Html5 Canvas中支持对text文本进行渲染的示例代码（图）-H5教程

html5 canvas中支持对text文本进行渲染;
直接的理解就是把text绘制在画布上，并像图形一样处理它（可以加shadow、gradient、pattern、color fill等等）；
既然它的本质是文字，就会具有文字所特有的一些属性；本篇的侧重点也在于此；
不过，在最后会增加一些图形填充效果在text上应用的实例；

context.font:

[font style] [font weight] [font size] [font face]

字体属性的设置与css中的类似；
例：

context.font = "italic bold 24px serif"; context.font = "normal lighter 50px cursive";

context.measureText(message):
当我们提供一个文本message,调用此方法，
它会依据当前context设置的字体、大小等，返回一个文本的度量信息对象TextMetrics；
当前html5 canvas中TextMetrics对象，仅包含一个属性，就是width;
可以用来确定当前给定字符串文本的在当前环境下的宽度；
例如：

var metrics = context.measureText(message);
var textWidth = metrics.width;

fillText([text],[x],[y],[maxWidth]):
参数的意义：
text:要在canvas上要渲染的文本内容；
x,y:代表开始渲染的点的位置坐标；
maxWidth:代表最大宽度；

与之搭配的设置文本的颜色属性：fillStyle

strokeText([text],[x],[y],[maxWidth]):
参数的意义与fillText相同；与fillText相比，它指渲染文字的轮廓；
与之搭配的设置文本的颜色属性：strokeStyle

Canvas中有对文本对齐方式的支持,包括两个选项：水平Horizontal alignment与竖直Vertical alignment；
context.textAlign：文字水平对齐方式。可取属性值: start, end, left,right, center。默认值:start.
context.textBaseline：文字竖直对齐方式。可取属性值：top, hanging, middle,alphabetic, ideographic, bottom。默认值：alphabetic.

Horizontal alignment选项:center|start|end|left|right
例：context.textAlign = "center";

当我们把一段文本渲染在canvas上时，文本本身显示在画布上，会占据一个矩形块（看不见的矩形，我们暂且称其为IBox(invisible bounding box)）；

这里提到的对齐方式，都是针些这个文本所占据的这个IBox来操作的（IBox有，上，下，左，右四条边线）；

把字符串“HA”在画布的中心点位置(两条黑色直线相交点为中心)；textAlign为默认值，应用不同的textBaseline所产生的效果如下图：

把字符串“HA”在画布的中心点位置(两条黑色直线相交点为中心)；textBaseline为默认值，应用不同的textAlign所产生的效果如下图：

大家可以细细品味一下，它们的区别……

实例：

基本属性展示：

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>test</title>
<script type="text/javascript" src="modernizr-latest.js"></script>
<script type="text/javascript">
window.addEventListener("load", eventWindowLoaded, false);

function eventWindowLoaded() {
canvasApp();
}

function canvasSupport() {
return Modernizr.canvas;
}

function eventWindowLoaded() {
canvasApp();
}

function canvasApp() {
var message = "your text";
var fillOrStroke = "fill";
var fontSize = "50";
var fontFace = "serif";
var textFillColor = "#ff0000";
var textBaseline = "middle";
var textAlign = "center";
var fontWeight = "normal";
var fontStyle = "normal";

if(!canvasSupport()) {
return;
}

var theCanvas = document.getElementById("canvasOne");
var context = theCanvas.getContext("2d");
var formElement = document.getElementById("textBox");
formElement.addEventListener('keyup', textBoxChanged, false);
formElement = document.getElementById("fillOrStroke");
formElement.addEventListener('change', fillOrStrokeChanged, false);
formElement = document.getElementById("textSize");
formElement.addEventListener('change', textSizeChanged, false);
formElement = document.getElementById("textFillColor");
formElement.addEventListener('change', textFillColorChanged, false);
formElement = document.getElementById("textFont");
formElement.addEventListener('change', textFontChanged, false);
formElement = document.getElementById("textBaseline");
formElement.addEventListener('change', textBaselineChanged, false);
formElement = document.getElementById("textAlign");
formElement.addEventListener('change', textAlignChanged, false);
formElement = document.getElementById("fontWeight");
formElement.addEventListener('change', fontWeightChanged, false);
formElement = document.getElementById("fontStyle");
formElement.addEventListener('change', fontStyleChanged, false);

drawScreen();

function drawScreen() {
context.fillStyle = "yellow";
context.fillRect(0, 0, theCanvas.width, theCanvas.height);

context.lineWidth = 1;
context.beginPath();
context.moveTo(theCanvas.width / 2, 0);
context.lineTo(theCanvas.width / 2, theCanvas.height);
context.stroke();
context.closePath();

context.beginPath();
context.moveTo(0, theCanvas.height/2);
context.lineTo(theCanvas.width, theCanvas.height/2);
context.stroke();
context.closePath();

