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Html5 Canvas Text

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：

使用实例：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8" />
    <title>test</title>
    <!--<script type="text/javascript" src="modernizr-latest.js"></script>-->
    <script src="Js/modernizr-2.6.2.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;
                }
            }


            //添加阴影效果
            //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);
            //}

            //添加渐变效果
            //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);
            //}

            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 class="color" 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>

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来源地址；

http://www.cnblogs.com/amtf/archive/2012/01/18/2324343.html

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原文地址：https://www.cnblogs.com/tianma3798/p/5563485.html