Obavezno pročitajte prije nego što napravite bilo što s oznakom platna, čak i ako to već znate.

Pregled

HTML element platna koristi se za crtanje "rasterske" grafike na web aplikaciji. Canvas API pruža dva konteksta crtanja: 2D i 3D, a u ovom ćemo vodiču razgovarati o 2D (što ću zbog jednostavnosti uputiti na Canvas API).

Prije nego što započnem, želim da znate jednu vrlo važnu točku. Canvas je API rasterske grafike - manipulirate stvarima na razini piksela. To znači da osnovni softver ne poznaje model koji koristite za prikaz konteksta - ne zna crtate li pravokutnik ili krug.

API Canvas podijelio sam u zasebne dijelove, da biste jedan po jedan žderali:

API putanje
Stilovi crtanja
Gradijenti i obrasci
Izravna manipulacija pikselima i slike
Transformacije
Hit Regions
Stanje i metoda clip ()

Postaviti

Da biste pokrenuli svoj Canvas tutorial, stvorite HTML datoteku i JS datoteku povezanu s njom.

  Canvas Demo   This will be displayed if your browser doesn't support the canvas element. The closing tag is necessary.

U vašoj canvas-demo.jsdatoteci,

// canvas-demo.js const demoCanvas = document.getElementById(’canvas-demo’).getContext(’2d’); window.onload = function() {// make sure to use onload /* Add code here as we go!!! @nodocs */ }

Staze

Putovi su skup točaka u mreži 2D piksela na platnu. Crtaju se uz pomoć ovog API-ja. Svaki oblik u putanji koji nacrtate W3C dokumentacija naziva "podputa".

beginPath()i closePath(): Svi oblici koje nacrtate dodaju se u trenutnu putanju. Ako nazovete strokeili fillkasnije, primijenit će to na sve oblike koje ste nacrtali u trenutnoj putanji. Da biste to spriječili, crtež dijelite pozivanjem beginPathi closePath.

// Calling this isn't necessary, but a good practice. demoCanvas.beginPath(); /* * Drawing code, copy and paste each example (separately) here */ demoCanvas.closePath();// this is required if you want to draw // in a separate path later

moveTo(x,y) : Označava konstrukciju novog oblika koji započinje u točki (x, y).
lineTo(x,y): Crta crtu od posljednje točke u trenutnom obliku do položene točke. Ako nije stvoren nijedan oblik (putem moveTo), tada se stvara novi s početkom u (x, y) (baš kao i moveTo).
quadraticCurveTo(cpx1,cpy1,x,y)i bezierCurveTo(cpx1,cpy1,cpx2,cpy2,x,y): Crta kvadratnu / kubičnu bezierovu krivulju koja započinje od posljednje točke u obliku, prolazi kroz kontrolne točke ( cpx1,cpy1i cpx2,cpy2) i završava na x,y. Bezierova krivulja je samo "glatka" krivulja koja prolazi kroz srednje "kontrolne" točke s danim završnim točkama. Imajte na umu da krivulja ne mora točno prolaziti kroz kontrolne točke - može se izravnati.
arcTo(x1,y1,x2,y2,radius): Ovo je malo komplicirana metoda za upotrebu. Pretpostavimo da je trenutna točka na putu x0,y0- tada arcToće povući luk koji ima dvije tangente koje povezuju ova dva para točaka (x1,y1) & (x0,y0)i (x1,y1) & (x2,y2). Polumjer luka bit će onaj zadani. Što je veći radijus, to će luk biti dalje udaljen x1,y1, (vidi Primjer 1.2 za vizualnu jasnoću). Ako još niste koristili moveTo, tada x0,y0će prema zadanim postavkama biti 0,0.
arc(x,y,radius,startAngle,endAngle,counterclockwise): Povezuje trenutnu točku na putu (prema zadanim postavkama 0,0) s početkom luka. Crta luk iz središta x,ypolumjera radius, od startAngledo endAngle. (Napomena: Za razliku od matematike olovke i papira, kutovi su opisani u smjeru kazaljke na satu u Canvas API-ju); ali pod četiri posebna uvjeta - (x0,y0)jednaki (x1,y1), (x1,y1)jednaki (x2,y2), (x0,y0),(x1,y1),(x2,y2)kolinearni su ili ako radiusje nula, tada će poziv na arcbiti jednak lineTo(x1,y1)i umjesto toga će se povući crta.
rect(x,y,w,h): Crta pravokutnik s gornjim lijevim kutom x,yi širine wi visine h.

