Úplný prehľad o HTML Canvas

Musíte si prečítať predtým, ako so štítkom na plátne niečo urobíte, aj keď to už viete.

Prehľad

Element plátna HTML sa používa na kreslenie „rastrovej“ grafiky do webovej aplikácie. Canvas API poskytuje dva kontexty kreslenia: 2D a 3D a v tejto príručke si povieme niečo o 2D (pre zjednodušenie ho budem ďalej odkazovať na Canvas API).

Než začnem, chcem, aby ste poznali jeden veľmi dôležitý bod. Canvas je rozhranie API pre rastrovú grafiku - manipulujete s vecami na úrovni pixelov. To znamená, že základný softvér nepozná model, ktorý používate na zobrazenie kontextu - nevie, či kreslíte obdĺžnik alebo kruh.

Rozdelil som Canvas API do samostatných častí, aby ste pohltili jeden po druhom:

  • Cesta API
  • Kreslenie štýlov
  • Prechody a vzory
  • Priama manipulácia s pixelmi a obrázky
  • Premeny
  • Zasiahnite regióny
  • Stav a metóda clip ()

Nastaviť

Ak chcete spustiť výukový program Canvas, vytvorte súbor HTML a súbor JS, ktorý je s ním prepojený.

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

Vo vašom canvas-demo.jssúbore

// 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 */ }

Cesty

Cesty sú zbierkou bodov v mriežke 2D pixelov na plátne. Sú nakreslené pomocou tohto API. Každý tvar v ceste, ktorú nakreslíte, sa v dokumentácii W3C nazýva „čiastková cesta“.

  • beginPath()a closePath(): Všetky tvary, ktoré nakreslíte, sa pridajú do aktuálnej cesty. Ak zavoláte strokealebo fillneskôr, použije sa to na všetky tvary, ktoré ste nakreslili na aktuálnej ceste. Aby ste tomu zabránili, rozdelíte kresbu tak, že zavoláte beginPatha 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) : Znamená konštrukciu nového tvaru, ktorý začína v bode (x, y).
  • lineTo(x,y): Nakreslí čiaru od posledného bodu aktuálneho tvaru po odovzdaný bod. Ak nebol vytvorený žiadny tvar (via moveTo), vytvorí sa nový od (x, y) (rovnako ako moveTo).
  • quadraticCurveTo(cpx1,cpy1,x,y)a bezierCurveTo(cpx1,cpy1,cpx2,cpy2,x,y): Nakreslí kvadratickú / kubickú Bézierovu krivku začínajúcu od posledného bodu tvaru, prechádzajúcu kontrolnými bodmi ( cpx1,cpy1a cpx2,cpy2) a končiacu bodom x,y. Bézierova krivka je iba „hladká“ krivka, ktorá prechádza strednými „kontrolnými“ bodmi s danými koncovými bodmi. Upozorňujeme, že krivka nemusí prechádzať presne cez kontrolné body - je možné ju vyhladiť.
  • arcTo(x1,y1,x2,y2,radius): Toto je mierne komplikovaná metóda. Predpokladajme, že aktuálny bod v ceste je x0,y0- potom arcTonakreslí oblúk, ktorý má dve dotyčnice spájajúce tieto dva páry bodov (x1,y1) & (x0,y0)a (x1,y1) & (x2,y2). Polomer oblúka bude daný. Čím väčší je polomer, tým ďalej bude oblúk ďalej x1,y1, (vizuálna jasnosť je uvedená v príklade 1.2). Ak ste ich ešte nepoužili moveTo, x0,y0predvolene budú 0,0.
  • arc(x,y,radius,startAngle,endAngle,counterclockwise): Pripojí aktuálny bod v ceste (predvolene 0,0) k začiatku oblúka. Kreslí oblúk od stredu x,ypolomeru radius, od startAngledo endAngle. (Poznámka: Na rozdiel od matematiky perom a papierom sú uhly v Canvas API popísané v smere hodinových ručičiek); ale za štyroch špeciálnych podmienok - (x0,y0)rovná sa (x1,y1), (x1,y1)rovná sa (x2,y2), (x0,y0),(x1,y1),(x2,y2)je kolineárna, alebo ak radiusje nula, potom bude volanie na arcekvivalentné lineTo(x1,y1)a namiesto toho sa nakreslí čiara.
  • rect(x,y,w,h): Nakreslí obdĺžnik s ľavým horným rohom x,ya so šírkou wa výškou h.

Príklad 1.1:

Teraz musíme vyskúšať ukážku - nakreslíme niekoľko náhodných vodorovných čiar a potom náčrt oka. Výsledok bude vyzerať ako niečo vľavo. Nezabudnite si prejsť kód a pohrať sa s kódom.

