Edit: Demo Code is found in the "parallelogram explorer" tab here: http://atmdev01.procloud.net/geometry_tools9/
So I'm calling the following javascript function at document load to draw the perimeter of a parallelogram, and it is working just fine to do that. The issue is when I call the function from the touchmove handler to allow an iPad user to adjust the size of the parallelogram, the canvas is not properly redrawing the shape. In fact it is not responding at all. I've run some alerts to verify that this function is actually being run and it is.
Could it be an issue with the way I'm clearing the canvas (the "canvas.width = canvas.width + 0" method) or simply with refresh rates on the iPad?
The interesting part is that it's working perfectly in a desktop browser using mousemove, but not on an iPad using touchmove. Please advise...
(the "corners" in the code are the areas where the user can touch and drag to resize the parallelogram)
this.drawSides = function() {
var order = [1, 2, 4, 3];
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var firstCorner = this.getCornerObj(1);
var secondCorner = this.getCornerObj(2);
var firstCornerOpp = this.getCornerObj(firstCorner.opp);
var secondCornerOpp = this.getCornerObj(secondCorner.opp);
/* Clear the canvas and draw a parallelogram with the provided corners */
canvas.width = canvas.width + 0; //clears the canvas faster than clearRect
ctx.beginPath();
for (i in order) {
if (i < order.length) {
var cornerObj = this.getCornerObj(order[i]);
if (order[i] > 1) {
var prevObj = this.getCornerObj(order[i-1]);
ctx.moveTo(prevObj.x + (prevObj.width)/2, prevObj.y + (prevObj.height)/2);
ctx.lineTo(cornerObj.x + (cornerObj.width)/2, cornerObj.y + (cornerObj.height)/2);
}
}
}
ctx.lineTo(firstCorner.x + (firstCorner.width)/2, firstCorner.y + (firstCorner.height)/2);
ctx.strokeStyle = "#300";
ctx.stroke();
}
The canvas isn't cleared properly with canvas.width = canvas.width; in safari.
Keep in mind that it's less computationally expensive to clear the canvas using context.clearRect() than it is to reset the canvas size. This is really important for mobile devices and tablets.
Reference: http://www.html5rocks.com/en/tutorials/canvas/performance/#toc-clear-canvas
I've resolved this issue. Turns out the issue was that I wasn't properly updating my cornerObjects' x and y coordinates on touchmove. (the code excerpt above has no issue)
Also, for future reference, canvas.width = canvas.width + 0; works just fine on Safari and the iPad.
Related
i'm working with an hybrid Android app and I need to take a picture from camera, process the image and then show the results obtained on another window that should contain a small preview of the picture taken and some text with the results.
Here is the relevant code that is not currently working as I need:
From script of first window:
.
.
.
CameraPreview.takePicture(function(imgData){
var mImage = new Image();
mImage.onload = function(){
var canvas = document.createElement('canvas');
canvas.width = mImage.width;
canvas.height = mImage.height;
var context = canvas.getContext('2d');
context.drawImage(mImage, 0, 0);
var imgData = context.getImageData(0, 0, canvas.width, canvas.height);
.
.(Process imgData[])
.
localStorage.setItem("resultImage", mImage.src);
window.open('result.html','_blank','location=no');
};
mImage.src = 'data:image/jpeg;base64,'+imgData;
});
Then, "result.html" pops up, which has the following html content:
...
<body>
...
<canvas id="resultCanvas" style="position: absolute; top: 0%; left: 0%;"></canvas>
...
</body>
...
And the following javascript code:
...
var mImage = new Image();
mImage.onload = function(){
var canvas = document.getElementById('resultCanvas');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight*0.3; // 1/3 of phone screen size
var context = canvas.getContext('2d');
context.drawImage(mImage);
navigator.notification.alert('Height = '+mImage.height+'\n'+'Width ='+mImage.width, function(){},'Image','Done');
};
mImage.src = localStorage.getItem("resultImage");
...
The alert window shows the 640x480 of the image, but the canvas is transparent where it should contain the picture taken.
