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Drawing graphics
The browser contains some very powerful graphics programming tools, from the Scalable Vector Graphics (SVG) language, to APIs for drawing on HTML <sheet> elements, (see The Sail API and WebGL). This article provides an introduction to sail, and further resources to allow y'all to larn more.
Graphics on the Spider web
As we talked about in our HTML Multimedia and embedding module, the Web was originally just text, which was very boring, so images were introduced — offset via the <img> chemical element and after via CSS backdrop such as background-prototype, and SVG.
This however was still non plenty. While you could use CSS and JavaScript to animate (and otherwise manipulate) SVG vector images — as they are represented by markup — at that place was still no style to practise the same for bitmap images, and the tools available were rather limited. The Web even so had no fashion to finer create animations, games, 3D scenes, and other requirements commonly handled by lower level languages such as C++ or Java.
The situation started to amend when browsers began to back up the <canvas> chemical element and associated Sail API — Apple tree invented it in around 2004, and other browsers followed by implementing it in the years that followed. Every bit you'll come across beneath, canvas provides many useful tools for creating 2nd animations, games, data visualizations, and other types of app, especially when combined with some of the other APIs the web platform provides.
The below case shows a simple 2nd canvass-based bouncing assurance blitheness that we originally met in our Introducing JavaScript objects module:
Around 2006–2007, Mozilla started piece of work on an experimental 3D canvass implementation. This became WebGL, which gained traction amid browser vendors, and was standardized around 2009–2010. WebGL allows you to create real 3D graphics inside your spider web browser; the below case shows a unproblematic rotating WebGL cube:
This article will focus mainly on second sail, as raw WebGL code is very complex. Nosotros will even so show how to employ a WebGL library to create a 3D scene more easily, and you can notice a tutorial covering raw WebGL elsewhere — see Getting started with WebGL.
Note: Basic sheet functionality is supported well across browsers, with the exception of IE eight and below for 2nd canvas, and IE 11 and beneath for WebGL.
Active learning: Getting started with a <canvas>
If you desire to create a 2D or 3D scene on a web folio, you lot need to start with an HTML <sail> element. This chemical element is used to define the surface area on the folio into which the image will be fatigued. This is as simple every bit including the element on the page:
<canvass width = "320" height = "240" > </canvas > This volition create a canvass on the page with a size of 320 by 240 pixels.
Inside the canvas tags, you can put some fallback content, which is shown if the user'due south browser doesn't support canvas.
<sail width = "320" height = "240" > <p > Your browser doesn't back up canvas. Boo hoo! </p > </sail > Of course, the above bulletin is really unhelpful! In a real example you'd want to relate the fallback content to the sail content. For example, if you were rendering a constantly updating graph of stock prices, the fallback content could be a static image of the latest stock graph, with alt text maxim what the prices are in text.
Creating and sizing our canvas
Let's commencement by creating our own canvass that nosotros draw hereafter experiments on to.
- First make a local copy of the 0_canvas_start directory. It contains three files:
- "index.html"
- "script.js"
- "style.css"
- Open "index.html", and add the following lawmaking into it, just below the opening
<body>tag:We take added a<sail class = "myCanvas" > <p > Add together suitable fallback here. </p > </canvas >classto the<canvass>element so it volition be easier to select if we accept multiple canvases on the page, merely we have removed thewidthandheightattributes for at present (you could add them back in if you lot wanted, just nosotros will set them using JavaScript in a below section). Canvases with no explicit width and height default to 300 pixels wide by 150 pixels high. - At present open up "script.js" and add the post-obit lines of JavaScript: Here nosotros have stored a reference to the canvas in the
const canvas = certificate. querySelector ( '.myCanvas' ) ; const width = canvas.width = window.innerWidth; const height = canvas.height = window.innerHeight;canvasconstant. In the second line we set both a new abidingwidthand the canvass'widthholding equal toWindow.innerWidth(which gives us the viewport width). In the tertiary line we set both a new constantheightand the canvas'elevationholding equal toWindow.innerHeight(which gives the states the viewport height). And then now we have a sheet that fills the entire width and height of the browser window! Yous'll also run into that we are chaining assignments together with multiple equals signs — this is allowed in JavaScript, and it is a good technique if you want to make multiple variables all equal to the same value. We wanted to brand the sheet width and top easily accessible in the width/elevation variables, as they are useful values to have bachelor for later (for example, if yous want to describe something exactly halfway across the width of the sheet).
Note: Y'all should generally set up the size of the image using HTML attributes or DOM properties, as explained above. You could use CSS, but the trouble then is that the sizing is washed after the sail has rendered, and simply like any other image (the rendered canvas is just an prototype), the image could become pixelated/distorted.
Getting the canvas context and final setup
We demand to do i concluding thing earlier we can consider our canvas template finished. To draw onto the canvas nosotros demand to get a special reference to the drawing area called a context. This is washed using the HTMLCanvasElement.getContext() method, which for basic usage takes a single string as a parameter representing the blazon of context you want to recollect.
