j][2],posicoes[j][3]);
}
});
I have some icons (Adicionar isto:) what I want add to a photo (Editar isto:). When the user clicks on the icons, a copy is made beside the photo. More click make more copies. These copies are draggable. The user choose the position of the objects and click in a button to save ("Salvar Mudanças"). This button makes a copy of the original photo with the new objects on it (inside a canvas).
At final of the JS code I put little photos (same as original) as little objects added to the photo in the canvas, but just because I don't know how to copy each object added. How can I do that?
Second question, when click on the button, the positions are not the same in the canvas, they are pull to down a little (and left too). Why is this happening if I put padding and margin 0px in CSS code?
Third question, the variable 'base' I used $('#foto') to get by id, but I can't use it at canvas arguments. When I use document.getElementById('foto') I can. What is the difference among them?
----editing ----
My aim with this is to make a mobile application, through cordova/phonegap. The user must be able to get a picture from the device or a selfie with the cam (it's already ok). When the picture/photo is gotten, some new objects like a crown's nose, colorful hat and other silly things appear on the window (it's ok, just change style="display: none;" to be displayed), when the user clicks on these things they make a copy at the bottom of the photo and these copies must be draggable (it's nice too). Then, with jQuery-UI I'm getting the position of all dragged objects into the photo, the user clicks on a button "Salvar Mudanças" ("save changes"), and this action copy the photo plus objects to a canvas area (this is the way I got to make the app). After, the user clicks on another button to share the modified image (in canvas) through whatsapp, facebook, email and any other way which the device could do.
Here is an alternate way of appending icons onto an image using just a canvas
Carve out a toolbar area at the top of an html5 canvas and fill it with your desired icons.
Put the destination image below the toolbar.
Listen for mouse events.
Allow the user to icons from the toolbar onto the image below.
When the user "drops" the dragging icon by releasing the mouse, create a duplicate of the dragged icon at the dropped position.
Here is annotated code and a Demo:
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
function reOffset(){
var BB=canvas.getBoundingClientRect();
offsetX=BB.left;
offsetY=BB.top;
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }
var isDown=false;
var startX,startY;
ctx.lineWidth=0.50;
ctx.strokeStyle='lightgray';
ctx.fillStyle='white';
// vars for icons, etc
var tbar={width:cw,height:35}
var tw=30;
var th=30;
var dragging=null;
var dropped=[];
var icons=[
{name:'Flower',x:tw*0,y:2,url:'https://dl.dropboxusercontent.com/u/139992952/multple/flower.png'},
{name:'Star',x:tw*1,y:2,url:'https://dl.dropboxusercontent.com/u/139992952/multple/star.png'},
{name:'Plane',x:tw*2,y:2,url:'https://dl.dropboxusercontent.com/u/139992952/multple/plane2.png'},
{name:'Mario',x:tw*3,y:2,url:'https://dl.dropboxusercontent.com/u/139992952/multple/marioStanding.png'},
];
var thumbs=[];
var mainImg=new Image();
mainImg.crossOrigin='anonymous';
mainImg.onload=start;
mainImg.src='https://dl.dropboxusercontent.com/u/139992952/multple/husky.jpg';
var imgCount=icons.length+1;
for(var i=0;i<icons.length;i++){
var icon=icons[i];
icon.index=i;
icon.img=new Image();
icon.img.crossOrigin='anonymous';
icon.img.onload=start;
icon.img.src=icon.url;
}
// start is called when each image is fully loaded
function start(){
// wait for all images to load
if(--imgCount>0){return;}
// create
for(var i=0;i<icons.length;i++){
var icon=icons[i];
thumbs.push(thumb(icon.img,tw,th));
}
