You are previewing HTML5: Up and Running.

HTML5: Up and Running

Cover of HTML5: Up and Running by Mark Pilgrim Published by O'Reilly Media, Inc.
  1. HTML5: Up and Running
    1. SPECIAL OFFER: Upgrade this ebook with O’Reilly
    2. Preface
      1. Diving In
      2. Conventions Used in This Book
      3. Using Code Examples
      4. A Note on the Editions of This Book
      5. Safari® Books Online
      6. How to Contact Us
    3. 1. How Did We Get Here?
      1. Diving In
      2. MIME Types
      3. A Long Digression into How Standards Are Made
      4. An Unbroken Line
      5. A Timeline of HTML Development from 1997 to 2004
      6. Everything You Know About XHTML Is Wrong
      7. A Competing Vision
      8. What Working Group?
      9. Back to the W3C
      10. Postscript
      11. Further Reading
    4. 2. Detecting HTML5 Features
      1. Diving In
      2. Detection Techniques
      3. Modernizr: An HTML5 Detection Library
      4. Canvas
      5. Canvas Text
      6. Video
      7. Video Formats
      8. Local Storage
      9. Web Workers
      10. Offline Web Applications
      11. Geolocation
      12. Input Types
      13. Placeholder Text
      14. Form Autofocus
      15. Microdata
      16. Further Reading
    5. 3. What Does It All Mean?
      1. Diving In
      2. The Doctype
      3. The Root Element
      4. The <head> Element
      5. New Semantic Elements in HTML5
      6. A Long Digression into How Browsers Handle Unknown Elements
      7. Headers
      8. Articles
      9. Dates and Times
      10. Navigation
      11. Footers
      12. Further Reading
    6. 4. Let’s Call It a Draw(ing Surface)
      1. Diving In
      2. Simple Shapes
      3. Canvas Coordinates
      4. Paths
      5. Text
      6. Gradients
      7. Images
      8. What About IE?
      9. A Complete Example
      10. Further Reading
    7. 5. Video on the Web
      1. Diving In
      2. Video Containers
      3. Video Codecs
      4. Audio Codecs
      5. What Works on the Web
      6. Licensing Issues with H.264 Video
      7. Encoding Ogg Video with Firefogg
      8. Batch Encoding Ogg Video with ffmpeg2theora
      9. Encoding H.264 Video with HandBrake
      10. Batch Encoding H.264 Video with HandBrake
      11. Encoding WebM Video with ffmpeg
      12. At Last, the Markup
      13. What About IE?
      14. A Complete Example
      15. Further Reading
    8. 6. You Are Here (And So Is Everybody Else)
      1. Diving In
      2. The Geolocation API
      3. Show Me the Code
      4. Handling Errors
      5. Choices! I Demand Choices!
      6. What About IE?
      7. geo.js to the Rescue
      8. A Complete Example
      9. Further Reading
    9. 7. The Past, Present, and Future of Local Storage for Web Applications
      1. Diving In
      2. A Brief History of Local Storage Hacks Before HTML5
      3. Introducing HTML5 Storage
      4. Using HTML5 Storage
      5. HTML5 Storage in Action
      6. Beyond Named Key/Value Pairs: Competing Visions
      7. Further Reading
    10. 8. Let’s Take This Offline
      1. Diving In
      2. The Cache Manifest
      3. The Flow of Events
      4. The Fine Art of Debugging, a.k.a. “Kill Me! Kill Me Now!”
      5. Let’s Build One!
      6. Further Reading
    11. 9. A Form of Madness
      1. Diving In
      2. Placeholder Text
      3. Autofocus Fields
      4. Email Addresses
      5. Web Addresses
      6. Numbers As Spinboxes
      7. Numbers As Sliders
      8. Date Pickers
      9. Search Boxes
      10. Color Pickers
      11. And One More Thing...
      12. Further Reading
    12. 10. “Distributed,” “Extensibility,” and Other Fancy Words
      1. Diving In
      2. What Is Microdata?
      3. The Microdata Data Model
      4. Marking Up People
      5. Marking Up Organizations
      6. Marking Up Events
      7. Marking Up Reviews
      8. Further Reading
    13. A. The All-in-One Almost-Alphabetical Guide to Detecting Everything
      1. List of Elements
      2. Further Reading
    14. Index
    15. About the Author
    16. Colophon
    17. SPECIAL OFFER: Upgrade this ebook with O’Reilly
















[a] Internet Explorer support requires the third-party explorercanvas library.