In this case nosotros want a 2nd sheet, so add the post-obit JavaScript line beneath the others in "script.js":
const ctx = canvas. getContext ( '2d' ) ; Note: other context values you could choose include webgl for WebGL, webgl2 for WebGL 2, etc., but we won't need those in this article.
So that's it — our canvas is now primed and ready for cartoon on! The ctx variable now contains a CanvasRenderingContext2D object, and all drawing operations on the canvas will involve manipulating this object.
Allow'southward do 1 last thing earlier we move on. Nosotros'll color the canvas background blackness to give you a starting time taste of the canvass API. Add the post-obit lines at the bottom of your JavaScript:
ctx.fillStyle = 'rgb(0, 0, 0)' ; ctx. fillRect ( 0 , 0 , width, height) ; Here we are setting a fill up color using the canvas' fillStyle property (this takes color values just like CSS properties practice), then drawing a rectangle that covers the entire area of the canvas with thefillRect method (the first two parameters are the coordinates of the rectangle's height left-mitt corner; the last ii are the width and height y'all want the rectangle drawn at — we told y'all those width and height variables would be useful)!
OK, our template is done and it's fourth dimension to move on.
2D canvas nuts
Equally we said to a higher place, all cartoon operations are done by manipulating a CanvasRenderingContext2D object (in our case, ctx). Many operations demand to be given coordinates to pinpoint exactly where to depict something — the acme left of the canvas is bespeak (0, 0), the horizontal (x) axis runs from left to right, and the vertical (y) axis runs from acme to bottom.
Drawing shapes tends to be washed using the rectangle shape primitive, or by tracing a line forth a certain path and so filling in the shape. Below we'll show how to do both.
Uncomplicated rectangles
Let's start with some simple rectangles.
- First of all, accept a copy of your newly coded sail template (or make a local re-create of the 1_canvas_template directory if you didn't follow the to a higher place steps).
- Next, add together the following lines to the bottom of your JavaScript: If yous save and refresh, you lot should meet a red rectangle has appeared on your canvas. Its top left corner is 50 pixels away from the tiptop and left of the canvas border (as divers by the offset two parameters), and it is 100 pixels wide and 150 pixels alpine (equally defined by the third and fourth parameters).
ctx.fillStyle = 'rgb(255, 0, 0)' ; ctx. fillRect ( 50 , 50 , 100 , 150 ) ; - Let'southward add some other rectangle into the mix — a green i this time. Add the following at the bottom of your JavaScript: Save and refresh, and you'll see your new rectangle. This raises an important point: graphics operations like drawing rectangles, lines, and and so forth are performed in the order in which they occur. Think of it similar painting a wall, where each coat of paint overlaps and may fifty-fifty hide what'southward underneath. You tin can't do anything to change this, and then you have to think advisedly nigh the society in which you draw the graphics.
ctx.fillStyle = 'rgb(0, 255, 0)' ; ctx. fillRect ( 75 , 75 , 100 , 100 ) ; - Note that you lot can depict semi-transparent graphics past specifying a semi-transparent color, for instance past using
rgba(). Theavalue defines what'southward called the "alpha aqueduct, " or the amount of transparency the color has. The higher its value, the more than it will obscure whatever'due south backside information technology. Add the following to your code:ctx.fillStyle = 'rgba(255, 0, 255, 0.75)' ; ctx. fillRect ( 25 , 100 , 175 , fifty ) ; - Now endeavour drawing some more rectangles of your own; have fun!
Strokes and line widths
So far nosotros've looked at drawing filled rectangles, merely you lot can also draw rectangles that are just outlines (called strokes in graphic design). To set up the color you lot want for your stroke, you use the strokeStyle property; drawing a stroke rectangle is done using strokeRect.
- Add the post-obit to the previous example, again beneath the previous JavaScript lines:
ctx.strokeStyle = 'rgb(255, 255, 255)' ; ctx. strokeRect ( 25 , 25 , 175 , 200 ) ; - The default width of strokes is 1 pixel; you can arrange the
lineWidthproperty value to alter this (it takes a number representing the number of pixels wide the stroke is). Add the following line in between the previous two lines:
At present you lot should see that your white outline has become much thicker! That'southward information technology for at present. At this point your example should look like this:
Drawing paths
If y'all want to draw anything more than complex than a rectangle, you need to draw a path. Basically, this involves writing code to specify exactly what path the pen should move forth on your sail to trace the shape y'all want to describe. Canvas includes functions for cartoon directly lines, circles, Bézier curves, and more than.
Permit'south kickoff the department off by making a fresh copy of our canvas template (1_canvas_template), in which to draw the new example.
We'll be using some common methods and properties beyond all of the below sections:
-
beginPath()— start drawing a path at the point where the pen currently is on the sheet. On a new canvas, the pen starts out at (0, 0). -
moveTo()— motility the pen to a different point on the sail, without recording or tracing the line; the pen "jumps" to the new position. -
fill()— draw a filled shape by filling in the path you've traced and then far. -
stroke()— draw an outline shape past drawing a stroke along the path you lot've drawn so far. - You can also use features like
lineWidthandfillStyle/strokeStylewith paths as well equally rectangles.