Primjer 1.1:

Sada moramo isprobati demonstraciju - nacrtati ćemo nekoliko slučajnih vodoravnih crta, a zatim skicu oka. Rezultat će izgledati poput nečega s lijeve strane. Ne zaboravite proći kroz kod i poigrati se s kodom.

/* Draw horizontal subpaths (shapes) in one path. */ // Draw a pattern of vertically stack horizontal // lines. demoCanvas.moveTo(10, 10);// start at (10,10) demoCanvas.lineTo(110, 10); demoCanvas.moveTo(10, 20);// 10 pts below demoCanvas.lineTo(180, 20); demoCanvas.moveTo(10, 30); demoCanvas.lineTo(150, 30); demoCanvas.moveTo(10, 40); demoCanvas.lineTo(160, 40); demoCanvas.moveTo(10, 50); demoCanvas.lineTo(130, 50); // try removing this moveTo, the quad-curve will then // start from from (130, 50), due to the lineTo. demoCanvas.moveTo(10, 100);// quad-curve starts from here demoCanvas.quadraticCurveTo(110, 55, 210, 100);// curve upward demoCanvas.moveTo(10, 100);// back here, let's draw one below demoCanvas.quadraticCurveTo(110, 145, 210, 100);// curve below // that forms the eye outline demoCanvas.moveTo(132.5, 100);// remove this, a horizontal line will be // drawn from (210, 100) to (132.5, 100) because arc() connects the last // point to the start of the arc. demoCanvas.arc(110, 100, 22.5, 0, 2*Math.PI, false);// pupil (circle) /* We'll talk about this shortly */ demoCanvas.stroke();// draws (by outlining our shapes in the path)

Primjer 1.2:

U donjem primjeru kreiram kubnu krivulju (s vizualnim smjernicama), arcTopozive u sredini desno i čopora sa arc()dolje lijevo. Kontrolne točke (u kubičnoj krivulji) su oblici kutova prema tri smjernice.

(x1,y1)jer arcToje kut koji tvore dvije tangente.

// comment this block out if you can see the cubic curve demoCanvas.moveTo(100, 100); demoCanvas.lineTo(150, 10); demoCanvas.moveTo(250, 100); demoCanvas.lineTo(200, 190); demoCanvas.moveTo(150, 10); demoCanvas.lineTo(200, 190) demoCanvas.moveTo(100, 100); demoCanvas.bezierCurveTo(150, 10, 200, 190, 250, 100); // arcTo() is too complicated to use // demoCanvas.stroke(); demoCanvas.closePath(); demoCanvas.beginPath(); demoCanvas.moveTo(200, 200);// comment out above line (and comment this line), // then the arc's tangent will come from (0,0)!! Try it. demoCanvas.arcTo(100, 300, 300, 300, 100); demoCanvas.moveTo(200, 200); demoCanvas.arcTo(100, 300, 300, 300, 50); demoCanvas.moveTo(100, 300); demoCanvas.lineTo(300, 300); demoCanvas.moveTo(100, 300); demoCanvas.lineTo(200, 200); demoCanvas.moveTo(50, 300); // packman demoCanvas.arc(50, 300, 35, Math.PI/6, 11*Math.PI/6, false); demoCanvas.lineTo(50, 300); demoCanvas.stroke();

Stilovi crtanja

Do sada smo crtali jednostavne tanke linije. Stilovi crtanja pomoći će nam da svoj crtež učinimo puno boljim.

Imajte na umu da na isti put ne možete primijeniti dva različita stila. Na primjer, ako želite nacrtati crvenu i plavu crtu - morat ćete stvoriti novi put za crtanje plave. Ako ne izradite novu putanju, nakon strokedrugog poziva nakon postavljanja boje stila prikaza na plavu, oba će retka biti obojena plavom bojom. Stoga se stilovi primjenjuju na sve podpute, bez obzira jesu li već izglađeni ili ne.

