HTML5 Canvas深度解析：打造互动式Web应用

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"《HTML5 Canvas》是O'Reilly出版社出版的一本关于HTML5画布技术的高清完整教程，由Steve Fulton和Jeff Fulton撰写。本书深入介绍了如何利用HTML5 Canvas进行网页上的图形绘制、文本渲染、图像处理和动画制作，特别适合希望开发交互式Web游戏的读者。" 在HTML5中，Canvas是一个强大的绘图API，它允许开发者通过JavaScript代码直接在网页上绘制图形。这本书详细讲解了以下关键知识点： 1. **Canvas基本概念**：介绍Canvas元素的使用方法，包括如何在HTML中声明Canvas元素，以及如何通过JavaScript获取Canvas画布的上下文对象（通常为2D渲染上下文）。 2. **绘图基础**：讲解了线条绘制、曲线路径、填充和描边规则，以及如何使用颜色和渐变来增强图形的视觉效果。 3. **形状与路径**：详细阐述如何绘制矩形、圆形、弧线等基本形状，以及如何创建和操作复杂的路径，实现自定义的几何形状。 4. **文本渲染**：讨论了在Canvas上动态生成和渲染文本的技术，包括字体选择、大小调整、对齐方式以及文本测量等。 5. **图像处理**：介绍如何加载、显示和操作图像，包括裁剪、缩放、旋转和组合图像，以及使用Canvas进行简单的图像滤镜效果。 6. **动画制作**：探讨了基于帧的动画、时间驱动动画以及物理模拟等动画技术，帮助读者创建流畅的交互体验。 7. **事件与交互**：讲解如何监听用户与Canvas的交互，如点击、拖动等，以及如何响应这些事件进行动态更新。 8. **性能优化**：提供了一些最佳实践，如避免不必要的重绘、使用局部更新和图像数据缓冲等，以提高Canvas应用的性能。 9. **Web游戏开发**：结合实际案例，阐述如何利用Canvas构建互动游戏，包括游戏逻辑、碰撞检测和用户输入处理等。 10. **其他高级特性**：可能涵盖了WebGL（3D图形渲染）、OffscreenCanvas（后台渲染）以及Web Workers（并行计算）等更进阶的话题，以扩展Canvas的应用范围。本书对于希望提升网页交互性和动态性的开发者来说是一份宝贵的资源，无论你是初学者还是有经验的前端工程师，都能从中获得实用的知识和技巧。通过阅读和实践，你将能够充分利用HTML5 Canvas创造出引人入胜的网页内容和互动体验。

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xviii | Preface

Acknowledgments

First, Steve would like to thank his beautiful wife, Dawn, for the amazing patience,

guidance, and support she lovingly provided before, during, and after this book was

written. Steve would also like to thank his girls—Rachel, Daphnie, and Katie—for all

their enthusiastic support and for not getting too frustrated every time they asked him

to play and Daddy said, “Sure, yeah, in just a couple minutes” because his head was

buried in these pages. He’d also like to thank his mom and dad, plus his sisters, Mari

and Carol for everything they taught us; and his uncle Richard and cousin John for all

their love and support. Also thanks to Sue, Morgan, and Lauren Miller; Jen, Eric, Sarah,

and Paige Garnica; Dietrich; Chantal Martin; and Ryan and Justin Fulton.

Jeff would like to thank his amazing wife, Jeanne, and his two wonderful boys, Ryan

and Justin, for putting up with him writing this second book in two years. The writing

process is a time- and energy-consuming endeavor that demands patience and under-

standing from those in close proximity to the temperamental author. Jeff would also

like to thank his mom and dad, as well as sisters Mari and Carol, for the morale and

babysitting support that was needed during the crucial writing times. Also, special

thanks to the Perry and Backlar clans for all of their love and support.

The authors would also like to acknowledge all the fine people at O’Reilly, especially

Mike Loukides, who took the chance on us for this book; and Simon St.Laurent, who

led us out of the wilderness; our copyeditor, Marlowe Shaeffer, who made the text

sparkle in her own special way; and our production editor, Kristen Borg, for finishing

the job.

We’d also like to thank our technical reviewers, Raffaele Cecco, Shelley Powers, and

Andres Pagella.