If I replace the "context.drawImage()" statement with "context.fillrect()" I can see the black rectangle drawn on screen, so it looks that the canvas is there, but somehow I can not make it draw the desired image.
Hope I explained myself correctly.
I really appreciate any help. Thanks!
Well, finally I solved it, first I tryied using an "img" element instead of a canvas, and that works fine, but the problem is I need to make some drawings over the picture, so insisting on code I posted before, I realized the problem was on the use of drawImage() method in the second script, it is necessary to specify position, so replacing:
context.drawImage(mImage);
with
context.drawImage(mImage,0,0);
solved everything.
I have this problem with scrolling an tiled image, but only on mobile (android) browsers..
The image is "scrolled" by repainting it on itself, I had a version using an offscreen buffer but no difference. The coordinates are parsed to integers, as the touch events give floats. If this is not done, the effect is even worse. The canvas width/height attr are set equal to the css style width/height so the canvas is not scaled.
When i repeatedly scroll the canvas following touchmove events using the following code, the rendered image gets blurred more and more on mobile chrome browser - not so on desktop browsers.
See the image attached, the tiles in the middle are the affected, those at the borders get redrawn.
What am I doing wrong? Is there another way of scrolling a canvas?
worldRenderer.prototype.offset = function(dx,dy) {
dx = parseInt(dx);
dy = parseInt(dy)
this.ctx.drawImage(this.canvas, dx, dy);
var obounds = this.calcVisible();
this.options.offsetx += dx;
this.options.offsety += dy;
var bounds = this.calcVisible();
var x0 = bounds[0];
var x1 = bounds[2];
if (dx>0) {
this.paintRect(this.ctx, [bounds[0],bounds[1],obounds[0],bounds[3]]);
x0=obounds[0];
} else if (dx<0) {
this.paintRect(this.ctx, [obounds[2],bounds[1],bounds[2],bounds[3]]);
x1=obounds[2];
}
if (dy>0) {
this.paintRect(this.ctx, [x0,bounds[1],x1,obounds[1]] );
} else if (dy<0) {
this.paintRect(this.ctx, [x0,obounds[3],x1,bounds[3]]) ;
}
}
After being a long time lurker, this is my first post here! I've been RTFMing and searching everywhere for an answer to this question to no avail. I will try to be as informative as I can, hope you could help me.
This code is for my personal webpage.
I am trying to implement some sort of a modern click-map using HTML5 and jQuery.
In the website you would see the main image and a hidden canvas with the same size at the same coordinates with this picture drawn into it.
When the mouse hovers the main picture, it read the mouse pixel data (array of r,g,b,alpha) from the image drawn onto the canvas. When it sees the pixel color is black (in my case I only check the RED value, which in a black pixel would be 0) it knows the activate the relevant button.
(Originally, I got the idea from this article)
The reason I chose this method, is for the page to be responsive and dynamically change to fit different monitors and mobile devices. To achieve this, I call the DrawCanvas function every time the screen is re-sized, to redraw the canvas with the new dimensions.
Generally, this works OK. The thing is ,there seems to be an inconsistent behavior in Chrome and IE(9). When I initially open the page, I sometimes get no pixel data (0,0,0,0), until i re-size the browser. At first I figured there's some loading issues that are making this happen so I tried to hack it with setTimeout, it still doesn't work. I also tried to trigger the re-size event and call the drawCanvas function at document.ready, still didn't work.
What's bothering me is most, are the inconsistencies. Sometimes it works, sometimes is doesn't. Generally, it is more stable in chrome than in IE(9).