A typical, simple path-drawing operation would wait something similar and so:
ctx.fillStyle = 'rgb(255, 0, 0)' ; ctx. beginPath ( ) ; ctx. moveTo ( fifty , 50 ) ; // draw your path ctx. fill ( ) ; Cartoon lines
Allow's draw an equilateral triangle on the canvas.
- First of all, add the following helper role to the lesser of your code. This converts degree values to radians, which is useful considering whenever you lot need to provide an angle value in JavaScript, it will nearly always be in radians, but humans commonly think in degrees.
function degToRad ( degrees ) { render degrees * Math. PI / 180 ; } - Next, start off your path by adding the following below your previous improver; here we set a color for our triangle, beginning cartoon a path, and and then motility the pen to (fifty, 50) without drawing anything. That'southward where we'll start drawing our triangle.
ctx.fillStyle = 'rgb(255, 0, 0)' ; ctx. beginPath ( ) ; ctx. moveTo ( 50 , fifty ) ; - Now add the following lines at the bottom of your script: Let'south run through this in order: First we draw a line across to (150, 50) — our path at present goes 100 pixels to the correct along the x centrality. Second, we work out the pinnacle of our equilateral triangle, using a chip of simple trigonometry. Basically, we are drawing the triangle pointing down. The angles in an equilateral triangle are e'er sixty degrees; to work out the top nosotros can split up information technology downwards the middle into two right-angled triangles, which volition each have angles of 90 degrees, threescore degrees, and 30 degrees. In terms of the sides:
ctx. lineTo ( 150 , 50 ) ; const triHeight = 50 * Math. tan ( degToRad ( sixty ) ) ; ctx. lineTo ( 100 , l + triHeight) ; ctx. lineTo ( 50 , l ) ; ctx. make full ( ) ;- The longest side is chosen the hypotenuse
- The side adjacent to the sixty degree angle is called the next — which nosotros know is 50 pixels, as it is half of the line we only drew.
- The side reverse the threescore caste angle is called the opposite, which is the elevation of the triangle we want to summate.
One of the basic trigonometric formulae states that the length of the side by side multiplied by the tangent of the angle is equal to the opposite, hence we come upward with 50 * Math.tan(degToRad(60)). We utilize ourdegToRad()function to convert 60 degrees to radians, asMath.tan()expects an input value in radians. - With the superlative calculated, we draw another line to
(100, 50 + triHeight). The 10 coordinate is simple; it must be halfway betwixt the previous 2 X values nosotros prepare. The Y value on the other hand must be 50 plus the triangle summit, every bit we know the top of the triangle is 50 pixels from the top of the canvas. - The next line draws a line back to the starting point of the triangle.
- Last of all, we run
ctx.fill()to cease the path and fill in the shape.
Drawing circles
Now let's wait at how to depict a circle in sheet. This is accomplished using the arc() method, which draws all or role of a circle at a specified point.
- Let's add together an arc to our canvass — add the post-obit to the lesser of your code:
ctx.fillStyle = 'rgb(0, 0, 255)' ; ctx. beginPath ( ) ; ctx. arc ( 150 , 106 , 50 , degToRad ( 0 ) , degToRad ( 360 ) , imitation ) ; ctx. fill ( ) ;arc()takes 6 parameters. The first ii specify the position of the arc's center (10 and Y, respectively). The tertiary is the circumvolve's radius, the quaternary and 5th are the commencement and end angles at which to draw the circumvolve (so specifying 0 and 360 degrees gives us a full circle), and the sixth parameter defines whether the circle should be drawn counterclockwise (anticlockwise) or clockwise (falseis clockwise).Note: 0 degrees is horizontally to the right.
- Let'southward try adding another arc: The blueprint here is very similar, only with two differences:
ctx.fillStyle = 'yellowish' ; ctx. beginPath ( ) ; ctx. arc ( 200 , 106 , l , degToRad ( - 45 ) , degToRad ( 45 ) , true ) ; ctx. lineTo ( 200 , 106 ) ; ctx. fill ( ) ;- Nosotros have ready the last parameter of
arc()totrue, meaning that the arc is fatigued counterclockwise, which means that even though the arc is specified as starting at -45 degrees and ending at 45 degrees, we draw the arc effectually the 270 degrees not inside this portion. If you were to changetruetofalseand then re-run the code, but the 90 degree piece of the circle would be drawn. - Before calling
fill(), we draw a line to the middle of the circle. This means that nosotros go the rather prissy Pac-Man-style cutout rendered. If yous removed this line (try it!) so re-ran the code, you'd become simply an border of the circle chopped off between the start and end signal of the arc. This illustrates another important betoken of the sheet — if you try to fill an incomplete path (i.e. one that is not airtight), the browser fills in a directly line between the start and finish bespeak and and so fills information technology in.
- Nosotros have ready the last parameter of
That's information technology for now; your final example should look similar this:
Note: The finished code is available on GitHub equally 3_canvas_paths.
Notation: To find out more most avant-garde path drawing features such every bit Bézier curves, cheque out our Drawing shapes with canvas tutorial.