// draw the toolbar & image
draw();
// listen for mouse events
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUpOut(e);});
$("#canvas").mouseout(function(e){handleMouseUpOut(e);});
}
// create thumbnails for each image (to be used as icons in the toolbar)
function thumb(img,w,h){
var iw=img.width;
var ih=img.height;
var s=Math.min((w/iw),(h/ih))
var c=document.createElement('canvas');
c.width=iw*s;
c.height=ih*s;
c.getContext('2d').drawImage(img,0,0,iw,ih,0,0,iw*s,ih*s);
return(c);
}
// draw the toolbar, image & any clone thumbnails added to the image
function draw(){
ctx.clearRect(0,0,cw,ch);
ctx.fillRect(0,0,cw,tbar.height);
ctx.strokeRect(0,0,cw,tbar.height);
ctx.drawImage(mainImg,0,tbar.height);
for(var i=0;i<icons.length;i++){
var icon=icons[i];
ctx.drawImage(thumbs[icon.index],icon.x,icon.y);
}
for(var i=0;i<dropped.length;i++){
var icon=dropped[i];
ctx.drawImage(thumbs[icon.thumbIndex],icon.x,icon.y);
}
if(dragging){
ctx.drawImage(thumbs[dragging.thumbIndex],dragging.x,dragging.y);
}
}
//
function handleMouseDown(e){
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
// get the mouse position
startX=parseInt(e.clientX-offsetX);
startY=parseInt(e.clientY-offsetY);
// hit test the toolbar icons and any dropped icons
var mx=startX;
var my=startY;
// hit test toolbar icons
if(mx>0 && mx<tw*icons.length && my>2 && my<2+tw){
var icon=icons[parseInt(mx/tw)];
dragging={thumbIndex:icon.index,x:icon.x,y:icon.y};
dragging.source='icons';
return;
}
// hit test dropped icons
for(var i=0;i<dropped.length;i++){
var icon=dropped[i];
if(mx>icon.x && mx<icon.x+tw && my>icon.y && my<icon.y+th){
dragging=dropped[i];
dragging.source='dropped';
dragging.droppedIndex=i;
return;
}
}
}
// Add any
function handleMouseUpOut(e){
if(!dragging){return;}
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
//
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// add the icon to its dropped position
if(dragging.source=='icons'){
if(dragging.y>tbar.height){
dropped.push(dragging);
}
// remove the dropped icon if it's back in the toolbar
}else if(dragging.y<tbar.height){
dropped.splice(dragging.droppedIndex,1);
}
// clear dragging
dragging=null;
// redraw
draw();
}
// move any icon that's being dragged
function handleMouseMove(e){
if(!dragging){return;}
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
// get mouse position
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// move the dragged icon by the distance the mouse
// has moved since the last mousemove event
var dx=mouseX-startX;
var dy=mouseY-startY;
startX=mouseX;
startY=mouseY;
dragging.x+=dx;
dragging.y+=dy;
// redraw
draw();
}
// save just the image with dropped icons
$('#save').click(function(){
var c=document.createElement('canvas');
c.width=cw;
c.height=ch-tbar.height;
c.getContext('2d').drawImage(canvas,0,tbar.height,cw,ch-tbar.height,0,0,c.width,c.height);
var img=new Image();
img.onload=function(){
document.body.appendChild(img);
}
img.src=c.toDataURL();
});
body{ background-color: ivory; }
#canvas{border:1px solid lightgray; }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<button id='save'>Save</button>
<h4>Drag from top toolbar & icon will be drop-copied<br>Drag dropped icons around image<br>Drag dropped icon back to toolbar to delete.</h4>
<canvas id="canvas" width=300 height=300></canvas>
It's a little tough to follow, but I suspect you're asking about compositing all of the existing DOM images onto a single canvas context. For that you'll probably want to look at the globalCompositeOperation property on the 2dcanvas:
globalCompositeOperation - the property
Compositing Tutorial - basic outline of how the different types work
Also your 3rd question: $('#foto') returns a jquery object, and document.getElementById returns a DOM object. To get the actual DOM element you would do something like: var theEl = $('#foto')[0]