Here is the deprecated code:
<script type="text/javascript">
$(document).ready(function(){setTimeout(function() {
// Get main image object
var mapWrapper = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
// Create a hidden canvas the same size as the main image and append it to main div
var canvas = document.createElement('canvas');
canvas.height = mapWrapper.clientHeight;
canvas.width = mapWrapper.clientWidth;
canvas.fillStyle = 'rgb(255,255,255)';
canvas.style.display = 'none';
canvas.id = 'hiddencvs';
$('#map_wrapper').append(canvas);
// Draw the buttons image into the canvas
drawCanvas(null);
$("#map_wrapper").mousemove(function(e){
var canvas = document.getElementById('hiddencvs');
var context = canvas.getContext('2d');
var pos = findPos(this);
var x = e.pageX - pos.x;
var y = e.pageY - pos.y;
// Get pixel information array (red, green, blue, alpha)
var pixel = context.getImageData(x,y,1,1).data;
var red = pixel[0];
var main_img = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
if (red == 0)
{
...
}
else {
...
}
});
},3000);}); // End DOM Ready
function drawCanvas(e)
{
// Get context of hidden convas and set size according to main image
var cvs = document.getElementById('hiddencvs');
var ctx = cvs.getContext('2d');
var mapWrapper = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
cvs.width = mapWrapper.clientWidth;
cvs.height = mapWrapper.clientHeight;
// Create img element for buttons image
var img = document.createElement("img");
img.src = "img/main-page-buttons.png";
// Draw buttons image inside hidden canvas, strech it to canvas size
ctx.drawImage(img, 0, 0,cvs.width,cvs.height);
}
$(window).resize(function(e){
drawCanvas(e);
}
);
function findPos(obj)
{
...
}
</script>
I'd appreciate any help!
Thanks!
Ron.
You don't wait for the image to be loaded so, depending on the cache, you may draw an image or not in the canvas.
You should do this :
$(function(){
var img = document.createElement("img");
img.onload = function() {
var mapWrapper = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
...
// your whole code here !
...
}
img.src = "img/main-page-buttons.png";
});
I am making a PhoneGap application with jQuery Mobile UI framework. I need a page where users will be able to draw stuff on the screen. I used this for reference and it works great in Ripple Emulator. However, on my actual device, a Nexus 4, instead of one line per touchmove, I get two lines. Is there something wrong with what I am doing?
EDIT: I found a similar problem reported in github. It seems to be the problem with Android's browser. The two lines were due to overlapping canvas elements. The only solution is to have canvas size less than 256px. Here's the link:
https://github.com/jquery/jquery-mobile/issues/5107
Here's my code
// start canvas code
var canvas = null; //canvas object
var context = null; //canvas's context object
var clearBtn = null; //clear button object
var buttonDown = false;
function captureDraw(){
canvas = document.getElementById('canvas');
clearBtn = document.getElementById('clearBtn');
setCanvasDimension();
initializeEvents();
context = canvas.getContext('2d');
}
function setCanvasDimension() {
//canvas.width = 300;
// canvas.width = window.innerWidth;
// canvas.height = window.innerHeight; //setting the height of the canvas
}
function initializeEvents() {
canvas.addEventListener('touchstart', startPaint, false);
canvas.addEventListener('touchmove', continuePaint, false);
canvas.addEventListener('touchend', stopPaint, false);
clearBtn.addEventListener('touchend', clearCanvas,false);
}
function clearCanvas() {
context.clearRect(0,0,canvas.width,canvas.height);
}
function startPaint(evt) {
if(!buttonDown)
{
context.beginPath();
context.moveTo(evt.touches[0].pageX, evt.touches[0].pageY);
buttonDown = true;
}
evt.preventDefault();
}
function continuePaint(evt) {
if(buttonDown)
{
context.lineTo(evt.touches[0].pageX,evt.touches[0].pageY);
context.stroke();
}
}
function stopPaint() {
buttonDown = false;
}
// end canvas code
Thanks!
Not an actual answer, but I found out that this is a known bug since Android 4.1.1. There have been many solutions like overriding offset-x: visible to the parent div of the canvas element, but it didn't work for me. See https://code.google.com/p/android/issues/detail?id=35474 for more information.
Other solution is keeping your canvas size below 256px. This is certainly a weird bug!
Based on Creating an HTML 5 canvas painting application I created a HTML5 canvas painting application. It works fine, but after creating each object I just need to drag the objects.
Working demo
How to implement drag and drop of the figures?