Text
Canvas as well has features for drawing text. Let'due south explore these briefly. Get-go by making another fresh copy of our canvas template (1_canvas_template) in which to draw the new case.
Text is drawn using two methods:
-
fillText()— draws filled text. -
strokeText()— draws outline (stroke) text.
Both of these accept three properties in their bones usage: the text string to draw and the 10 and Y coordinates of the betoken to start drawing the text at. This works out as the bottom left corner of the text box (literally, the box surrounding the text y'all draw), which might confuse y'all as other drawing operations tend to beginning from the elevation left corner — behave this in mind.
At that place are likewise a number of properties to help control text rendering such every bit font, which lets you specify font family, size, etc. It takes as its value the same syntax equally the CSS font property.
Try adding the post-obit cake to the bottom of your JavaScript:
ctx.strokeStyle = 'white' ; ctx.lineWidth = 1 ; ctx.font = '36px arial' ; ctx. strokeText ( 'Canvas text' , 50 , 50 ) ; ctx.fillStyle = 'red' ; ctx.font = '48px georgia' ; ctx. fillText ( 'Canvas text' , l , 150 ) ; Here nosotros draw two lines of text, i outline and the other stroke. The terminal example should expect like so:
Note: The finished lawmaking is available on GitHub as 4_canvas_text.
Have a play and come across what you can come with! You can detect more information on the options available for canvas text at Cartoon text.
Drawing images onto canvas
It is possible to render external images onto your canvass. These can be uncomplicated images, frames from videos, or the content of other canvases. For the moment we'll simply wait at the case of using some unproblematic images on our canvass.
- As before, brand some other fresh copy of our canvass template (1_canvas_template) in which to draw the new example. Images are drawn onto sheet using the
drawImage()method. The simplest version takes three parameters — a reference to the paradigm y'all want to render, and the X and Y coordinates of the image'south tiptop left corner. - Let's outset by getting an epitome source to embed in our canvas. Add the following lines to the bottom of your JavaScript: Here we create a new
const image = new Image ( ) ; prototype.src = 'firefox.png' ;HTMLImageElementobject using theImage()constructor. The returned object is the same type as that which is returned when you grab a reference to an existing<img>element). We then ready itssrcattribute to equal our Firefox logo prototype. At this indicate, the browser starts loading the image. - We could now try to embed the epitome using
drawImage(), just we need to make certain the epitome file has been loaded showtime, otherwise the code will fail. We tin achieve this using theloadevent, which volition only be fired when the image has finished loading. Add together the following block beneath the previous ane:If you load your example in the browser at present, you should see the image embedded in the canvas.paradigm. addEventListener ( 'load' , ( ) => ctx. drawImage (image, 20 , 20 , 185 , 175 , 50 , 50 , 185 , 175 ) ) ; - Simply there'south more! What if we want to display merely a part of the paradigm, or to resize it? We can do both with the more circuitous version of
drawImage(). Update yourctx.drawImage()line like then:ctx. drawImage (paradigm, 20 , 20 , 185 , 175 , 50 , fifty , 185 , 175 ) ;- The commencement parameter is the paradigm reference, as before.
- Parameters two and iii define the coordinates of the superlative left corner of the area you desire to cut out of the loaded image, relative to the top-left corner of the image itself. Nothing to the left of the starting time parameter or above the 2nd volition exist fatigued.
- Parameters 4 and 5 define the width and height of the area we want to cut out from the original image we loaded.
- Parameters 6 and 7 define the coordinates at which you want to draw the meridian-left corner of the cut-out portion of the image, relative to the height-left corner of the canvas.
- Parameters 8 and nine define the width and height to depict the cut-out surface area of the image. In this example, we take specified the same dimensions every bit the original slice, but you could resize it by specifying different values.
The final example should expect like and so:
Notation: The finished code is bachelor on GitHub equally 5_canvas_images.
Loops and animations
We have then far covered some very basic uses of 2D sheet, but actually you won't experience the full power of canvas unless you update or animate it in some way. Later on all, canvas does provide scriptable images! If you aren't going to change annihilation, then yous might also just employ static images and save yourself all the work.
Creating a loop
Playing with loops in sheet is rather fun — you can run sheet commands inside a for (or other type of) loop merely like any other JavaScript code.
Let's build a simple example.
- Make some other fresh re-create of our canvas template (1_canvas_template) and open it in your code editor.