Drawing on canvas after megapix rendering is reversed

I have a page which allows you to browse in an image, then draw on it and save both the original and the annotated version. I am leveraging megapix-image.js and exif.js to help in rendering images from multiple mobile devices properly. It works great, except in certain orientations. For example, a vertical photo taken on an iPhone4s is considered orientation 6 by exif and gets flipped accordingly by megapix-image so it's rendered nicely on the canvas. For some reason, when I draw on it afterward, it seems like the drawing is reversed. Mouse and touch both behave the same way. The coordinates look right to me (meaning they match a working horizontal pic and a non-working vertical pic), as does the canvas height and width when megapix-image.js flips it. This leads me to believe it has something to do with the context, but honestly, I am not really sure. I have a JS fiddle of the part of my work that shows the behavior. Just browse in a vertically taken pic from a mobile device or take a pic in vertical format on a mobile device and use it. I think all will show this same behavior.
The final rendering is done like this:
function RenderImage(file2) {
if (typeof file2[0].files[0] != 'undefined') {
EXIF.getData(file2[0].files[0], function () {
orientation = EXIF.getTag(this, "Orientation");
var file = file2[0].files[0];
var mpImg = new MegaPixImage(file);
var resCanvas1 = document.getElementById('annoCanvas');
mpImg.render(resCanvas1, {
maxWidth: 700,
maxHeight: 700,
orientation: orientation
});
});
}
}
But the full jsfiddle is here:
http://jsfiddle.net/awebster28/Tq3qU/6/
Does anyone have any clues for me?
If you look at the lib you are using there is a transformCoordinate function that is used to set the right transform before drawing.
And they don't save/restore the canvas (boooo!!!) so it remains with this transform after-wise.
Solution for you is to do what the lib should do : save the context before the render and restore it after :
function RenderImage(file2) {
// ... same code ...
var mpImg = new MegaPixImage(file);
var eData = EXIF.pretty(this);
// Render resized image into canvas element.
var resCanvas1 = document.getElementById('annoCanvas');
var ctx = resCanvas1.getContext('2d');
ctx.save();
//setting the orientation flips it
mpImg.render(resCanvas1, {
maxWidth: 700,
maxHeight: 700,
orientation: orientation
});
ctx.restore();
//...
}
I ended up fixing this by adding another canvas to my html (named "annoCanvas2"). Then, I updated megapix-image.js to include this function, which draws the contents of the new canvas to a fresh one:
function drawTwin(sourceCanvas)
{
var id = sourceCanvas.id + "2";
var destCanvas = document.getElementById(id);
if (destCanvas !== null) {
var twinCtx = destCanvas.getContext("2d");
destCanvas.width = sourceCanvas.width;
destCanvas.height = sourceCanvas.height;
twinCtx.drawImage(sourceCanvas, 0, 0, sourceCanvas.width, sourceCanvas.height);
}
}
Then, just after the first is rotated and flipped and rendered, I rendered the resulting canvas to my "twin". Then I had a nice canvas, with my updated image that I could then draw on and also save!
var tagName = target.tagName.toLowerCase();
if (tagName === 'img') {
target.src = renderImageToDataURL(this.srcImage, opt, doSquash);
} else if (tagName === 'canvas') {
renderImageToCanvas(this.srcImage, target, opt, doSquash);
//------I added this-----------
drawTwin(target);
}
I was glad to have it fixed so I met my deadline, but I am still not sure why I had to do this. If anyone out there can explain it, I'd love to know why.

drag or move image inside a canvas using touchmove in HTML5 for touch devices

I'm creating a HTML5 game which can be made to run on Android also. I went through few articles and didn't get the solution yet. I have a image which i'm generating through javascript and I want to move this image using touchmove so that I can run it in my Android device. This is the code:
gameCanvas.addEventListener("touchmove", touchXY, true);
function touchXY(e) {
if (!e)
var e = event;
e.preventDefault();
avatarX = e.targetTouches[0].pageX - gameCanvas.offsetLeft;
avatarY = e.targetTouches[0].pageY - gameCanvas.offsetTop;
}
This is not working. I got this code from https://developer.apple.com/library/archive/documentation/AudioVideo/Conceptual/HTML-canvas-guide/AddingMouseandTouchControlstoCanvas/AddingMouseandTouchControlstoCanvas.html
And this is my canvas:
<canvas id="gameCanvas" onclick="setUpGame();" width="400" height="300"></canvas>
This is my image:
avatarImage.src = "img/avatar.png";
gameCanvas.getContext("2d").drawImage(avatarImage, Math.random() * 100, Math.random() * 100);
I just want to move the image inside the canvas.
I wrote a full example, hopefully it's not too verbose.
<!DOCTYPE html>
<html>
<head>
<meta charset='UTF-8'>
</head>
<body>
<canvas id='canvas' width='512' height='512'></canvas>
<script>
var c=document.getElementById('canvas'),
ctx=c.getContext('2d'),
activeBox='none',
//populate the map with objects
box=[
{
x:256,
y:128,
width:32,
height:64
},
{
x:128,
y:64,
width:64,
height:64
},
{
x:32,
y:32,
width:32,
height:32
},
];
function draw(){
//clear the screen, draw population
ctx.clearRect(0,0,c.width,c.height);
for(var i=0;i<box.length;i++){
ctx.fillRect(box[i].x,box[i].y,box[i].width,box[i].height);
}
//repeat at 60fps if possible, pause if window looses focus
requestAnimationFrame(draw);
}
function startTouch(e){
//this makes it easier to write control flow later and keeps XY relative to canvas
var xTouch=e.touches[0].pageX-c.offsetLeft,
yTouch=e.touches[0].pageY-c.offsetTop;
//its best to go through this loop in touchstart, because it only happens once per touch
for(var i=0;i<box.length;i++){
if(xTouch>box[i].x&&xTouch<box[i].x+box[i].width){
if(yTouch>box[i].y&&yTouch<box[i].y+box[i].height){
activeBox=i;
}
}
}
}
function moveTouch(e){
//grab a box by the center
if(activeBox!='none'){
box[activeBox].x=e.changedTouches[0].pageX-box[activeBox].width/2;
box[activeBox].y=e.changedTouches[0].pageY-box[activeBox].height/2;
}
}
function endTouch(){
//clear active so that dragging empty space wont move the last active box
activeBox='none';
}
canvas.addEventListener('touchstart',startTouch);
canvas.addEventListener('touchmove',moveTouch);
canvas.addEventListener('touchend',endTouch);
window.addEventListener('load',draw);
</script>
</body>
</html>
I used fillRect for simplicity, but if you want to replace it with drawImage you'll need to create a new element for each and add a source property to the box object array. Here's a partial example.
//you need a new one of these for every image
var img=new Image();
img.src='http://www.w3schools.com/images/w3logotest2.png';
var box={
source:img,
x:Math.floor((Math.random()*256)+1),
y:Math.floor((Math.random()*256)+1)
};
//make sure the image doesnt load before the script
window.onload=function(){
ctx.drawImage(img,box.x,box.y);
}