U demoCanvastu svrhu definirano je nekoliko svojstava 2D kontekstnog objekta :

lineWidth: Debljina crtanih linija. Prema zadanim postavkama ovo je 1; stoga su dva gornja primjera koristila obris debljine 1 piksela.
lineCap: Ovo je kapica nanesena na krajeve potputa (oblika). To je niz i može imati tri važeće vrijednosti: "stražnji dio", "okrugli", "kvadratni" (vidi Primjer 1.3 za vizualnu jasnoću). "Stražnjica" će završiti linije bez kapice - što rezultira krutim, pravokutnim krajevima poput tankih pravokutnika. "Okrugli" dodaje polukrug na krajeve dajući glatke krajeve. "Kvadrat" dodaje kvadrat na kraj, ali izgleda kao "stražnjica". "Okrugli" i "kvadratni" dodaju malo dodatne duljine svakoj podputi.
lineJoin: Ovo odlučuje kako će se spojiti dvije preklapajuće linije. Na primjer, ako želite stvoriti strelicu s desne strane (>), tada možete promijeniti način oblikovanja kuta s ovim svojstvom. Ovo ima tri valjane vrijednosti: "okruglo", "koso" i "mitre". U primjeru 1.4 provjerite kako mijenjaju kutove. (Zadana vrijednost je "mitra"). "Okrugli" će oblikovati kružne kutove, dok će "koso" stvoriti krute trostrane kutove, a "mitra" će stvoriti oštar rub.
miterLimit: Kada lineJoin="miter", ovo određuje najveću udaljenost c / w unutarnjeg i vanjskog kuta crte. Pogledajte primjer 1.4 (b) za vizualnu jasnoću. Ako je granična vrijednost previsoka, oštre strelice mogu imati veliko zajedničko područje c / b dvije crte. Ako se pređe graničnik mitre, zaslon se vraća u kosi spoj.

Primjeri 1.3 i 1.4:

U primjeru 1.3 s lijeve strane možete vidjeti kako su okrugle i kvadratne linije s ograničenim crtama duže od zadanog ograničenja. (NAPOMENA: Što je linija deblja, to je veće povećanje duljine)

U primjeru 1.4 (a) možete vidjeti kako funkcioniraju spojevi u obliku kružnice i kosa. Stvorene linije identične su u gornjem i donjem dijelu. Samo su lineJoinsvojstva različita.

U primjeru 4.1 (b) možete vidjeti kako funkcionira mitered spoj i što se događa ako se proslijedi duljina mitere.

Definirana su dodatna svojstva stila prikaza:

font: Ovaj niz definira kako želite oblikovati tekst. Na primjer, demoCanvas.font="10px Times New Roman"valjana je vrijednost fonta.
textAlign: Valjane vrijednosti su - "početak", "kraj", "lijevo", "desno" i "središte". Zadana postavka je "start".
textBaseline: Valjane vrijednosti su - „gornji”, „viseći”, „srednji”, „abecedni”, „ideografski”, „donji”. Zadana vrijednost je "abecedno".

Actual drawing methods

In the examples till now, you might have noticed I’ve used demoCanvas.stroke() before closing each path. The stroke method does that actual drawing partly in those examples.

stroke : This method draws the outline around each subpath (shapes) according to the lineWidth and related properties.
fill : This method fills the interior of the shape traced by the path. If the path is not closed, then it will close it automatically by connecting the last point to the first point.

demoCanvas.moveTo(10,10); demoCanvas.lineTo(50, 50); demoCanvas.lineTo(10, 50); demoCanvas.fill();

The above code does not close the triangle (10,10),(50,50),(10,50) but calling fill() fills it as expected.

clearRect(x,y,w,h) : Clears the pixels in the rectangle formed with the given parameters.
strokeRect(x,y,w,h) : Equivalent to calling rect and then stroke . It doesn’t add the rectangle to the current path — hence, you can change the style later and call stroke without affecting the rectangle formed.
fillRect(x,y,w,h) : Equivalent to calling rect and then fill . This also doesn’t add the rectangle to the current path.
strokeText(text,x,y,maxWidth) and fillText(text,x,y,maxWidth) : Writes the text at (x,y) according to the strokeStyle / fillStyleproperty. maxWidth is optional and defines the maximum length in pixels that you want the text to occupy. If the text is longer, then it is scaled to a smaller font. measureText("text").width can be used to find the display width of a piece of text, based on the current font.

NOTE: fillStyle and strokeStyle are the properties that can be set to any CSS color string to set the fill & stroke colors.

Gradients and Patterns

Out of the box, the 2D context provides linear and radial gradients. The createLinearGradient and createRadialGradient methods return CanvasGradient objects, which can then be modified what we want.

createLinearGradient(x0,y0,x1,y1) : Constructs a linear gradient that runs on the line x0,y0 to x1,y1 .
createRadialGradient(x0,y0,r0,x1,y1,r1) : Constructs a radial gradient that runs in the cone (of circles) with the top (inner circle) of radius r0and bottom (outer circle) of radius r1 . The first color would have a radius of r0 .