- Add together the following line to the lesser of your JavaScript. This contains a new method,
interpret(), which moves the origin indicate of the canvas:This causes the coordinate origin (0, 0) to exist moved to the center of the canvas, rather than being at the peak left corner. This is very useful in many situations, similar this i, where nosotros want our pattern to be fatigued relative to the middle of the canvas.ctx. translate (width/ 2 , peak/ 2 ) ; - Now add the post-obit lawmaking to the lesser of the JavaScript: Here we are implementing the aforementioned
function degToRad ( degrees ) { return degrees * Math. PI / 180 ; } function rand ( min, max ) { return Math. floor (Math. random ( ) * (max-min+ 1 ) ) + (min) ; } permit length = 250 ; let moveOffset = xx ; for ( permit i = 0 ; i < length; i++ ) { }degToRad()part we saw in the triangle example in a higher place, arand()function that returns a random number between given lower and upper bounds,lengthandmoveOffsetvariables (which we'll find out more about later), and an emptyforloop. - The idea hither is that nosotros'll depict something on the canvas within the
forloop, and iterate on it each time and then nosotros can create something interesting. Add the post-obit code within yourforloop:And so on each iteration, we:ctx.fillStyle = ` rgba( ${ 255 -length} ,0, ${ 255 -length} ,0.9) ` ; ctx. beginPath ( ) ; ctx. moveTo (moveOffset,moveOffset) ; ctx. lineTo (moveOffset+length,moveOffset) ; const triHeight = length/ 2 * Math. tan ( degToRad ( lx ) ) ; ctx. lineTo (moveOffset+ (length/ ii ) ,moveOffset+triHeight) ; ctx. lineTo (moveOffset,moveOffset) ; ctx. make full ( ) ; length-- ; moveOffset += 0.vii ; ctx. rotate ( degToRad ( five ) ) ;- Ready the
fillStyleto be a shade of slightly transparent purple, which changes each time based on the value oflength. As you'll see later on the length gets smaller each time the loop runs, and then the effect here is that the color gets brighter with each successive triangle fatigued. - Begin the path.
- Move the pen to a coordinate of
(moveOffset, moveOffset); This variable defines how far nosotros want to move each time we draw a new triangle. - Draw a line to a coordinate of
(moveOffset+length, moveOffset). This draws a line of lengthlengthparallel to the X centrality. - Calculate the triangle'due south height, equally earlier.
- Draw a line to the downwards-pointing corner of the triangle, and then draw a line back to the offset of the triangle.
- Call
fill()to fill in the triangle. - Update the variables that describe the sequence of triangles, so we can exist ready to describe the next ane. We decrease the
lengthvalue by one, so the triangles become smaller each time; increasemoveOffsetby a small-scale amount and then each successive triangle is slightly farther away, and utilize some other new office,rotate(), which allows us to rotate the entire canvas! Nosotros rotate it by 5 degrees before drawing the side by side triangle.
- Ready the
That's it! The last example should look like so:
At this betoken, we'd like to encourage you to play with the example and make information technology your own! For example:
- Draw rectangles or arcs instead of triangles, or fifty-fifty embed images.
- Play with the
lengthandmoveOffsetvalues. - Innovate some random numbers using that
rand()part nosotros included above only didn't use.
Animations
The loop example we built in a higher place was fun, but really you need a constant loop that keeps going and going for any serious canvas applications (such as games and existent time visualizations). If you remember of your canvas as beingness like a movie, you really want the brandish to update on each frame to show the updated view, with an platonic refresh charge per unit of sixty frames per second so that movement appears squeamish and smooth to the human being eye.
There are a few JavaScript functions that volition allow you to run functions repeatedly, several times a second, the best ane for our purposes here existence window.requestAnimationFrame(). It takes i parameter — the name of the function yous desire to run for each frame. The next time the browser is fix to update the screen, your function will get called. If that function draws the new update to your animation, then calls requestAnimationFrame() again merely before the end of the function, the blitheness loop will continue to run. The loop ends when you stop calling requestAnimationFrame() or if y'all call window.cancelAnimationFrame() afterward calling requestAnimationFrame() only before the frame is called.
Note: It'southward good practise to call cancelAnimationFrame() from your main code when you're washed using the animation, to ensure that no updates are still waiting to be run.
The browser works out complex details such as making the blitheness run at a consistent speed, and not wasting resources animative things that tin can't be seen.
To meet how it works, let's speedily wait over again at our Bouncing Balls example (see information technology alive, and also see the source code). The code for the loop that keeps everything moving looks similar this:
function loop ( ) { ctx.fillStyle = 'rgba(0, 0, 0, 0.25)' ; ctx. fillRect ( 0 , 0 , width, acme) ; for ( const brawl of balls) { ball. draw ( ) ; ball. update ( ) ; ball. collisionDetect ( ) ; } requestAnimationFrame (loop) ; } loop ( ) ; Nosotros run the loop() role one time at the bottom of the code to showtime the cycle, drawing the outset animation frame; the loop() office and so takes charge of calling requestAnimationFrame(loop) to run the next frame of the animation, again and over again.
Note that on each frame nosotros are completely immigration the canvas and redrawing everything. For every brawl present we describe it, update its position, and check to come across if it is colliding with any other balls. Once you lot've drawn a graphic to a sail, at that place's no way to dispense that graphic individually like y'all tin with DOM elements. You lot tin't move each ball around on the canvas, because one time it'southward fatigued, information technology's part of the canvas, and is not an individual attainable element or object. Instead, y'all have to erase and redraw, either past erasing the entire frame and redrawing everything, or past having code that knows exactly what parts need to be erased and just erases and redraws the minimum surface area of the canvas necessary.
Optimizing animation of graphics is an unabridged specialty of programming, with lots of clever techniques bachelor. Those are across what we need for our example, though!