[b] Mozilla Firefox 3.0 support requires a compatibility shim.

In addition to drawing lines on a canvas, you can also draw text on a canvas. Unlike text on the surrounding web page, there is no box model. That means none of the familiar CSS layout techniques are available: no floats, no margins, no padding, no word wrapping. (Maybe you think that’s a good thing!) You can set a few font attributes, then you pick a point on the canvas and draw your text there.

Unlabeled axes drawn on a canvas

Figure 4-4. Unlabeled axes drawn on a canvas

The following font attributes are available on the drawing context (see Simple Shapes):

  • font can be anything you would put in a CSS font rule. That includes font style, font variant, font weight, font size, line height, and font family.

  • textAlign controls text alignment. It is similar (but not identical) to a CSS text-align rule. Possible values are start, end, left, right, and center.

  • textBaseline controls where the text is drawn relative to the starting point. Possible values are top, hanging, middle, alphabetic, ideographic, and bottom.

textBaseline is tricky, because text is tricky. (Well, English text isn’t tricky, but you can draw any Unicode character you like on a canvas, and Unicode is tricky.) The HTML5 specification explains the different text baselines:[5]

The top of the em square is roughly at the top of the glyphs in a font, the hanging baseline is where some glyphs like आ are anchored, the middle is half-way between the top of the em square and the bottom of the em square, the alphabetic baseline is where characters like Á, ÿ, f, and Ω are anchored, the ideographic baseline is where glyphs like 私 and 達 are anchored, and the bottom of the em square is roughly at the bottom of the glyphs in a font. The top and bottom of the bounding box can be far from these baselines, due to glyphs extending far outside the em square (see Figure 4-5 ).

Text baselines

Figure 4-5. Text baselines

For simple alphabets like English, you can safely stick with top, middle, or bottom for the textBaseline property.

Let’s draw some text! Text drawn inside the canvas inherits the font size and style of the <canvas> element itself, but you can override this by setting the font property on the drawing context:

context.font = "bold 12px sans-serif";   
context.fillText("x", 248, 43);
context.fillText("y", 58, 165);

The fillText() method draws the actual text:

context.font = "bold 12px sans-serif";
context.fillText("x", 248, 43);   
context.fillText("y", 58, 165);

For the text in the upper-left corner, say we want the top of the text to be at y=5. But we’re lazy—we don’t want to measure the height of the text and calculate the baseline. Instead, we can set textBaseline to top and pass in the upper-left coordinate of the text’s bounding box:

context.textBaseline = "top";
context.fillText("( 0 , 0 )", 8, 5);

Now for the text in the lower-right corner. Let’s say we want the bottom-right corner of the text to be at coordinates (492,370)—just a few pixels away from the bottom-right corner of the canvas—but again, we don’t want to measure the width or height of the text. We can set textAlign to right and textBaseline to bottom, then call fillText() with the bottom-right coordinates of the text’s bounding box:

context.textAlign = "right";
context.textBaseline = "bottom";
context.fillText("( 500 , 375 )", 492, 370);

Figure 4-6 shows the result.

Labeled axes on a canvas

Figure 4-6. Labeled axes on a canvas

Oops! We forgot the dots in the corners. We’ll see how to draw circles a little later; for now we’ll cheat a little and draw them as rectangles (see Simple Shapes):

context.fillRect(0, 0, 3, 3);   
context.fillRect(497, 372, 3, 3);

And that’s all she wrote! Figure 4-7 shows the final product.

A canvas coordinates diagram on a canvas

Figure 4-7. A canvas coordinates diagram on a canvas

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