The CanvasGradient has one method: addColorStop(offset,color) . The gradient starts at 0 and ends at 1. The color at the position of offset will be set using this method. For example, addColorStop(.5, "green") will make the middle color green. Colors b/w two adjacent stops will be interpolated (mixed).

Example 1.6:

In the example on the left, you can see how linear and radial gradients work.

var linearGrad = demoCanvas.createLinearGradient(5,5,100,5); linearGrad.addColorStop(0, "blue"); linearGrad.addColorStop(.5, "green"); linearGrad.addColorStop(1, "red"); demoCanvas.strokeStyle=linearGrad; demoCanvas.lineWidth=50; demoCanvas.moveTo(5,5); demoCanvas.lineTo(100,5); demoCanvas.stroke();// change strokeStyle(l10) to fillStyle(l10) // and stroke() to fill(). Then, change lineTo(100,5) to rect(5,5,95,50). // Results should be almost same. demoCanvas.closePath(); demoCanvas.beginPath(); var radialGrad = demoCanvas.createRadialGradient(50,50,10,50,50,40); radialGrad.addColorStop(0, "blue"); radialGrad.addColorStop(.5, "green"); radialGrad.addColorStop(1, "red"); demoCanvas.fillStyle=radialGrad; demoCanvas.arc(50,50,30,0,2*Math.PI,false); demoCanvas.fill();

You might wonder what if x0,y0 and x1,y1 given to the linear/radial gradient are not equal to the line/arc we create? See Example 1.7

Example 1.7

var linearGrad = demoCanvas.createLinearGradient(5,5,100,5); linearGrad.addColorStop(0, "blue"); linearGrad.addColorStop(.5, "green"); linearGrad.addColorStop(1, "red"); demoCanvas.strokeStyle=linearGrad; demoCanvas.lineWidth=50; demoCanvas.moveTo(50,5); demoCanvas.lineTo(155,5); demoCanvas.stroke();// change strokeStyle(l10) to fillStyle(l10) // and stroke() to fill(). Then, change lineTo(100,5) to rect(5,5,95,50). // Results should be almost same. demoCanvas.closePath(); demoCanvas.beginPath(); var radialGrad = demoCanvas.createRadialGradient(50,50,10,50,50,40); radialGrad.addColorStop(0, "blue"); radialGrad.addColorStop(.5, "green"); radialGrad.addColorStop(1, "red"); demoCanvas.fillStyle=radialGrad; demoCanvas.arc(60,60,30,0,2*Math.PI,false); demoCanvas.fill();

Direct pixel manipulation & Images

The ImageData object can be used to manipulate individual pixels. It has three properties:

width : The width of the image data in device-display pixels.
height : The height of the image data in device-display pixels.
data : This is a Uint8ClampedArray (MDN doc here) which contains the individual pixel data in a series of (R,G,B,A) bytes for the top-most pixel to the bottom-right pixel. So the nth pixel’s red value would be at data[y*width+x] , green would be at data[y*width+x+1] , blue would be at data[y*width+x+2] , and the alpha would be at data[y*width+x+3] .

NOTE: A RGBA value can be used to represent a color — where R,G,B are the amounts of red, green, and blue and A is the opacity (alpha value). In the Canvas, these elements can have any integer value in [0, 255].

You can get a ImageData object with the following methods in the Canvas API:

createImageData(sw,sh) : This creates an ImageData object of width and height sw and sh , defined in CSS pixels. All the pixels will be initialized to transparent black (hex R,G,B=0, and also A=0).

CSS pixels might map to a different number of actual device pixels exposed by the object itself

createImageData(data) : Copies the given image-data and returns the copy.
getImageData(sx,sy,sw,sh) : Returns a copy of the canvas’s pixels in the rectangle formed by sx,sy,sw,sh in a ImageData object. Pixels outside the canvas are set to transparent black.
putImageData(imagedata,dx,dy,dirtyX,dirtyY,dirtyWidth,dirtyHeight): (The last four ‘dirty’ arguments are optional). Copies the pixel values in imagedata into the canvas rectangle at dx,dy . If you provide the last four arguments, it will only copy the dirty pixels in the image data (the rectangle formed at dirtyX,dirtyY of dimensions dirtyWidth*dirtyHeight ). Not passing the last four arguments is the same as calling putImageData(imagedata,dx,dy,0,0,imagedata.width,imagedata.height).