In general, the process of doing a canvas animation involves the following steps:
- Clear the canvas contents (e.thousand. with
fillRect()orclearRect()). - Salve state (if necessary) using
save()— this is needed when y'all want to save settings you've updated on the canvas earlier continuing, which is useful for more advanced applications. - Depict the graphics you are animating.
- Restore the settings you saved in pace 2, using
restore() - Call
requestAnimationFrame()to schedule drawing of the adjacent frame of the animation.
Note: We won't cover salvage() and restore() here, but they are explained nicely in our Transformations tutorial (and the ones that follow it).
A simple graphic symbol animation
Now let's create our ain simple animation — we'll get a character from a certain rather awesome retro computer game to walk beyond the screen.
- Make another fresh re-create of our sheet template (1_canvas_template) and open up it in your code editor.
- At the lesser of the JavaScript, add the following line to once once again make the coordinate origin sit in the middle of the canvass:
ctx. translate (width/ 2 , elevation/ two ) ; - Now let's create a new
HTMLImageElementobject, set itssrcto the image we desire to load, and add anonloadconsequence handler that volition cause thedescribe()function to burn down when the image is loaded:const prototype = new Image ( ) ; image.src = 'walk-right.png' ; image.onload = draw; - Now we'll add some variables to keep rails of the position the sprite is to be fatigued on the screen, and the sprite number nosotros desire to brandish. Let's explain the spritesheet image (which we have respectfully borrowed from Mike Thomas' Create a sprite sail walk cycle using CSS animation). The image looks like this:
let sprite = 0 ; let posX = 0 ;
It contains six sprites that make upwards the whole walking sequence — each ane is 102 pixels wide and 148 pixels high. To display each sprite cleanly we will have to use drawImage()to chop out a single sprite image from the spritesheet and display only that role, like we did in a higher place with the Firefox logo. The 10 coordinate of the slice will take to be a multiple of 102, and the Y coordinate will always be 0. The slice size volition always be 102 past 148 pixels. - At present let's insert an empty
draw()role at the bottom of the code, ready for filling up with some code: - The rest of the code in this section goes inside
depict(). Offset, add the post-obit line, which clears the canvas to gear up for drawing each frame. Notice that we have to specify the top-left corner of the rectangle as-(width/2), -(height/2)because we specified the origin position aswidth/ii, acme/2earlier on.ctx. fillRect ( - (width/ two ) , - (elevation/ 2 ) , width, superlative) ; - Next, we'll draw our image using drawImage — the 9-parameter version. Add together the following: As you can see:
ctx. drawImage (image, (sprite* 102 ) , 0 , 102 , 148 , 0 +posX, - 74 , 102 , 148 ) ;- Nosotros specify
imageevery bit the prototype to embed. - Parameters 2 and 3 specify the superlative-left corner of the slice to cut out of the source image, with the X value as
spritemultiplied past 102 (wherespriteis the sprite number between 0 and v) and the Y value always 0. - Parameters 4 and 5 specify the size of the slice to cut out — 102 pixels past 148 pixels.
- Parameters 6 and 7 specify the top-left corner of the box into which to draw the slice on the canvass — the X position is 0 +
posX, meaning that we can modify the drawing position by altering theposXvalue. - Parameters eight and 9 specify the size of the image on the canvas. We just desire to go on its original size, and so we specify 102 and 148 every bit the width and peak.
- Nosotros specify
- Now nosotros'll alter the
spritevalue after each describe — well, after some of them anyhow. Add the following block to the bottom of thedraw()part:Nosotros are wrapping the whole cake inif (posX % 13 === 0 ) { if (sprite === 5 ) { sprite = 0 ; } else { sprite++ ; } }if (posX % thirteen === 0) { ... }. We use the modulo (%) operator (also known every bit the residual operator) to check whether theposXvalue can be exactly divided by 13 with no rest. If and so, we move on to the next sprite by incrementingsprite(wrapping to 0 later we're done with sprite #5). This finer means that nosotros are only updating the sprite on every 13th frame, or roughly about v frames a second (requestAnimationFrame()calls us at up to lx frames per 2nd if possible). We are deliberately slowing down the frame rate because we only take six sprites to work with, and if we display i every 60th of a second, our character will move way likewise fast! Inside the outer block nosotros utilise anif ... elsestatement to check whether thespritevalue is at v (the concluding sprite, given that the sprite numbers run from 0 to 5). If we are showing the last sprite already, nosotros resetspriteback to 0; if not we just increment it by i. - Next we need to work out how to change the
posXvalue on each frame — add the following lawmaking block just below your final i.We are using anotherif (posX > width/ two ) { let newStartPos = - ( (width/ ii ) + 102 ) ; posX = Math. ceil (newStartPos) ; console. log (posX) ; } else { posX += 2 ; }if ... elsestatement to encounter if the value ofposXhas become greater thanwidth/2, which means our character has walked off the right border of the screen. If and so, we calculate a position that would put the character just to the left of the left side of the screen. If our graphic symbol hasn't yet walked off the edge of the screen, we incrementposXby ii. This will brand him move a little flake to the right the side by side fourth dimension nosotros draw him. - Finally, we need to make the animation loop past calling
requestAnimationFrame()at the bottom of thedraw()function:window. requestAnimationFrame (draw) ;
That's information technology! The final example should look like so:
A simple drawing awarding
As a final blitheness case, we'd similar to show you a very simple cartoon application, to illustrate how the blitheness loop can exist combined with user input (similar mouse move, in this example). We won't get you to walk through and build this one; nosotros'll just explore the most interesting parts of the code.
