Develop Reference

Clip by mask defined by pixel image or prevent drawing outside of that mask

I'm two days into js,html and css programming. So very newbie!
Following and building upon this TUTORIAL
Q1: How can I add this male into the background (see figuere 1.) and prohibit any strokes outside of the borders?
Adding image to background was no biggy!
function make_base()
{
base_image = new Image();
base_image.src = 'img/bmapFront.gif';
base_image.onload = function(){
context.drawImage(base_image, 0,0);
}
}
There is a context.clip function, not sure if I can use pixel form as clipping path. Making tons of "image substractions" isn't the best way.
Any suggestions
Edit:
Did the Job for me: VeryHelpful
var frontPath = new Path2D ("M 133.41,17.00 C 141.37,2.41 160.66, !VERY LONG! ")
context.clip(frontPath);
Messy strokes!
He should look like this. Then I want to save him.

Although there is such a thing as ctx.clip(), this is sometimes not what's wanted as it's impractical to use a path.
The solution that I like involves creating a virtual empty canvas onto which you draw your pixel image. Through various manipulations, like using ctx.getImageData and similar to make sure you only get one kind of color or apply other filters only once, you can obtain an image that seems to be empty (alpha of 0, mostly) in the places where you want to clip other images or paths out.
At that point, you'd use ctx.globalCompositeOperation = 'source-atop', or pick another one you might want to use from mdn's list of globalCompositeOperations.
At this point, you can just draw this virtual canvas image onto the main canvas

How to make area of image clickable

This is not a: "Do all the work for me!" kind of question. I just wanna know which approach you think would be suitable for this challenge.
I have this map:
As you can see by the blue marker, I've roughly drawned some selections/areas of the map. Theese areas I want to serve as links.
But I don't quite know how to grasp this challenge, since all of the areas have quite odd shapes.
I have looked at cords, but it seems like a huge job with all of the twists and turns that I would need to do.
I would be awesome if I could just slice up the areas in Photoshop and save each of them as .png and just tell my page to ignore the transparent area! But that's just wishfull thinking I suppose.
I hope that one of you have a suggestion that I've overlooked.

Give a try to these -
http://polymaps.org/
http://www.amcharts.com/javascript-maps/
Raphael JS
You can try making an SVG version of your map and then implement it's clickiness with one of these libraries depending on which one you choose.
Here's one tutorial to do this with Raphael JS - http://parall.ax/blog/view/2985/tutorial-creating-an-interactive-svg-map

Make an image for each clickeable zone, like this:
Register to the click event of the img element from the page, this way:
var getAreaFromXY = function(x,y) {
// for each section colored map
// get pixel color on x,y (see http://stackoverflow.com/questions/8751020/how-to-get-a-pixels-x-y-coordinate-color-from-an-image)
// if the color is red, that is the zone
};
$(".post-text img").click(function(e) {
var area = getAreaFromXY(e.offsetX, e.offsetY);
});

Three mouse detection techniques for HTML5 canvas, none adequate

I've built a canvas library for managing scenes of shapes for some work projects. Each shape is an object with a drawing method associated with it. During a refresh of the canvas, each shape on the stack is drawn. A shape may have typical mouse events bound which are all wrapped around the canvas' own DOM mouse events.
I found some techniques in the wild for detecting mouseover on individual shapes, each of which works but with some pretty serious caveats.
A cleared ghost canvas is used to draw an individual shape by itself. I then store a copy of the ghost canvas with getImageData(). As you can imagine, this takes up a LOT of memory when there are many points with mouse events bound (100 clickable shapes on a 960x800 canvas is ~300MB in memory).
To sidestep the memory issue, I began looping over the pixel data and storing only addresses to pixels with non-zero alpha. This worked well for reducing memory, but dramatically increased the CPU load. I only iterate on every 4th index (RGBA), and any pixel address with a non-zero alpha is stored as a hash key for fast lookups during mouse moves. It still overloads mobile browsers and Firefox on Linux for 10+ seconds.
I read about a technique where all shapes would be drawn to one ghost canvas using color to differentiate which shape owned each pixel. I was really happy with this idea, because it should theoretically be able to differentiatate between millions of shapes.
Unfortunately, this is broken by anti-aliasing, which cannot be disabled on most canvas implementations. Each fuzzy edge creates dozens of colors which might be safely ignored except that /they can blend/ with overlapping shape edges. The last thing I want to happen when someone crosses the mouse over a shape boundary is to fire semi-random mouseover events for unrelated shapes associated with colors that have emerged from the blending due to AA.
I know that this not a new problem for video game developers and there must be fast algorithms for this kind of thing. If anyone is aware of an algorithm that can resolve (realistically) hundreds of shapes without occupying the CPU for more than a few seconds or blowing up RAM consumption dramatically, I would be very grateful.
There are two other Stack Overflow topics on mouseover detection, both of which discuss this topic, but they go no further than the 3 methods I describe.
Detect mouseover of certain points within an HTML canvas? and
mouseover circle HTML5 canvas.
EDIT: 2011/10/21
I tested another method which is more dynamic and doesn't require storing anything, but it's crippled by a performance problem in Firefox. The method is basically to loop over the shapes and: 1) clear 1x1 pixel under mouse, 2) draw shape, 3) get 1x1 pixel under mouse. Surprisingly this works very well in Chrome and IE, but miserably under Firefox.
Apparently Chrome and IE are able to optimize if you only want a small pixel area, but Firefox doesn't appear to be optimizing at all based on the desired pixel area. Maybe internally it gets the entire canvas, then returns your pixel area.
Code and raw output here: http://pastebin.com/aW3xr2eB.

