The document is an overview of HTML5 Canvas, presenting its definition as an immediate-mode API for two-dimensional vector graphics. It covers the Canvas 2D context and various functionalities including drawing, transformations, and styles, along with tips for optimization and practical tricks. The document concludes with demonstrations to reinforce the discussed concepts.

1. Intro to HTML5 Canvas
Juho Vepsäläinen

2. Gonna Talk About These
Topics You Betcha
Canvas (Overview)
Canvas 2D Context (The Beefcake)
Canvas Tricks (For Fun and Profit)
Canvas Demos (To Recap Concepts Discussed)

3. Canvas
Overview

4. http://dev.w3.org/html5/2dcontext/
Just Google it. :)

5. Definition
Immediate-mode API and associated utility methods
for drawing two-dimensional vector graphics
to a raster drawing area.
Draw and forget API and utils for 2D drawing.

6. Markup
<canvas></canvas>
<canvas width="800" height="600"></canvas>
<canvas width="800" height="600">No canvas for you!</canvas>

7. Element Access
Attributes: width, height
Method: getContext (for drawing), toDataURL (for
saving)

8. Show some apps and libs now.

9. Canvas 2D Context
The Beefcake

10. Get Some Context
var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");

11. Context Functionality
It's a state machine
Read-only ref back to canvas (attr)
save/restore + ops
Ops: transformations, compositing, colors and styles,
line caps/joins, shadows, rects, paths, text, drawing
images, pixel manipulation + misc. crud I won't cover

12. Context for Dummies
1. Set some states (transformation, color, ie.)
2. Draw (lines, whatnot)
3. ???
4. Profit

13. Default Context
(0, 0) x
y

14. Transformed Context
(0, 0) x
ctx.scale(x, y)
ctx.rotate(rad)
ctx.translate(x, y)
y ctx.transform/setTransform(a,b,c,d,e,f) (set resets)

15. Compositing
ctx.globalAlpha = 0.38;
ctx.globalCompositeOperation = "source-over";

16. Colors and Styles - stroke/
fill
ctx.strokeStyle = "black";
ctx.fillStyle = "yellow";

17. Colors and Styles -
Gradients
ctx.createLinearGradient(x0,y0,x1,y1)
ctx.createRadialGradient(x0,y0,r0,x1,y1,r1)
ctx.addColorStop(offset, color);

18. Colors and Styles - Patterns
ctx.createPattern(catImg, 'repeat-x');

19. Line Caps/Joins
ctx.lineWidth = 12;
ctx.lineCap = "square";
ctx.lineJoin = "miter";
ctx.miterLimit = 10;

20. Shadows
ctx.shadowOffsetX = 5;
ctx.shadowOffsetY = 5;
ctx.shadowBlur = 3;
ctx.shadowColor = 'grey';

21. Rectangles
ctx.clearRect/fillRect/strokeRect(x,y,w,h)

22. Paths
ctx.beginPath();
ctx.moveTo(x, y); // initial pos
// define curve
ctx.lineTo/quadraticCurveTo/bezierCurveTo/
arcTo/arc/rect
...
ctx.closePath();
ctx.fill/stroke/clip();

23. Text
ctx.font = "24px sans-serif ";
ctx.textAlign = "center";
ctx.fillText/strokeText(text,x,y,maxWidth);

24. Drawing Images
ctx.drawImage(img/canvas/video, lots of alternatives);
Supports compositing! Use this to your advantage.

25. Pixel Manipulation
ctx.createImageData/getImageData/putImageData
var data = ctx.getImageData(0, 0, w, h);
var realData = data.data;
for(var y = 0, pos = 0; y < h; y++) {
for(var x = 0; x < w; x++, pos+=4) {
realData[pos + 2] *= 0.5; // modify Blue channel
}
}
data.data = realData;
Friggin' slow! Avoid if possible. Optimize usage.

26. Canvas Tricks
For Fun and Profit

27. Blurred Lines
Basic idea: Use line shadow, offset actual line
so that it isn't visible.

28. Multiple Layers
Basic idea: CSS z-index + absolute positioning.

29. Erasing
Basic idea: Use destination-out compositing op.

30. CSS Fun
Basic idea: Play around with CSS opacity
and transformations (incurs perf penalty).

31. Canvas Demos
To recap concepts discussed

32. Demo time.