For all integer values of x and y where dirtyX ≤ x < dirtyX+dirtyWidth and dirtyY ≤ y < dirtyY+dirtyHeight, copy the four channels of the pixel with coordinate (x, y) in the imagedata data structure to the pixel with coordinate (dx+x, dy+y) in the underlying pixel data of the canvas.

Example 1.8:

I’ve filled the whole 400x400 canvas with random colors (fully opaque) using the getImageData/putImageData methods.

Note that using beginPath/closePath isn’t necessary to use the ImageData API — because your not using the Canvas API to form shapes/curves.

/* replace this line with demoCanvas.createImageData(390,390) instead. */ var rectData = demoCanvas.getImageData(10, 10, 390, 390); for (var y=0; y<390; y++) { for (var x=0; x<390; x++) { const offset = 4*(y*390+x);// 4* because each pixel is 4 bytes rectData.data[offset] = Math.floor(Math.random() * 256);// red rectData.data[offset+1] = Math.floor(Math.random() * 256);// green rectData.data[offset+2] = Math.floor(Math.random() * 256);// blue rectData.data[offset+3] = 255;// alpha, fully opaque } } demoCanvas.putImageData(rectData, 10, 10); /* beginPath/closePath aren't required for this code */

Images can be drawn onto the canvas directly. The drawImage can be used in three different ways to do so. It requires a CanvasImageSource as the pixel source.

A CanvasImageSource can be one of the following — HTMLImageElement, HTMLCanvasElement, HTMLVideoElement. To copy into the canvas, you can use a . You could also copy an existing canvas or the screenshot of a video!!!

drawImage(image,dx,dy) : Copies the image-source into the canvas at (dx,dy). The whole image is copied.
drawImage(image,dx,dy,dw,dh) : Copies the image-source into the rectangle in the canvas at (dx,dy) of size (dw,dh). It will be scaled down or scaled up if necessary.
drawImage(image,sx,sy,sw,sh,dx,dy,dw,dh) : Copies the rectangle in the image source sx,sy,sw,sh into the rectangle in the canvas dx,dy,dw,dhand scales up or down if required. However, if the rectangle sx,sy,sw,shhas parts outside the actual source — then the source rectangle is clipped to include the inbound parts and the destination rectangle is clipped in the same proportion; however, you shouldn’t pass any out-of-bounds rectangle — keep it simple, stupid.

Example 1.9:

var image = document.getElementById('game-img'); demoCanvas.drawImage(image, 50, 50, 200, 200, 100, 100, 200, 200); /* beginPath/closePath aren't required for this code */

NOTE: Add this to your HTML —

Transformations

Now we’re getting to the exciting parts of the Canvas API!!!

The Canvas uses a transformation matrix to transform the input (x, y) coordinates into the displayed (x, y) coordinates. Note that pixels drawn before the transformation are not transformed — they are untouched. Only stuff drawn after applying the transformation will be changed.

There are three in-built transformation methods:

scale(xf,yf) : This method scales the input by xf in the horizontal direction and yf in the vertical direction. If you want to magnify an image by a factor of m , then pass xf=yf=m . To stretch/squeeze an image horizontally by m , xf=m,yf=1 . To stretch/squeeze an image vertically by m , xf=1,yf=m .
rotate(angle) : Rotates the input by an angle of angle in the clockwise direction, in radians.
translate(dx,dy) : Shifts the input by dx,dy .

Example 2.0:

var image = document.getElementById('game-img'); demoCanvas.drawImage(image, 0, 0, 400, 400); demoCanvas.rotate(Math.PI / 6); demoCanvas.scale(2, 2); demoCanvas.translate(10, 10); demoCanvas.drawImage(image, 0, 0, 400, 400);

In Example 2.0, notice how the original image is intact. Only the second image (overlay) is transformed by three methods — rotate, scale, transform.

To revert all transformations:

demoCanvas.setTransform(1, 0, 0, 0, 0, 1); // sets the transform to the identity matrix

NOTE:

Changing the order of transformation can affect the final result.
For advanced users, you may want to look at the transform and setTransform methods. This will let you set the 3D transformation matrix directly.
getImageData and putImageData are not affected by the transform. That means if you draw a black rectangle using putImageData , it won’t be transformed (rotated/scaled/translated).
As changing the transform only works for drawings done after applying it, you can’t scale/rotate/translate the existing canvas directly (nor does getImageData and then putImageData work). You may have to create another hidden canvas of the same size — and then copy the image-data into the 2nd canvas, then use drawImage on the 2nd canvas.
Check this example: //canvasdemo2d.github.io/ (source: //github.com/canvasdemo2d/canvasdemo2d.github.io). Move your cursor over the canvas and see what it does. It won’t work on mobile phones, unfortunately. The cascading effect is due to the fact that I am translating the canvas w.r.t mouse using drawImage . drawImagethen writes to the same canvas it’s reading from, which causes the repeating pattern!

