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Sample code that supports rendering text in Html5 Canvas (picture)

黄舟Original: 2017-03-28 15:50:193170browse

html5 canvas supports rendering text;
The direct understanding is to draw the text on the canvas, and like Treat it the same as graphics (you can add shadow, gradient, pattern, color fill, etc.);
Since its essence is text, it will have some attributes unique to text ;The focus of this article is also here;
However, at the end, some examples of applying graphic fill effects to text will be added;

context.font:

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

The setting of font attributes is similar to that in css;
Example:

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

context.measureText(message):
When we Provide a text message and call this method.
It will return the metric information of a text based on the font, size, etc. set by the current contextObjectTextMetrics;
The TextMetrics object in the current html5 canvas only contains one attribute, which is width;
can be used to determine the current given string text The width in the current environment;
For example:

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

fillText([text],[x],[y],[maxWidth]):
##Parameter meaning:
text: required The text content to be rendered on the canvas;
x,y: represents the position coordinates of the point where rendering starts;
maxWidth: represents the maximum width;

Set the color attribute of the text with it: fillStyle

strokeText([text ],[x],[y],[maxWidth]):
The meaning of the parameter is the same as fillText; compared with fillText, it refers to the outline of the rendered text;
The matching color attribute of the text is set: strokeStyle

##Canvas has support for text alignment. Includes two options: horizontal Horizontal alignment and vertical Vertical alignment;

context.textAlign: text horizontal alignment. Possible attribute values: start, end, left, right, center. Default value: start.
context.textBaseline: Text vertical alignment. Possible attribute values: top, hanging, middle, alphabetic, ideographic, bottom. Default value: 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(&#39;keyup&#39;, textBoxChanged, false);
formElement = document.getElementById("fillOrStroke");
formElement.addEventListener(&#39;change&#39;, fillOrStrokeChanged, false);
formElement = document.getElementById("textSize");
formElement.addEventListener(&#39;change&#39;, textSizeChanged, false);
formElement = document.getElementById("textFillColor");
formElement.addEventListener(&#39;change&#39;, textFillColorChanged, false);
formElement = document.getElementById("textFont");
formElement.addEventListener(&#39;change&#39;, textFontChanged, false);
formElement = document.getElementById("textBaseline");
formElement.addEventListener(&#39;change&#39;, textBaselineChanged, false);
formElement = document.getElementById("textAlign");
formElement.addEventListener(&#39;change&#39;, textAlignChanged, false);
formElement = document.getElementById("fontWeight");
formElement.addEventListener(&#39;change&#39;, fontWeightChanged, false);
formElement = document.getElementById("fontStyle");
formElement.addEventListener(&#39;change&#39;, 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>
<div 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>
</div>
</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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