Develop Reference

Responsive Heatmaps

The Problem
I've encountered a slight problem where I've written some code where I've automated a script to track the users interactions with a UI, including mousemove & click events. If I didn't have to worry about making it responsive, then I could probably call it a day & ship the work that I've done already, however my brain is seeking some wizardry knowledge, I'm struggling to make it super responsive. Here's just a quick example of the kinda thing that I'm working on, it's nothing genius, if anything I it's mostly heatmap.js that's doing the heavy lifting. Currently I'm just seeing if I can do this as a proof of concept more than anything else...
The Code
So currently, I'm tracking the event.pageX & event.pageY values to store exactly where an event took place, I'm also storing the window.innerWidth & window.innerHeight values to try & work out some function that would allow me to offset the positions based on the size(s) of other devices.
E.g. If you look at the sample image above, that's perfect for a static page, but if I were to say make the page a little more narrow, you can see here that it's doesn't line up with the image above:
Anyway, without blabbering on too much, here's some sample code:
// A lot of other code...
var setupHeatMaps = function (pages, heatMaps) {
pages.forEach(function (page) {
page.addEventListener("click", function (event) {
heatMaps.push({ x: event.pageX, y: event.pageY, value: 10000 });
onStateChange();
});
// Don't collect ALL mouse movements, that'd be crazy, so collect
// every 1/10 mouse movements.
var counter = 0;
page.addEventListener("mousemove", function (event) {
if (counter === 10) {
heatMaps.push({ x: event.pageX, y: event.pageY, value: 20 });
onStateChange();
counter = 0;
} else {
counter ++;
}
});
});
};
// A lot of other code...
// Curried function so that it can be passed around without exposing the state...
var renderHeatMaps = function (heatMaps) {
return function () {
var max = heatMaps.length;
var points = heatMaps;
var parent = getParentElement();
var styleObj = window.getComputedStyle(parent);
var div = document.createElement("div");
var body = document.querySelector("body");
var background = document.createElement("div");
// This element needs to sit in front of the
// background element, hence the higher z-index value.
div.style.position = "absolute";
div.style.zIndex = 9;
div.style.left = "0px";
div.style.top = "-80px";
div.style.width = "100vw";
// Even though this element will sit behind the element
// that's created above, we will still want this element to
// sit in front of 99% of the content that's on the page.
background.style.position = "fixed";
background.style.top = "0px";
background.style.left = "0px";
background.style.height = "100vh";
background.style.width = "100vw";
background.style.zIndex = 5;
background.style.backgroundColor = "rgba(255, 255, 255, 0.35)";
background.setAttribute("id", "quote-customer-heat-map-background");
var heightInPx = styleObj.getPropertyValue("height");
var rawHeight = parseInt(heightInPx.replace("px", ""));
var newHeight = parseInt((rawHeight + 80));
div.style.height = newHeight + "px";
div.setAttribute("id", "quote-customer-heat-map-foreground");
body.style.paddingBottom = "0px";
body.appendChild(background);
body.appendChild(div);
var heatMap = h337.create({
container: div,
radius: 45
});
heatMap.setData({ max: max, data: points });
};
};
// A lot of other code...
As the pages elements that you can see being used in setupHeatMaps changes in width, viewing this data gets offset quite badly. I'll be honest, I've spent a lot of time yesterday thinking about this issue & I've still not thought of anything that seems reasonable.
Alternatively
I have wondered if I should somehow just store the page as an image, with the heatmap overplayed, that way I wouldn't really have to worry about the heatmap being responsive. But then I need to figure out some other things... E.g. Versioning this data, so in the event that a user views a page on their phone, it'll stored that data separately to data that was collected from a previous session where they were on a laptop or on a desktop device.
Conclusion
I'll be honest, I'm not entirely sure what the best course of action is, have any of you guys encountered anything like this before? Have you guys thought of something genius that solves an issue like this?
P.S. I would share a lot more code, however there's an immense amount of code that goes into this overall solution, so I kinda can't share all of it, I'm pretty sure the guys at Stackoverflow would hate me for that! 😅 - You can also tell that I'm doing this as a POC because normally I'd just offload the rendering of the background element & the div element to the underlying framework rather than do it programatically like this, keep that in mind, this is just a POC.

