Una lectura obligada antes de hacer cualquier cosa con la etiqueta de lienzo, incluso si ya la conoce.

Visión general

El elemento de lienzo HTML se utiliza para dibujar gráficos "rasterizados" en una aplicación web. Canvas API proporciona dos contextos de dibujo: 2D y 3D, y en esta guía, hablaremos sobre el 2D (que lo referiré a Canvas API para simplificar).

Antes de comenzar, quiero que sepa un punto muy importante. Canvas es una API de gráficos de trama: manipula cosas a nivel de píxel. Eso significa que el software subyacente no conoce el modelo que está utilizando para mostrar su contexto; no sabe si está dibujando un rectángulo o un círculo.

He dividido la API de Canvas en trozos separados, para que pueda engullir uno por uno:

API de ruta
Estilos de dibujo
Gradientes y patrones
Manipulación directa de píxeles e imágenes
Transformaciones
Regiones de golpe
Estado y el método clip ()

Preparar

Para iniciar su tutorial de Canvas, cree un archivo HTML y un archivo JS vinculado a él.

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

En su canvas-demo.jsarchivo,

// 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 */ }

Caminos

Las rutas son una colección de puntos en la cuadrícula de píxeles 2D en el lienzo. Se dibujan con la ayuda de esta API. Cada forma en una ruta que dibuje se denomina "subruta" en la documentación del W3C.

beginPath()y closePath(): todas las formas que dibuja se agregan a la ruta actual. Si llama strokeo fillmás adelante, se aplicará a todas las formas que ha dibujado en la ruta actual. Para evitarlo, divide tu dibujo llamando beginPathy 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) : Significa la construcción de una nueva forma que comienza en el punto (x, y).
lineTo(x,y): Dibuja una línea desde el último punto de la forma actual hasta el punto pasado. Si no se creó ninguna forma (vía moveTo), se crea una nueva comenzando en (x, y) (como moveTo).
quadraticCurveTo(cpx1,cpy1,x,y)y bezierCurveTo(cpx1,cpy1,cpx2,cpy2,x,y): Dibuja una curva Bézier cuadrática / cúbica comenzando desde el último punto de la forma, pasando por los puntos de control ( cpx1,cpy1y cpx2,cpy2) y terminando en x,y. Una curva de Bezier es simplemente una curva "suave" que pasa por puntos intermedios de "control" con puntos finales determinados. Tenga en cuenta que la curva no tiene que pasar exactamente a través de los puntos de control, se puede suavizar.
arcTo(x1,y1,x2,y2,radius): Este es un método un poco complicado de usar. Suponga que el punto actual en la ruta es x0,y0- luego arcTodibujará un arco que tiene dos tangentes que conectan estos dos pares de puntos (x1,y1) & (x0,y0)y (x1,y1) & (x2,y2). El radio del arco será el dado. Cuanto mayor sea el radio, más lejos estará el arco x1,y1(consulte el Ejemplo 1.2 para obtener claridad visual). Si aún no lo moveToha utilizado , lo x0,y0será de forma predeterminada 0,0.
arc(x,y,radius,startAngle,endAngle,counterclockwise): Conecta el punto actual en la ruta (por defecto 0,0) al comienzo del arco. Dibuja el arco desde el centro x,ydel radio radius, desde startAnglehasta endAngle. (Nota: a diferencia de las matemáticas con lápiz y papel, los ángulos se describen en el sentido de las agujas del reloj en la API de Canvas); pero en cuatro condiciones especiales: (x0,y0)iguales (x1,y1), (x1,y1)iguales (x2,y2), (x0,y0),(x1,y1),(x2,y2)son colineales, o si radiuses cero, la llamada a arcserá equivalente a lineTo(x1,y1)y se dibujará una línea en su lugar.
rect(x,y,w,h): Dibuja un rectángulo con la esquina superior izquierda x,yy de ancho wy alto h.

Ejemplo 1.1:

Ahora tenemos que probar una demostración: vamos a dibujar algunas líneas horizontales aleatorias y luego un boceto de un ojo. El resultado se verá como algo a la izquierda. No olvide revisar el código y jugar con el código.