If I understand the question correctly, you want to detect when the mouse enters/leaves a shape on the canvas, correct?
If so, then you can use simple geometric calculations, which are MUCH simpler and faster than looping over pixel data. Your rendering algorithm already has a list of all visible shapes, so you know the position, dimension and type of each shape.
Assuming you have some kind of list of shapes, similar to what #Benjammmin' is describing, you can loop over the visible shapes and do point-inside-polygon checks:
// Track which shape is currently under the mouse cursor, and raise
// mouse enter/leave events
function trackHoverShape(mousePos) {
var shape;
for (var i = 0, len = visibleShapes.length; i < len; i++) {
shape = visibleShapes[i];
switch (shape.type ) {
case 'arc':
if (pointInCircle(mousePos, shape) &&
_currentHoverShape !== shape) {
raiseEvent(_currentHoverShape, 'mouseleave');
_currentHoverShape = shape;
raiseEvent(_currentHoverShape, 'mouseenter');
return;
}
break;
case 'rect':
if (pointInRect(mousePos, shape) &&
_currentHoverShape !== shape) {
raiseEvent(_currentHoverShape, 'mouseleave');
_currentHoverShape = shape;
raiseEvent(_currentHoverShape, 'mouseenter');
}
break;
}
}
}
function raiseEvent(shape, eventName) {
var handler = shape.events[eventName];
if (handler)
handler();
}
// Check if the distance between the point and the shape's
// center is greater than the circle's radius. (Pythagorean theroem)
function pointInCircle(point, shape) {
var distX = Math.abs(point.x - shape.center.x),
distY = Math.abs(point.y - shape.center.y),
dist = Math.sqrt(distX * distX + distY * distY);
return dist < shape.radius;
}
So, just call the trackHoverShape inside your canvas mousemove event and it will keep track of the shape currently under the mouse.
I hope this helps.

From comment:
Personally I would just switch to using SVG. It's more what it was
made for. However it may be worth looking at EaselJS
source. There's a method Stage.getObjectUnderPoint(), and their demo's
of this seem to work perfectly fine.
I ended up looking at the source, and the library utilises your first approach - separate hidden canvas for each object.
One idea that came to mind was attempting to create some kind of a content-aware algorithm to detect anti-aliased pixels and with what shapes they belong. I quickly dismissed this idea.
I do have one more theory, however. There doesn't seem to be a way around using ghost canvases, but maybe there is a way to generate them only when they're needed.
Please note the following idea is all theoretical and untested. It is possible I may have overlooked something that would mean this method would not work.
Along with drawing an object, store the method in which you drew that object. Then, using the method of drawing an object you can calculate a rough bounding box for that object. When clicking on the canvas, run a loop through all the objects you have on the canvas and extract ones which bounding boxes intercept with the point. For each of these extracted objects, draw them separately onto a ghost canvas using the method reference for that object. Determine if the mouse is positioned over a non-white pixel, clear the canvas, and repeat.
As an example, consider I have drawn two objects. I will store the methods for drawing the rectangle and circle in a readable manner.
circ = ['beginPath', ['arc', 75, 75, 10], 'closePath', 'fill']
rect = ['beginPath', ['rect', 150, 5, 30, 40], 'closePath', 'fill']
(You may want to minify the data saved, or use another syntax, such as the SVG syntax)
As I am drawing these circles for the first time, I will also keep note of the dimensional values and use them to determine a bounding box (Note: you will need to compensate for stroke widths).
circ = {left: 65, top: 65, right: 85, bottom: 85}
rect = {left: 150, top: 5, right: 180, bottom: 45}
A click event has occurred on the canvas. The mouse point is {x: 70, y: 80}
Looping through the two objects, we find that the mouse coordinates fall within the circle bounds. So we mark the circle object as a possible candidate for collision.
Analysing the circles drawing method, we can recreate it on a ghost canvas and then test if the mouse coordinates fall on a non-white pixel.
After determining if it does or does not, we can clear the ghost canvas to prepare for any more objects to be drawn on it.
As you can see this removes the need to store 960 x 800 x 100 pixels and only 960 x 800 x2 at most.
This idea would best be implemented as some kind of API for automatically handling the data storage (such as the method of drawing, dimensions...).

Detecting shape coordinates in Canvas

I'm writing drag & drop functionality in my HTML5 Canvas application and am wondering how to detect if I'm clicking on a shape other than a rectangle or square, in which case I would do something like this inside of my 'mousedown' event handler:
if (evt._x > 13 && evt._x < 202 .... ) {}
I don't see how to easily do something like that with an arc like this:
ctx.arc(25, 25, 20, 0, (Math.PI/180)*360);
I hope that is clear, thank you in advance.

Just use isPointInPath, which checks if a given point is within the current drawing path. If you're drawing multiple shapes to the canvas, than a good technique is to associate each of your shapes with a "hidden" canvas, draw each path to its respective canvas, than test isPointInPath against each of these, offsetting the destination/mouse coordinates as needed. Theres no reason to resort to your own calculations for this.

First you check if the click is within a shape's bounding box (the smallest rectangle which fully encloses the shape). If it is, then you do the more complex math to determine if the click is within the shape itself. You'll have to implement this math yourself as I don't think there's anything built-in for it.

You'll get the formula you need here and also in Polygon article of Wikipedia.

This may sound stupid, but you can use <area> tags inside a <map> over an <img> to create interactive polygonal shapes. They have their own onclicks/mouseovers/etc. already implemented by all browsers.

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.