Perhaps you could change the setup and record what element an event is on and save the location data relative to that element, not the whole page.
You record all events on the page still but you save the data relative to the elements instead of the whole page, which prevents you from mashing the data together in your heatmap afterwards when you view
it over your website at various widths.
Say you you have a setup like this in pseudocode:
#1000px width
[html width 1000px, height 500px
[div width 800px, height 400px centered
[button1 width 100px, height 30px centered]
]
]
#500px width
[html width 500px, height 1000px
[div width 400px, height 800px centered
[button1 width 80px, height 30px centered]
]
]
Your users move over some element at all times. Just capture that data like this, example data from 2 users at the two different screen sizes moving the cursor towards the button in the center:
user interaction{
over element: html {dimensions: width 1000px, height 500px}
user position: {x 900, y 20}
}
user interaction{
over element: html {dimensions: width 1000px, height 500px}
user position: {x 850, y 60}
}
user interaction{
over element: div {width 800px, height 400px}
user position: {x 700, y 60}
}
user interaction{
over element: button1 {width 100px, height 30px}
user position: {x 90, y 10}
}
user interaction{
over element: html {dimensions: width 500, height 1000px}
user position: {x 450, y 100}
}
user interaction{
over element: div {width 400px, height 800px}
user position: {x 380, y 40}
}
user interaction{
over element: button1 {width 80px, height 30px}
user position: {x 60, y 10}
}
Then when you view your website draw the heat over all elements and calculate the relative position of the captured data.
So when you would view your site at #500px width the heat over your button1 would be as following:
[button 1 width 80px, height 30px
heat 1, x 72px y 10
heat 2, x 60px y 10
]
And you do the same for all other elements. I don't know how useful the data is like this, but it was for the sake of weird wizardry right?
Personally I'd just exclude data based on the screen width your viewing your heatmap at. That way you can use this setup and get useful heatmap data. So you'd exclude data based on if it was captured at a specific responsive width. So you could at the very least mash all user data together at high widths cause you'd know your interesting web elements would probably be centered still and the same size.
Its common to cut up your responsive design in 2 or 3 sizes, monitor, tablet and phone. You'd be surprised how similar you can keep your design layout across those 3. The more similar the more useful it will be to mix the data from different width that will fall into the specific media query range.
As long as you use the same technique for getting the width and height for saving your data as painting it later it will be fine. Even if you'd ignore margins and borders for instance your event would still capture on the element when its in the margin and that way you could get data like this: user position: {x -10, y -1} and still use that to paint your heat just fine on the element.
You could also give the option to mix and filter the user data across different size to the user, just call it experimental data mixing or something. You could potentially still get some very useful visual information on the most clicked elements for instance if you mix all user data regardless of screen size.
Another thing you could do is soft-mix the results. If your user data differs too much in width and height from the current element's dimensions, say more then 30%, you could exclude it. You could make this dynamic by letting the user set the sensitivity (feathering) of that percentage. That way you can still view all user heat on all smaller elements while it ignores the not very relevant user events on your larger more changeable elements.

My understanding of the issue is that the project works well, in terms of correctly performing the events, storing the information and displaying it, yet, there is a UX problem, because the system is offsetting (in time, I assume). I would use an idea which resembles very much compression. Imagine your monitor as a grid of pixels. It's perfectly clear that there are some precision problems, because even if we use the maximum precision our computer and its components allows us to use, if we take two adjacent pixels and want to click in between the two, we quickly realize that the precision our system offers is quite limited, which is axiomatically true for digital devices, which are unable to respect the Hausdorf separation.
Now that we have explored the limitations with our little thought experiment, we can acknowledge that such limitations exist. Hence, in principle, lowering the number of pixels, but not the number of events would not introduce more problems, it would decrease spatial precision though for a certain amount (the amount that you deem to be still acceptable, off course). So, if you have x * y resolution applied for the map, then you can think about your resolution as "every 5 pixels in with and height counts". That would make a mapping between actual pixels and imaginary pixels that would form the basis of our compression, decreasing the clickable places 25x.
This would allow you to think about the monitor as a set of regions, the number of regions being significantly smaller than the actual number of pixels (25x in our example). That would allow you to avoid storing each coordination/click separately and instead, you could always store the (x, y, n), that is, the center of the region where the event occurred and the number that event was repeated.
This way if the user clicks on a pixel (x', y'), which is located in a region whose center is (x, y), then you would only have new data if a click did not occur in that region yet. Otherwise, if a click occurred there (not necessarily at the exact same pixel, but in the region), then you would just increase n. As a result, instead of a very large set of raw pixel-event data you would have a much smaller set of pixel-event-number data.
The drawback of this approach, off course is that it somewhat reduces geometrical precision, but it should contribute to the optimization of data processing.
I confess that I'm not very experienced with Heatmap.js, so I'm unsure whether it has API support for what I am suggesting. If it has, then trying to use it would make sense. If it is not optimized properly, or does not support such a feature, then I would implement the heatmap feature using a canvas or an svg, depending on the actual needs.