//Text
context.textBaseline = textBaseline;
context.textAlign = textAlign;
context.font = fontWeight + " " + fontStyle + " " + fontSize + "px " + fontFace;
var xPosition = (theCanvas.width / 2);
var yPosition = (theCanvas.height / 2);
switch(fillOrStroke) {
case "fill":
context.fillStyle = textFillColor;
context.fillText(message, xPosition, yPosition);
break;
case "stroke":
context.strokeStyle = textFillColor;
context.strokeText(message, xPosition, yPosition);
break;
case "both":
context.fillStyle = textFillColor;
context.fillText(message, xPosition, yPosition);
context.strokeStyle = "#000000";
context.strokeText(message, xPosition, yPosition);
break;
}
}

function textBoxChanged(e) {
var target = e.target;
message = target.value;
drawScreen();
}

function fillOrStrokeChanged(e) {
var target = e.target;
fillOrStroke = target.value;
drawScreen();
}

function textSizeChanged(e) {
var target = e.target;
fontSize = target.value;
drawScreen();
}

function textFillColorChanged(e) {
var target = e.target;
textFillColor = "#" + target.value;
drawScreen();
}

function textFontChanged(e) {
var target = e.target;
fontFace = target.value;
drawScreen();
}

function textBaselineChanged(e) {
var target = e.target;
textBaseline = target.value;
drawScreen();
}

function textAlignChanged(e) {
var target = e.target;
textAlign = target.value;
drawScreen();
}

function fontWeightChanged(e) {
var target = e.target;
fontWeight = target.value;
drawScreen();
}

function fontStyleChanged(e) {
var target = e.target;
fontStyle = target.value;
drawScreen();
}

}
</script>
</head>
<body>
<p style="position: absolute; top: 50px; left: 50px;">
<canvas id="canvasOne" width="400" height="150">
Your browser does not support HTML5 Canvas.
</canvas>
<form>
<span>Text</span>
<input id="textBox"/>
<br/>
<span>Fill or Stroke</span>
<select id="fillOrStroke">
<option value="fill">fill</option>
<option value="stroke">stroke</option>
<option value="both">both</option>
</select>
<br/>
<span>Font</span>
<select id="textFont">
<option value="serif">serif</option>
<option value="sans-serif">sans-serif</option>
<option value="cursive">cursive</option>
<option value="fantasy">fantasy</option>
<option value="monospace">monospace</option>
</select>
<br/>
<span>font size</span>
<input type="range" id="textSize" min="0" max="200"    value="30"/>
<br/>
<span>font color</span>
<input id="textFillColor" value="FF0000"/>
<br/>
<span>font weight</span>
<select id="fontWeight">
<option value="normal">normal</option>
<option value="bold">bold</option>
<option value="bolder">bolder</option>
<option value="lighter">lighter</option>
</select>
<br/>
<span>font style</span>
<select id="fontStyle">
<option value="normal">normal</option>
<option value="italic">italic</option>
<option value="oblique">oblique</option>
</select>
<br/>
<span>textBaseLine</span>
<select id="textBaseline">
<option value="middle">middle</option>
<option value="top">top</option>
<option value="hanging">hanging</option>
<option value="alphabetic">alphabetic</option>
<option value="ideographic">ideographic</option>
<option value="bottom">bottom</option>
</select>
<br/>
<span>TextAlign</span>
<select id="textAlign">
<option value="center">center</option>
<option value="start">start</option>
<option value="end">end</option>
<option value="left">left</option>
<option value="right">right</option>
</select>
</form>
</p>
</body>
</html>

阴影效果：

代换上面代码的drawScreen方法体

function drawScreen() {
context.fillStyle = "yellow";
context.fillRect(0, 0, theCanvas.width, theCanvas.height);
//Text
context.textBaseline = textBaseline;
context.textAlign = textAlign;
context.shadowColor = "#707070";
context.shadowOffsetX = 5;
context.shadowOffsetY = 5;
context.shadowBlur = 5;
context.font = fontWeight + " " + fontStyle + " " + fontSize + "px " + fontFace;
var xPosition = (theCanvas.width / 2);
var yPosition = (theCanvas.height / 2);
context.fillStyle = textFillColor;
context.fillText(message, xPosition, yPosition);
}

渐变效果：

代换View Code 代码段中的drawScreen方法

function drawScreen() {
context.fillStyle = "yellow";
context.fillRect(0, 0, theCanvas.width, theCanvas.height);
var gradient = context.createLinearGradient(0, 0, theCanvas.width, 0);
context.font = "italic bold 40px serif";
gradient.addColorStop(0, "#000000");
gradient.addColorStop(.5, "#FF0000");
gradient.addColorStop(1, "#00ff00");
var xPosition = (theCanvas.width / 2);
var yPosition = (theCanvas.height / 2);
context.fillStyle = gradient;
context.fillText("message", xPosition, yPosition);
}

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