Thanks to everyone at Electrotank, especially Jobe Makar, Matthew Weisner, and

Teresa Carrigan; as well as our friends at Jett Morgan, Creative Bottle, Producto Stu-

dios, Mattel, Mochi, Adobe, Microsoft, Zynga, The SPIL Group, Giles Thomas from

Learningwebgl.com, Ari Feldman, and Terry Paton, plus Ace The Super Villain, Bas

Alicante, egdcltd, Tony Fernando, SeuJogo, Hayes, Jose Garay, Richard Davey

(@PhotonStorm), Squize and nGfx (@GamingYourWay), and all our other friends at

8bitrocket.com. We would also like to give a huge shout out to the simply outstanding

team at 444 Deharo, especially the entire FV team and the Foo Fighters pod!

Finally, we'd like to thank all of our friends who helped along the way, including Ian

Legler, Brandon Crist, Eric Barth, Wesley Crews, Kenny Brown, Mike Foti, Evan

Pershing, Scott Johnson, Scott Lunceford, Kurt Legler, Ryan Legler, John Little, Matt

Hyatt, Varun Tandon, Mark Hall, Jason Neifeld, Mike Peters and The Alarm.

…and, last but not least, thanks to the W3C for coming up with such a kickass spec for

HTML5 Canvas.

Preface | xix

CHAPTER 1

Introduction to HTML5 Canvas

HTML5 is the current iteration of HTML, the HyperText Markup Language. HTML

was first standardized in 1993, and it was the fuel that ignited the World Wide Web.

HTML is a way to define the contents of a web page using tags that appear within pointy

brackets, < >.

HTML5 Canvas is an immediate mode bitmapped area of the screen that can be ma-

nipulated with JavaScript. Immediate mode refers to the way the canvas renders pixels

on the screen. HTML5 Canvas completely redraws the bitmapped screen on every

frame using Canvas API calls from JavaScript. As a programmer, your job is to set up

the screen display before each frame is rendered so that the correct pixels will be shown.

This makes HTML5 Canvas very different from Flash, Silverlight, or SVG, which op-

erate in retained mode. In this mode, a display list of objects is kept by the graphics

renderer, and objects are displayed on the screen according to attributes set in code

(i.e., the x position, y position, and alpha transparency of an object). This keeps the

programmer away from low-level operations, but gives her less control over the final

rendering of the bitmapped screen.

The basic HTML5 Canvas API includes a 2D context that allows a programmer to draw

various shapes, render text, and display images directly onto a defined area of the

browser window. You can apply colors; rotations; alpha transparencies; pixel manip-

ulations; and various types of lines, curves, boxes, and fills to augment the shapes, text,

and images you place onto the canvas.

In itself, the HTML5 Canvas 2D context is a display API used to render graphics on a

bitmapped area, but there is very little in that context to create applications using the

technology. By adding cross-browser-compatible JavaScript functionality for keyboard

and mouse inputs, timer intervals, events, objects, classes, sound, math functions, etc.,

you can learn to take HTML5 Canvas and create stunning animations, applications,

and games.

Here’s where this book comes in. We are going to break down the Canvas API into

digestible parts and then put it back together, demonstrating how to use it to create

applications. Many of the techniques you will learn in this book have been tried and

used successfully on other platforms, and now we are applying them to this exciting

new technology.

Browser Support for HTML5 Canvas

With the exception of Internet Explorer 8, HTML5 Canvas is supported in some way

by most modern web browsers, with specific feature support growing on an almost

daily basis. The best support seems to be from Google Chrome, followed closely by

Safari, Firefox, and Opera. We will utilize a JavaScript library named modernizr.js that

will help us figure out which browsers support which Canvas features. At the same

time, if you are worried about Internet Explorer, version 9 promises to have support

for Canvas. In the meantime, you can check out Google Chrome Frame (http://code

.google.com/chrome/chromeframe/), which delivers Canvas support for IE.

The Basic HTML Page

Before we get to Canvas, we need to talk a bit about the HTML5 standards we will be

using to create our web pages.

HTML is the standard language used to construct pages on the World Wide Web. We

will not spend much time on HTML, but it does form the basis of <canvas>, so we

cannot skip it entirely.

A basic HTML page is divided into sections, commonly <head> and <body>. The new

HTML5 specification adds a few new sections, such as <nav>, <article>, <header>, and

<footer>.

The <head> tag usually contains information that will be used by the HTML <body> tags

to create the HTML page. It is a standard convention to put JavaScript functions in the

<head>, as you will see later when we discuss the <canvas> tag. There may be reasons

to put some JavaScript in the <body>, but we will make every attempt to keep things

simple by having all JavaScript in the <head>.

Basic HTML for a page might look like Example 1-1.

Example 1-1. A basic HTML page

<!doctype html>

<head>

<title>CH1EX1: Basic Hello World HTML Page</title>

</head>

<body>

Hello World!

</body>

</html>

2 | Chapter 1: Introduction to HTML5 Canvas

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