While this solution may not be the most elegant solution ever, it does at the very least work, if anyone has any ideas or anything that's just plain ol' better, by all means chip in!
My Solution Explained
So I was thinking, I could render this via an iframe, sure it's not the most beautiful solution ever, but at least that means that there's no complicated positioning logic involved, it's relatively lightweight in terms of computational complexity compared to some other solutions I've looked at & that it works for all sorts of screen sizes, it essentially removes the need for this to be responsive... Kinda...
Sure that means that you'll have scroll bars left, right & centre, but it's simple, straight to the point & it means that I can produce an MVP in very little time, and realistically, I'd expect your average junior developer may have an easier time understanding what's going on? What do you guys think? 🙂
I'm trying to think of it from a user perspective too, realistically in this kinda application, I would much prefer true, raw accuracy over something looking so nice it makes me want to become a designer. I've even done some homework into this subject, as I've essentially been trying to build some session-replay software, it has been a bloody interesting project/feature I must say! 😀
With some styling, I've been able to accomplish something like this... Obviously this isn't a direct snippet of the website/application, but this is some marketing material that my boss has created, but the content within the 'laptop', that is an actual screenshot/snip of the web application! 😅 - So all in all, I think it looks okay? - You can see where the overflow happens & the right hand side of the screen is a bit cut off, but because everything else within the UI sorta behaves like they have a fixed position there, I personally think it seems to work pretty darn well.
Edit
I'd just like to thank everyone that spent their time, whether that was adding comments or providing answers, fantastic to see the dev community be so helpful! Thank you all guys! 😀
Blog Post
So I've written a little more on this subject matter here:
LinkedIn Post
Blog Post

Javascript & Canvas: Draw and delete lines to create a "breathing" circle

I would like to create an element, that shows a red circle. Once the user clicks on it, she can record her voice. In order to show the LIVE mode, I'd like to make the circle "breath" according to the incoming frequencies.
I'm experimenting with a <canvas> element. That means it creates a circle that gets bigger and smaller, depending on the variable arcrad. However, the lines are being drawn correctly, but they do not disappear afterwards. I tried to apply .clip() but can't get it to work...
if (arcrad <= 10) arcrad = 10;
analyserContext.beginPath();
analyserContext.arc(100,120,arcrad,0,2*Math.PI);
analyserContext.closePath();
analyserContext.lineWidth = 2;
analyserContext.strokeStyle = 'red';
analyserContext.stroke();
Any ideas - or completely different strategies for this use case?

Canvas will overdraw by default. For your animation you’ll need to clean the canvas at the start of each frame. Use something the following at the start of your drawing function:
analyserContext.clearRect(0,0,200,200);
assuming your canvas is 200 pixels wide and high. It’s worth pointing out that sometimes you don’t want to completely clear the animation field every frame. For example, if you were to draw a semi transparent rectangle over the frame at the beginning (instead of clearing it) then you’d end up with a basic ‘bullet time’ style effect.

It's a normal behavior. Once something it's drawn on the canvas, it's there forever. You have to think like if you were painting something: what has been done cannot be undone.
Luckily, you still have solutions:
1) redraw another circle on top of the first one with the background color. It's really not the recommend way, but it still can be useful
2) use clearRect method (see How to clear the canvas for redrawing)

There are numerous ways to clear a canvas pre drawing to create animation:
How to clear the canvas for redrawing
simplest in my mind:
canvas.width=canvas.width;
though can equally use clearRect (which is actually quicker and won't reset the entire canvas if that is an issue regarding transforms etc!) over the region or whole canvas.
Get the likes of:
http://jsfiddle.net/dw17jxee/

using clearRect() to overwrite text output on canvas?