/* 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)

Príklad 1.2:

V príklade nižšie vytvorím kubickú krivku (s vizuálnymi pokynmi), arcTohovory vpravo hore a pack-man s arc()vľavo dole. Kontrolné body (v kubickej krivke) sú rohy tvorené tromi vodidlami.

(x1,y1)lebo arcToje roh tvorený dvoma dotyčnicami.

// 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();

Štýly kreslenia

Doteraz sme kreslili jednoduché tenké cestičky. Štýly kreslenia nám pomôžu vylepšiť našu kresbu.

Upozorňujeme, že na jednu cestu nemôžete použiť dva rôzne štýly. Napríklad, ak chcete nakresliť červenú a modrú čiaru - budete musieť vytvoriť novú cestu, aby ste nakreslili modrú čiaru. Ak nevytvoríte novú cestu, potom pri strokedruhom volaní po nastavení farby štýlu zobrazenia na modrú farbu budú oba riadky sfarbené do modra. Preto sa štýly používajú na všetky podcesty, bez ohľadu na to, či už boli alebo neboli pohladené.

Na demoCanvastento účel je definovaných niekoľko vlastností 2D kontextového objektu :

  • lineWidth: Hrúbka nakreslených čiar. Predvolene je to 1; teda dva vyššie uvedené príklady používali obrys hrubý 1 pixel.
  • lineCap: Toto je čiapočka použitá na koncoch podchodov (tvarov). Je to reťazec a môže mať tri platné hodnoty: „zadok“, „okrúhly“, „štvorec“ (vizuálna prehľadnosť je uvedená v príklade 1.3). Zadok bude ukončovať čiary bez čiapky - výsledkom budú pevné, ortogonálne konce ako tenké obdĺžniky. „Guľatý“ pridáva na konce polkruh, aby poskytoval hladké konce. „Štvorec“ pridáva na koniec štvorec, ale vyzerá to ako „zadok“. „Okrúhle“ a „štvorcové“ pridávajú každej podceste trochu zvláštnej dĺžky.
  • lineJoin: Toto rozhodne o tom, ako sú spojené dve prekrývajúce sa čiary. Napríklad ak chcete vytvoriť šípku vpravo (>), môžete pomocou tejto vlastnosti zmeniť spôsob, akým je roh tvorený. Má tri platné hodnoty: „guľatý“, „skosený“ a „pokosový“. Skontrolujte príklad 1.4, ako menia rohy. (Predvolená hodnota je „mitre“). „Okrúhle“ budú vytvárať kruhové rohy, zatiaľ čo „skosenie“ vytvorí tvrdé trojstranné rohy a „pokos“ ostré hrany.
  • miterLimit: Kedy lineJoin="miter", toto rozhodne o maximálnej vzdialenosti medzi vnútorným a vonkajším rohom čiary. Vizuálna jasnosť je uvedená v príklade 1.4 (b). Ak je limit pokosu príliš vysoký, potom môžu mať ostré šípky veľkú spoločnú oblasť medzi týmito dvoma čiarami. Ak je prekročený limit pokosu, displej sa vráti späť do spojenia so skosením.

Príklady 1.3 a 1.4:

In Example 1.3 on the left, you can see how the round & square line-capped lines are longer than the default capping. (NOTE: The thicker the line, the greater the increase in length)

In Example 1.4(a), you can see how round and bevel joins work. The lines created are identical in the upper and lower parts. Only the lineJoinproperties are different.

In Example 4.1(b), you can see how a mitered join works, and what happens if the mitered length is passed.

Additional display style properties are defined:

  • font : This string defines how you want to style text. For example, demoCanvas.font="10px Times New Roman" is a valid font value.
  • textAlign : The valid values are — “start”, “end”, “left”, “right”, and “center”. The default is “start”.
  • textBaseline : The valid values are — “top”, “hanging”, “middle”, “alphabetic”, “ideographic”, “bottom”. The default is “alphabetic”.

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:

  1. Save the state of the canvas
  2. Clip the lower half region
  3. Draw your star (as if on the whole canvas)
  4. 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.

Further reading:

  • How to use Firebase for building Android multiplayer games
  • Ako synchronizovať vašu hernú aplikáciu s viacerými zariadeniami so systémom Android
  • Kruhové závislosti v JavaScripte

Shukant Pal je tvorcom jadra Silcos. Je zanieteným študentom a teraz trénuje pokročilý vývoj webových aplikácií. Má praktické skúsenosti s Reactom a jeho ekosystémom.

Všetky citácie sú prevzaté z dokumentov W3C pre Canvas 2D Context.

Hej, som Shukant Pal. Vo svojom voľnom čase vyvíjam veľa webových aplikácií. Sledujte ma na sociálnych sieťach.