Calling several functions using JavaScript's window.onload attribute

I have the following code in the index.html page of my site, which when the page loads, draws a number of images to the HTML5 canvas:
window.onload = function(){
var sources = {};
sources[0] = document.getElementById("building").src,
sources[1] = document.getElementById("chair").src,
sources[2] = document.getElementById("drink").src,
sources[3] = document.getElementById("food").src,
sources[4] = document.getElementById("fridge").src,
sources[5] = document.getElementById("land").src,
sources[6] = document.getElementById("money").src,
sources[7] = document.getElementById("oven").src,
sources[8] = document.getElementById("table").src,
sources[9] = document.getElementById("van").src,
sources[10] = document.getElementById("burger").src,
sources[11] = document.getElementById("chips").src,
sources[12] = document.getElementById("drink").src,
sources[13] = document.getElementById("franchiseFee").src,
sources[14] = document.getElementById("wages").src,
sources[15] = document.getElementById("admin").src,
sources[16] = document.getElementById("cleaners").src,
sources[17] = document.getElementById("electricity").src,
sources[18] = document.getElementById("insurance").src,
sources[19] = document.getElementById("manager").src,
sources[20] = document.getElementById("rates").src,
sources[21] = document.getElementById("training").src,
sources[22] = document.getElementById("water").src,
sources[23] = document.getElementById("burger").src,
sources[24] = document.getElementById("chips").src,
sources[25] = document.getElementById("drink").src,
sources[26] = document.getElementById("creditors").src,
sources[27] = document.getElementById("electricity").src,
sources[28] = document.getElementById("food").src,
sources[29] = document.getElementById("hirePurchase").src,
sources[30] = document.getElementById("loan").src,
sources[31] = document.getElementById("overdraft").src,
sources[32] = document.getElementById("payeTax").src,
sources[33] = document.getElementById("tax").src
loadImages(sources, drawImage);
};
Sources is the array that I'm using to hold the images in JavaScript so that they can be drawn to the canvas once they've been loaded from a hidden section in my HTML.
This function currently works exactly as it's intended- it has a call to the loadImages function, which loads the images from a hidden section in the HTML into the JavaScript array, and calls the drawImage function on each of the images in the array.
But I also have another function that I want to be called with the window.onload:
The function I want to add to window.onload is this:
function drawGameElements(){
/* Draw a line for the 'score bar'. */
context.moveTo(0, 25);
context.lineTo(1000, 25);
context.stroke();
/* Draw current level/ total levels on the left, and current score on the right. */
context.font = "11pt Calibri"; /* Text font & size */
context.strokeStyle = "black"; /* Font colour */
context.strokeText(currentLevel + "/" + totalLevels, 10, 15);
context.strokeText(currentScore, 950, 15);
}
I tried adding a call to the function just below the loadImages(sources, drawImage); line in window.onload = function(){};
So that I now have:
window.onload = function(){
...
loadImages(sources, drawImage);
drawGameElements();
};
Although this partially works, in that it draws the line across the top of the canvas for the 'score bar' and writes "1/3" for the levels on the left hand side of the canvas just above the line, for some reason it doesn't draw the current score on the right hand side.
Also, as soon as I click on one of the images that's been drawn to the canvas, to drag and drop it around the canvas, the 'score bar' then disappears from the canvas completely.
Does anyone know why this is? How can I get the score bar to remain visible throughout the duration of the game, no matter what else happens on the canvas? Also, how can I get the currentScore variable to be displayed?
I would instead use a callback inside of loadImages to call drawGameElements. That way drawGameElements only runs after loadImages is finished. It sounds like you've created a race condition with your two functions.
If I where you, I would create the score bar outside of the canvas. Set its position to absolute and let it hover above the canvas at the desired location. That way you can just access it as HTML elements and do not need to worry about redrawing.
The major advantage of using HTML elements for these displays is that you don't need to redraw them, for example after moving the images around on the canvas. You also don't have to worry about how to refresh the values (on a canvas, just painting new values over the old ones will not be enough). This way you can just store the values in HTML elements and go wild on the canvas, knowing the score will always stay on top.

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