I'm making a little game on jsbin and everything so far is going well but I'm have a slight problem. The goal of the game is to click the randomly appearing circle as many times as possible in one minute. I want it to output the time left and the score in the corners, and I have done so. The problem is that they are overwriting each other. This is because to prevent flickering I decided not to use
c.clearRect(0,0,canvas.width,canvas.height);
instead drawing a clearRect just over the circle when its clicked. I want to do a similar thing with text. I used this line:
c.clearRect(0,fontSize,c.measureText(timeLeft),fontSize);
this should work but it has no effect. I've tried everything, but I don't know what's wrong with this line. My only other theroy is that is in the wrong spot in the code, but I haven't found a problem with that.
Here is the link to the current version I'm working on:
http://jsbin.com/touchgame/10/edit
Thanks!

measureText() returns an object with a width property, so you need to use c.measureText(timeLeft).width.
Also, you decreased the timeLeft and then called clearRect, which will clear a rectangle based on the new width for timeLeft. You want to clear based on the width for the "old" timeLeft value and then decrease the timeLeft:
if (timeLeft < 1){
c.clearRect(0,fontSize,c.measureText(timeLeft).width + 5,fontSize*1.5);
timeLeft--;
//clear over time output
console.log(c.measureText(timeLeft));
}
This should work. Because the way drawing text works, it's not trivial to know exactly the bounding box of the text, so that's why we clear a larger area than fontSize. You could use c.textBaseline = 'top', which would place the text with the top coordinate at the y you specify instead of the baseline of the text at y.
Finally, I think it's an easier approach to clear the canvas completely and redraw everything when you want to update the graphics. If you clear the canvas and then immediately redraw evrything you won't get any flickering. The performance hit of redrawing everything is usually neglible in most cases, and it makes things a lot simpler.

Is there any way to accelerate the mousemove event?

I wrote a little drawing script (canvas) for this website: http://scri.ch/
When you click on the document, every mousemove event basically executes the following:
- Get coordinates.
- context.lineTo() between this point and the previous one
- context.stroke() the line
As you can see, if you move the cursor very fast, the event isn’t triggering enough (depending on your CPU / Browser / etc.), and a straight line is traced.
In pseudocode:
window.addEventListener('mousemove', function(e){
myContext.lineTo(e.pageX, e.pageY);
myContext.stroke();
}, false);
This is a known problem, and the solution is fine, but I would like to optimize that.
So instead of stroke() each time a mousemove event is triggered, I put the new coordinates inside an array queue, and regularly draw / empty it with a timer.
In pseudocode:
var coordsQueue = [];
window.addEventListener('mousemove', function(e){
coordsQueue.push([e.pageX, e.pageY]);
}, false);
function drawLoop(){
window.setTimeout(function(){
var coords;
while (coords = coordsQueue.shift()) {
myContext.lineTo(coords[0], coords[1]);
}
myContext.stroke();
drawLoop();
}, 1000); // For testing purposes
}
But it did not improve the line. So I tried to only draw a point on mousemove. Same result: too much space between the points.
It made me realize that the first code block is efficient enough, it is just the mousemove event that is triggering too slowly.
So, after having myself spent some time to implement a useless optimization, it’s your turn: is there a way to optimize the mousemove triggering speed in DOM scripting?
Is it possible to “request” the mouse position at any time?
Thanks for your advices!

If you want to increase the reporting frequency, I'm afraid you're out of luck. Mice only report their position to the operating system n times per second, and I think n is usually less than 100. (If anyone can confirm this with actual specs, feel free to add them!)
So in order to get a smooth line, you'll have to come up with some sort of interpolation scheme. There's a whole lot of literature on the topic; I recommend monotone cubic interpolation because it's local, simple to implement, and very stable (no overshoot).
Then, once you've computed the spline, you can approximate it with line segments short enough so that it looks smooth, or you can go all-out and write your own Bresenham algorithm to draw it.
If all this is worth it for a simple drawing application... that's for you to decide, of course.

Cool site, unfortunately there is no way to request the current position of the mouse with JavaScript, the only hooks you have are the events you're already using. If you must have more control I'd look at using flash where you can change the frame rate and request the mouse position.
trace("Mouse X: " + _xmouse);
trace("Mouse Y: " + _ymouse);

Detect mouseover of certain points within an HTML canvas?

I've built an analytical data visualization engine for Canvas and have been requested to add tooltip-like hover over data elements to display detailed metrics for the data point under the cursor.
For simple bar & Gaant charts, tree graphs and node maps with simple square areas or specific points of interest, I was able to implement this by overlaying absolutely-positioned DIVs with :hover attributes, but there are some more complicated visualizations such as pie charts and a traffic flow rendering which has hundreds of separate areas defined by bezeir curves.
Is is possible to somehow attach an overlay, or trigger an event when the user mouses over a specific closed path?
Each area for which hover needs to be specified is defined as follows:
context.beginPath();
context.moveTo(segmentRight, prevTop);
context.bezierCurveTo(segmentRight, prevTop, segmentLeft, thisTop, segmentLeft, thisTop);
context.lineTo(segmentLeft, thisBottom);
context.bezierCurveTo(segmentLeft, thisBottom, segmentRight, prevBottom, segmentRight, prevBottom);
/*
* ...define additional segments...
*/
// <dream> Ideally I would like to attach to events on each path:
context.setMouseover(function(){/*Show hover content*/});
// </dream>
context.closePath();
Binding to an object like this is almost trivial to implement in Flash or Silverlight, since but the current Canvas implementation has the advantage of directly using our existing Javascript API and integrating with other Ajax elements, we are hoping to avoid putting Flash into the mix.
Any ideas?