Hit Regions

As of the time of writing (March 2019), support for hit regions is experimental in Chrome and on Firefox. Mobile browser don’t even support it at all. Hence, I will explain to you “what” could hit regions be used for.

Hit regions are used to catch pointer events on the canvas and know “where” the user clicked. For example, you could have two rectangles A & B — when the user clicks A, you want to perform action $A and when the user clicks B, you want to perform action $B. Let’s walk through the whole process!

A hit region is related to these three things:

Path: The current path when the hit region was created (for example, a rectangle). All pointer events inside the path are routed to that hit region.
Id: An unique id string to identify the hit region by the event handler.
Control: An alternative DOM element ( HTMLButtonElement , for example) that gets the pointer events instead.

NOTE: The path is automatically provided by the canvas when adding a new hit region. Only one — id or control — is needed to form a hit region.

Methods for manipulating the hit-region list of a canvas are:

addHitRegion(options) : Takes a HitRegionOptions object and forms a hit-region enclosed by the current path. The options argument should be a string id property or a HTMLElementcontrol property.
removeHitRegion(id) : Removes the hit region with the id id so that it no longer receives any pointer events.
clearHitRegions() : Removes all hit regions.

demoCanvas.fillStyle = 'red'; demoCanvas.rect(10,10,60,60); demoCanvas.fill();// first rectangle demoCanvas.addHitRegion({ id: 'btn1' }); demoCanvas.fillStyle = 'blue'; demoCanvas.rect(10,110,60,60); demoCanvas.fill(); demoCanvas.addHitRegion({ id: 'btn2' }); document.getElementById('demo-canvas').onpointerdown = function(evt) { // demoCanvas is the 2d context, not the HTMLCanvasElement console.log('Hello id: ' + evt.region);// region is hitregion id } // This code might not work due to this being an // unsupported (new) feature of HTML5.

NOTE: Hit regions aren’t supported — but that doesn’t mean you have to use them to capture pointer events. You could create your “own hit-region list” and representations of boundaries of regions (cause you can’t get the current path from the canvas, too bad). In the document.getElementById('demo-canvas').onpointerdown method, get the current clientX,clientY properties and search through the hit region list. Based on the hit region that contains the point, you can perform the intended action.

States and the clip() method

State saving is a convenience provided by the W3C specification. You can save the current state of a canvas and restore it later.

You could also build such a system (partially) by writing your own JavaScript model. But you would have to save a quite of stuff: transformation matrix, hit-region list, style properties, and so on. Furthermore, you cannot revert the clipping area (we’ll get to the clipmethod in some time) directly.

NOTE: The save / restore methods do not save & restore the actual drawing/pixels. They only save other properties.

Hence, I would recommend heavily using the save & restore methods to go back and forth instead of erasing stuff on your own or making your own state-saving mechanism.

The CanvasRendering2DContext object has an associated state stack. The save method will push the current canvas state onto that stack, while the restore method will pop the latest state from the stack.

The Clipping Region

The clipping region is a specific region in which all drawings are to be done. Obviously, by default, the clipping region is the rectangle is the whole canvas. But you may want to draw in a specific region instead of the whole thing. For example, you may want to draw the lower half of a star formed by multiple lineTo methods.

So, for example, let’s say you know how to draw a star in the canvas. It touches all sides of the canvas. But now you want to only display the lower half of the star. In this scenario, you would:

Save the state of the canvas
Clip the lower half region
Draw your star (as if on the whole canvas)
Restore the canvas state

To clip a region, you have to call the clip() method which does the following:

The clip() method must create a new clipping region by calculating the intersection of the current clipping region and the area described by the path, using the non-zero winding number rule. Open subpaths must be implicitly closed when computing the clipping region, without affecting the actual subpaths. The new clipping region replaces the current clipping region.

When the context is initialized, the clipping region must be set to the rectangle with the top left corner at (0,0) and the width and height of the coordinate space.

— W3C Documentation for Canvas 2D Context

demoCanvas.save(); demoCanvas.rect(0, 200, 400, 200);// lower-half rectangle subpath demoCanvas.clip(); /* star drawing method */ demoCanvas.restore();

That’s all for now. I will write an article on animations with the canvas and how to write a custom interface completely on the canvas.