/* 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)

Ejemplo 1.2:

En el siguiente ejemplo, creo una curva cúbica (con pautas visuales), arcTollamadas en el centro a la derecha y un pack-man arc()en la parte inferior izquierda. Los puntos de control (en la curva cúbica) son las esquinas que forman las tres líneas guía.

(x1,y1)porque arcToes la esquina formada por las dos tangentes.

// 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();

Estilos de dibujo

Hasta ahora, hemos estado dibujando senderos sencillos y delgados. Los estilos de dibujo nos ayudarán a mejorar nuestro dibujo.

Tenga en cuenta que no puede aplicar dos estilos diferentes en la misma ruta. Por ejemplo, si desea dibujar una línea roja y una línea azul, tendrá que crear una nueva ruta para dibujar la azul. Si no crea una nueva ruta, al llamar por strokesegunda vez después de configurar el color de su estilo de visualización en azul, ambas líneas se colorearán en azul. Por tanto, los estilos se aplican a todos los subtrayectos, independientemente de que ya hayan sido trazados o no.

Algunas propiedades del objeto de contexto 2D demoCanvasse definen para este propósito:

lineWidth: El grosor de las líneas que se dibujan. Por defecto, es 1; por lo tanto, los dos ejemplos anteriores utilizaron un contorno de 1 píxel de grosor.
lineCap: Este es el límite que se aplica al final de los subtrayectos (formas). Es una cadena y puede tener tres valores válidos: "tope", "redondo", "cuadrado" (consulte el Ejemplo 1.3 para mayor claridad visual). “Butt” terminará las líneas sin tapa, lo que dará como resultado extremos rígidos y ortogonales como rectángulos delgados. “Redondo” agrega un semicírculo a los extremos para dar extremos suaves. "Cuadrado" añade un cuadrado al final, pero parece "trasero". "Redondo" y "cuadrado" añaden un poco de longitud adicional a cada subtrayecto.
lineJoin: Esto decide cómo se unen dos líneas superpuestas. Por ejemplo, si desea crear una flecha hacia la derecha (>), puede cambiar cómo se forma la esquina con esta propiedad. Tiene tres valores válidos: "redondo", "bisel" y "inglete". Consulte el Ejemplo 1.4 para ver cómo cambian las esquinas. (El valor predeterminado es "inglete"). "Redondo" formará esquinas circulares, mientras que "bisel" creará esquinas rígidas de tres lados y "inglete" formará un borde afilado.
miterLimit: Cuando lineJoin="miter", esto decide la distancia máxima entre las esquinas interior y exterior de la línea. Consulte el Ejemplo 1.4 (b) para obtener claridad visual. Si el límite de inglete es demasiado alto, las flechas afiladas pueden tener un área común grande entre las dos líneas. Si se supera el límite de inglete, la pantalla retrocede en una unión en bisel.

Ejemplo 1.3 y 1.4:

En el ejemplo 1.3 de la izquierda, puede ver cómo las líneas redondeadas y cuadradas con límite de línea son más largas que el límite predeterminado. (NOTA: Cuanto más gruesa sea la línea, mayor será el aumento de longitud)

En el ejemplo 1.4 (a), puede ver cómo funcionan las uniones redondas y biseladas. Las líneas creadas son idénticas en las partes superior e inferior. Solo las lineJoinpropiedades son diferentes.

En el ejemplo 4.1 (b), puede ver cómo funciona una unión a inglete y qué sucede si se pasa la longitud a inglete.

Se definen propiedades de estilo de visualización adicionales:

font: Esta cadena define cómo desea aplicar el estilo al texto. Por ejemplo, demoCanvas.font="10px Times New Roman"es un valor de fuente válido.
textAlign: Los valores válidos son - "inicio", "final", "izquierda", "derecha" y "centro". El valor predeterminado es "inicio".
textBaseline: Los valores válidos son - "superior", "pendiente", "medio", "alfabético", "ideográfico", "inferior". El valor predeterminado es "alfabético".

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.