The example can be institute on GitHub as 8_canvas_drawing_app, and you can play with it live beneath:
Let's look at the about interesting parts. First of all, we keep track of the mouse's 10 and Y coordinates and whether it is being clicked or non with three variables: curX, curY, and pressed. When the mouse moves, we fire a function set as the onmousemove effect handler, which captures the current X and Y values. We likewise utilize onmousedown and onmouseup upshot handlers to alter the value of pressed to true when the mouse button is pressed, and back to false again when it is released.
allow curX; let curY; let pressed = false ; // update mouse pointer coordinates document. addEventListener ( 'mousemove' , eastward => { curX = (window.Event) ? e.pageX : e.clientX + (document.documentElement.scrollLeft ? certificate.documentElement.scrollLeft : document.body.scrollLeft) ; curY = (window.Outcome) ? due east.pageY : due east.clientY + (document.documentElement.scrollTop ? document.documentElement.scrollTop : document.body.scrollTop) ; } ) ; canvas. addEventListener ( 'mousedown' , ( ) => pressed = true ) ; canvas. addEventListener ( 'mouseup' , ( ) => pressed = false ) ; When the "Clear canvas" button is pressed, we run a simple function that clears the whole canvas dorsum to black, the same way nosotros've seen before:
clearBtn. addEventListener ( 'click' , ( ) => { ctx.fillStyle = 'rgb(0,0,0)' ; ctx. fillRect ( 0 , 0 ,width,height) ; } ) ; The drawing loop is pretty simple this time effectually — if pressed is true, we depict a circle with a make full style equal to the value in the colour picker, and a radius equal to the value ready in the range input. We take to draw the circle 85 pixels above where we measured it from, because the vertical measurement is taken from the top of the viewport, merely we are drawing the circle relative to the top of the canvas, which starts below the 85 pixel-high toolbar. If we drew it with merely curY as the y coordinate, it would appear 85 pixels lower than the mouse position.
function depict ( ) { if (pressed) { ctx.fillStyle = colorPicker.value; ctx. beginPath ( ) ; ctx. arc (curX, curY- 85 , sizePicker.value, degToRad ( 0 ) , degToRad ( 360 ) , false ) ; ctx. fill ( ) ; } requestAnimationFrame (draw) ; } draw ( ) ; Note: The <input> range and color types are supported fairly well beyond browsers, with the exception of Internet Explorer versions less than ten; also Safari doesn't yet back up color. If your browser doesn't support these inputs, they will fall back to simple text fields and you'll just accept to enter valid color/number values yourself.
WebGL
It's now time to leave 2D backside, and have a quick wait at 3D canvas. 3D canvass content is specified using the WebGL API, which is a completely separate API from the 2d canvas API, even though they both render onto <canvas> elements.
WebGL is based on OpenGL (Open Graphics Library), and allows you to communicate directly with the computer's GPU. As such, writing raw WebGL is closer to low level languages such every bit C++ than regular JavaScript; it is quite complex but incredibly powerful.
Using a library
Because of its complexity, most people write 3D graphics code using a 3rd party JavaScript library such every bit Three.js, PlayCanvas, or Babylon.js. Most of these piece of work in a similar manner, providing functionality to create primitive and custom shapes, position viewing cameras and lighting, covering surfaces with textures, and more. They handle the WebGL for you, letting you work on a higher level.
Aye, using one of these means learning some other new API (a tertiary party one, in this example), but they are a lot simpler than coding raw WebGL.
Recreating our cube
Permit's await at a simple case of how to create something with a WebGL library. Nosotros'll cull Three.js, equally it is one of the most popular ones. In this tutorial we'll create the 3D spinning cube we saw before.
- To showtime with, make a local copy of threejs-cube/alphabetize.html in a new folder, then save a copy of metal003.png in the same binder. This is the image nosotros'll utilise as a surface texture for the cube after.
- Side by side, create a new file chosen
script.js, again in the same folder as earlier. - Next, you lot need to download the three.min.js library and save it in the same directory as earlier.
- Now we've got
three.jsattached to our page, we tin start to write JavaScript that makes use of it intoscript.js. Let'southward beginning by creating a new scene — add the following into your main.js file:Theconst scene = new 3.Scene ( ) ;Scene()constructor creates a new scene, which represents the whole 3D world we are trying to display. - Adjacent, we need a photographic camera so we can see the scene. In 3D imagery terms, the camera represents a viewer's position in the earth. To create a camera, add the following lines next: The
const camera = new Three.PerspectiveCamera ( 75 , window.innerWidth / window.innerHeight, 0.1 , m ) ; camera.position.z = 5 ;PerspectiveCamera()constructor takes four arguments:- The field of view: How wide the area in front of the camera is that should be visible onscreen, in degrees.
