HTML 5_Canvas
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HTML 5 Canvas
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Transcript of HTML 5_Canvas
HTML 5Canvas
Basic usage Drawing shapes Using images Applying styles and colors Transformations Compositing Basic animations Optimizing the canvas
supported in Firefox 1.5 and later, Opera 9 and later, newer versions of Safari, Chrome, and Internet Explorer 9.
<canvas id="tutorial" width="150" height="150"></canvas>
This looks a lot like the <img> element, the only difference is that it doesn't have the src and alt attributes. The <canvas> element has only two attributes - width andheight. These are both optional and can also be set using DOM properties. When no width and height attributes are specified, the canvas will initially be 300 pixelswide and 150 pixels high.
Fallback content<canvas id="stockGraph" width="150" height="150"> current stock price: $3.15 +0.15 </canvas> <canvas id="clock" width="150" height="150"> <img src="images/clock.png" width="150" height="150" alt=""/> </canvas>
The rendering context
2D rendering context. 3D context ("experimental-webgl") based on
OpenGL ES.
var canvas = document.getElementById('tutorial'); var ctx = canvas.getContext('2d');
Checking for support
var canvas = document.getElementById('tutorial'); if (canvas.getContext){ var ctx = canvas.getContext('2d'); // drawing code here } else { // canvas-unsupported code here }
A skeleton template<html> <head> <title>Canvas tutorial</title> <script type="text/javascript"> function draw(){ var canvas = document.getElementById('tutorial'); if (canvas.getContext){ var ctx = canvas.getContext('2d'); } } </script> <style type="text/css"> canvas { border: 1px solid black; } </style> </head> <body onload="draw();"> <canvas id="tutorial" width="150" height="150"></canvas> </body> </html>
Drawing shapes The grid Top left is 0,0 canvas only supports one primitive shape - rectangles. All other shapes must be
created by combining one or more paths fillRect(x,y,width,height) : Draws a filled rectangle
strokeRect(x,y,width,height) : Draws a rectangular outlineclearRect(x,y,width,height) : Clears the specified area and makes it fully transparent
Drawing paths beginPath()closePath()stroke()fill()moveTo(x, y)lineTo(x, y)arc(x, y, radius, startAngle, endAngle, anticlockwise)
Bezier and quadratic curvesquadraticCurveTo(cp1x, cp1y, x, y)bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)
Rectanglesrect(x, y, width, height)
Using images Importing images
Importing images is basically a two step process: Firstly we need a reference to a JavaScript Image object or
other canvas element as a source. It isn't possible to use images by simply providing a URL/path to them.
Secondly we draw the image on the canvas using the drawImage function.
drawImage drawImage(image, x, y, width, height) var img = new Image(); // Create new img element img.src = 'myImage.png'; // Set source path
Slicing drawImage(image, sx, sy, sWidth, sHeight, dx, dy,
dWidth, dHeight)
function draw() { var ctx = document.getElementById('canvas').getContext('2d'); var img = new Image(); img.onload = function(){ ctx.drawImage(img,0,0); ctx.beginPath(); ctx.moveTo(30,96); ctx.lineTo(70,66); ctx.lineTo(103,76); ctx.lineTo(170,15); ctx.stroke(); }; img.src = 'images/img.png'; }
Applying styles and colors fillStyle = color
strokeStyle = color ctx.fillStyle = "orange"; ctx.fillStyle = "#FFA500"; ctx.fillStyle = "rgb(255,165,0)";
Transparency globalAlpha = transparency value
ctx.fillStyle = "rgba(255,165,0,1)"; ctx.globalAlpha = 0.2;
Line styles lineWidth = value
lineCap = type ('butt','round','square')lineJoin = type ['round','bevel','miter']miterLimit = value
Gradients createLinearGradient(x1,y1,x2,y2)
createRadialGradient(x1,y1,r1,x2,y2,r2) var lineargradient = ctx.createLinearGradient(0,0,150,15
0); var radialgradient = ctx.createRadialGradient(75,75,0,75,
75,100); addColorStop(position, color)
Patterns createPattern(image,type) Type [repeat, repeat-x, repeat-y and no-repeat.]
var img = new Image(); img.src = 'someimage.png'; var ptrn = ctx.createPattern(img,'repeat');
Shadows shadowOffsetX = float
shadowOffsetY = floatshadowBlur = floatshadowColor = color
Transformations save()
restore() Translating
translate(x, y) Rotating
rotate(angle) Scaling
scale(x, y) Transforms
transform(m11, m12, m21, m22, dx, dy)
Compositing globalCompositeOperation
source-over (default) source-in source-out source-atop Lighter Xor destination-over destination-in destination-out destination-atop darker Copy
Clipping paths Clip()
Basic animationsBasic animation steps
These are the steps you need to take to draw a frame: Clear the canvas
Unless the shapes you'll be drawing fill the complete canvas (for instance a backdrop image), you need to clear any shapes that have been drawn previously. The easiest way to do this is using the clearRect method.
Save the canvas stateIf you're changing any setting (styles, transformations, etc) which affect the canvas state and want to make sure the original state is used each time a frame is drawn, you need to save it.
Draw animated shapesThe step where you do the actual frame rendering.
Restore the canvas stateIf you've saved the state, restore it before drawing a new frame.
Basic animations setInterval(animateShape,500); setTimeout(animateShape,500);
The new method requestAnimationFrame() allows modern browsers to stop
drawing graphics when a tab or window is not visible. Improving overall performance and batteries on mobile devices.
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