When the user clicks on the canvas, you have to check the coordinates (compare it to the coordinates for the objects), and see if it's on an object. E.g. You can test if a point (e.g. the coordinates for the mousedown even) is within a circle with this method:
function (pt) {
return Math.pow(pt.x - point.x,2) + Math.pow(pt.y - point.y,2) <
Math.pow(radius,2);
};
If the mousedown is on the object, you have to change the objects coordinates according to how the mouse is moved.
Here is an example, where you can drag a circle:
<!DOCTYPE html>
<html>
<head>
<script>
window.onload = function() {
drawCircle(circle);
element = document.getElementById('canvas');
element.addEventListener('mousedown', startDragging, false);
element.addEventListener('mousemove', drag, false);
element.addEventListener('mouseup', stopDragging, false);
element.addEventListener('mouseout', stopDragging, false);
}
function mouseX(e) {
return e.clientX - element.offsetLeft;
}
function mouseY(e) {
return e.clientY - element.offsetTop;
}
var Point = function (x, y) {
this.x = x;
this.y = y;
return this;
}
var Circle = function (point, radius) {
this.point = point;
this.radius = radius;
this.isInside = function (pt) {
return Math.pow(pt.x - point.x, 2) + Math.pow(pt.y - point.y, 2) <
Math.pow(radius, 2);
};
return this;
}
function startDragging(e) {
var p = new Point(e.offsetX, e.offsetY);
if(circle.isInside(p)) {
deltaCenter = new Point(p.x - circle.point.x, p.y - circle.point.y);
}
}
function drag(e) {
if(deltaCenter != null) {
circle.point.x = (mouseX(e) - deltaCenter.x);
circle.point.y = (mouseY(e) - deltaCenter.y);
drawCircle(circle);
}
}
function stopDragging(e) {
deltaCenter = null;
}
function drawCircle(circle) {
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.beginPath();
ctx.arc(circle.point.x, circle.point.y, circle.radius, 0, Math.PI*2, true);
ctx.fill();
}
var circle = new Circle(new Point(30, 40), 25);
var deltaCenter = null;
var element;
</script>
</head>
<body>
<canvas id="canvas" width="400" height="300"></canvas>
</body>
</html>
Try it on jsFiddle
The same effect can be accomplished using Raphael.js (http://raphaeljs.com/) with Joint.jS (http://www.jointjs.com/).
Shapes created with Raphael can be accessed like any DOM element and can be manipulated via attributes. It is an awesome framework.
Joint.js helps in connecting the shapes. They also have a diagramming library and can help create ERD, Statemachine and several common diagrams. The best part is that you can extend their diagram element and create your own custom elements. Its jaw-dropingly cool.
Checkout their demos with source code at http://www.jointjs.com/demos
If you are using the raphael as "raw" lib you must handle the undo/redo by yourself.
The graphiti lib did have Undo/Redo Stack inside and supports the export for SVG, PNG, JSON,...
Additional you have some kind of Viso like connectors and ports.
http://www.draw2d.org/graphiti/jsdoc/#!/example
Greetings
I don't think there's an easy way to do this.
If you're just dealing with lines, my approach would be to keep track of all lines created, with starting coordinates, ending coordinates and some kind of z-index. When the user starts a dragging action (onmousedown), you have to check if the point is near the line, and then update the object and redraw the canvas when the mouse is moved.
How can I tell if a point belongs to a certain line?
This gets a lot more complicated if you're dealing with complex objects though. You'll probably have to find a solution to check if a point is inside a path.
Objects drawn into HTML5 Canvas are turned into pixels and then forgotten. You can't adjust properties on them and have the canvas update to see the effects. You can remember them yourself, but the canvas will still have those pixels set, so you'd have to basically redraw the whole canvas (or at least some of it) when you adjust a property.
You might want to consider SVG for this application instead, SVG elements are remembered in the DOM and when their properties are updated the browser will update the graphic to reflect the changes.
If you must use canvas, then you're going to need to write quite a bit of code to handle mouse-hits, object properties, and repaints.