- The attribute ratio: Usually, this is the ratio of the scene's width divided by the scene'due south peak. Using another value volition distort the scene (which might exist what you want, simply usually isn't).
- The well-nigh plane: How shut to the photographic camera objects tin exist before we end rendering them to the screen. Think almost how when you move your fingertip closer and closer to the space between your eyes, eventually yous can't see it anymore.
- The far airplane: How far away things are from the camera before they are no longer rendered.
- The third vital ingredient is a renderer. This is an object that renders a given scene, as viewed through a given photographic camera. Nosotros'll create one for now using the
WebGLRenderer()constructor, but we'll not use it till later. Add the following lines next:The first line creates a new renderer, the second line sets the size at which the renderer will draw the camera'southward view, and the third line appends theconst renderer = new Iii.WebGLRenderer ( ) ; renderer. setSize (window.innerWidth, window.innerHeight) ; document.body. appendChild (renderer.domElement) ;<sheet>chemical element created past the renderer to the certificate's<body>. Now anything the renderer draws volition be displayed in our window. - Next, nosotros want to create the cube we'll display on the sail. Add the following chunk of code at the bottom of your JavaScript: There's a bit more to take in here, and then let'southward go through it in stages:
permit cube; const loader = new THREE.TextureLoader ( ) ; loader. load ( 'metal003.png' , texture => { texture.wrapS = THREE .RepeatWrapping; texture.wrapT = THREE .RepeatWrapping; texture.repeat. prepare ( 2 , 2 ) ; const geometry = new THREE.BoxGeometry ( 2.4 , ii.four , 2.iv ) ; const material = new THREE.MeshLambertMaterial ( { map : texture } ) ; cube = new THREE.Mesh (geometry, material) ; scene. add (cube) ; describe ( ) ; } ) ;- We outset create a
cubeglobal variable and so we can access our cube from anywhere in the lawmaking. - Adjacent, we create a new
TextureLoaderobject, then callload()on it.load()takes two parameters in this example (although it can have more): the texture we want to load (our PNG), and a part that will run when the texture has loaded. - Within this function nosotros use backdrop of the
textureobject to specify that we want a 2 ten 2 repeat of the image wrapped around all sides of the cube. Next, we create a newBoxGeometryobject and a newMeshLambertMaterialobject, and bring them together in aMeshto create our cube. An object typically requires a geometry (what shape information technology is) and a material (what its surface looks like). - Final of all, nosotros add our cube to the scene, so call our
draw()function to starting time off the animation.
- We outset create a
- Before we get to defining
draw(), we'll add a couple of lights to the scene, to liven things upward a bit; add the following blocks next:Anconst lite = new 3.AmbientLight ( 'rgb(255,255,255)' ) ; // soft white light scene. add (calorie-free) ; const spotLight = new THREE.SpotLight ( 'rgb(255,255,255)' ) ; spotLight.position. set ( 100 , 1000 , chiliad ) ; spotLight.castShadow = true ; scene. add (spotLight) ;AmbientLightobject is a kind of soft light that lightens the whole scene a fleck, like the sun when you are outside. TheSpotLightobject, on the other hand, is a directional axle of light, more like a flashlight/torch (or a spotlight, in fact). - Terminal of all, permit's add our
draw()function to the lesser of the code:This is fairly intuitive; on each frame, we rotate our cube slightly on its X and Y axes, then return the scene as viewed by our camera, then finally telephone callrole draw ( ) { cube.rotation.x += 0.01 ; cube.rotation.y += 0.01 ; renderer. render (scene, camera) ; requestAnimationFrame (draw) ; }requestAnimationFrame()to schedule drawing our next frame.
Permit'southward have another quick expect at what the finished product should look similar:
You can find the finished code on GitHub.
Note: In our GitHub repo yous can also discover some other interesting 3D cube example — Three.js Video Cube (see information technology live also). This uses getUserMedia() to take a video stream from a estimator web cam and projection it onto the side of the cube as a texture!
Summary
At this signal, you should take a useful idea of the nuts of graphics programming using Sheet and WebGL and what you can do with these APIs, besides every bit a good idea of where to go for further data. Accept fun!
See also
Here we have covered only the real nuts of sheet — at that place is and so much more than to learn! The beneath articles will take you further.
- Canvas tutorial — A very detailed tutorial series explaining what you lot should know about 2D canvas in much more particular than was covered here. Essential reading.
- WebGL tutorial — A series that teaches the basics of raw WebGL programming.
- Building upwards a basic demo with 3.js — basic Iii.js tutorial. We besides have equivalent guides for PlayCanvas or Babylon.js.
- Game evolution — the landing page for web games development on MDN. There are some really useful tutorials and techniques available here related to 2nd and 3D canvas — meet the Techniques and Tutorials menu options.
Examples
- Violent theremin — Uses the Web Audio API to generate sound, and canvas to generate a pretty visualization to go along with it.
- Vocalization alter-o-matic — Uses a canvas to visualize real-time audio data from the Web Sound API.
In this module
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Source: https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics
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