You could handle the mousemove event and get the x,y coordinates from the event. Then you'll probably have to iterate over all your paths to test if the point is over the path. I had a similar problem that might have some code you could use.
Looping over things in this way can be slow, especially on IE. One way you could potentially speed it up - and this is a hack, but it would be quite effective - would be to change the color that each path is drawn with so that it is not noticeable by humans but so that each path is drawn in a different color. Have a table to look up colors to paths and just look up the color of the pixel under the mouse.

Shadow Canvas
The best method I have seen elsewhere for mouseover detection is to repeat the part of your drawing that you want to detect onto a hidden, cleared canvas. Then store the ImageData object. You can then check the ImageData array for the pixel of interest and return true if the alpha value is greater than 0.
// slow part
ctx.clearRect(0,0,canvas.width,canvas.height);
ctx.fillRect(100,100,canvas.width-100,canvas.height-100);
var pixels = ctx.getImageData(0,0,canvas.width,canvas.height).data;
// fast part
var idx = 4 * (mouse_x + mouse_y * canvas.width) + 3;
if (pixels[idx]) { // alpha > 0
...
}
Advantages
You can detect anything you want since you're just repeating the context methods. This works with PNG alpha, crazy compound shapes, text, etc.
If your image is fairly static, then you only need to do this one time per area of interest.
The "mask" is slow, but looking up the pixel is dirt cheap. So the "fast part" is great for mouseover detection.
Disadvantages
This is a memory hog. Each mask is W*H*4 values. If you have a small canvas area or few areas to mask, it's not that bad. Use chrome's task manager to monitor memory usage.
There is currently a known issue with getImageData in Chrome and Firefox. The results are not garbage collected right away if you nullify the variable, so if you do this too frequently, you will see memory rise rapidly. It does eventually get garbage collected and it shouldn't crash the browser, but it can be taxing on machines with small amounts of RAM.
A Hack to Save Memory
Rather than storing the whole ImageData array, we can just remember which pixels have alpha values. It saves a great deal of memory, but adds a loop to the mask process.
var mask = {};
var len = pixels.length;
for (var i=3;i<len;i+=4) if ( pixels[i] ) mask[i] = 1;
// this works the same way as the other method
var idx = 4 * (mouse_x + mouse_y * canvas.width) + 3;
if (mask[idx]) {
...
}

This could be done using the method ctx.isPointInPath, but it is not implemented in ExCanvas for IE.
But another solution would be to use HTML maps, like I did for this little library : http://phenxdesign.net/projects/phenx-web/graphics/example.htm you can get inspiration from it, but it is still a little buggy.

I needed to do detect mouse clicks for a grid of squares (like cells of an excel spreadsheet). To speed it up, I divided the grid into regions recursively halving until a small number of cells remained, for example for a 100x100 grid, the first 4 regions could be the 50x50 grids comprising the four quadrants.
Then these could be divided into another 4 each (hence giving 16 regions of 25x25 each).
This requires a small number of comparisons and finally the 25x25 grid could be tested for each cell (625 comparisons in this example).

There is a book by Eric Rowell named "HTML5 CANVAS COOKBOOK". In that book there is a chapter named "Interacting with the Canvas: Attaching Event Listeners to Shapes and Regions". mousedown, mouseup, mouseover, mouseout, mousemove, touchstart, touchend and touchmove events can be implemented. I highly suggest you read that.

This can't be done (well, at least not that easily), because objects you draw on the canvas (paths) are not represented as the same objects in the canvas. What I mean is that it is just a simple 2D context and once you drawn something on it, it completely forgets how it was drawn. It is just a set of pixels for it.
In order to watch mouseover and the likes for it, you need some kind of vector graphics canvas, that is SVG or implement your own on top of existing (which is what Sam Hasler suggested)

I would suggest overlaying an image map with proper coordinates set on the areas to match your canvas-drawn items. This way, you get tooltips AND a whole lot of other DOM/Browser functionality for free.