HTML5之7 __Canvas: 縮放、旋轉、建立陰影-H5教程-PHP中文網

一. canvas API 的scale() 函數用於縮放.

scale() 帶有兩個參數，分別代表在x, y 兩個維度的值，每個參數在canvas 顯示圖像的時候，向其傳遞在本方向軸上影像要放大(或縮小) 的量，如果x為2, 就代表所繪製影像中全部元素都會變成兩倍寬，如果y 值為0.5 ,繪製出來的影像全部元素都會變成之前的一半高。

例如以下程序很方便地在canvas 上創建出新的樹.

<!DOCTYPE html>
<html>
<meta charset="UTF-8">
  <title>缩放示例</title>
  <canvas id="trails" style="border: 1px solid;"  width="400" height="600"> </canvas>
  <script>
        var gravel = new Image();
        gravel.src = "gravel.jpg";
        gravel.onload = function () {
            drawTrails();
        }
 
        function createCanopyPath(context) {
            context.beginPath();
            context.moveTo(-25, -50);
            context.lineTo(-10, -80);
            context.lineTo(-20, -80);
            context.lineTo(-5, -110);
            context.lineTo(-15, -110);
 
            context.lineTo(0, -140);
            context.lineTo(15, -110);
            context.lineTo(5, -110);
            context.lineTo(20, -80);
            context.lineTo(10, -80);
            context.lineTo(25, -50);
            context.closePath();
        }
 
        // 将绘制树的方法 放到 drawTree()，方便重用
        function drawTree(context) {
            var trunkGradient = context.createLinearGradient(-5, -50, 5, -50);
            trunkGradient.addColorStop(0, &#39;#663300&#39;);
            trunkGradient.addColorStop(0.4, &#39;#996600&#39;);
            trunkGradient.addColorStop(1, &#39;#552200&#39;);
            context.fillStyle = trunkGradient;
            context.fillRect(-5, -50, 10, 50);
 
            var canopyShadow = context.createLinearGradient(0, -50, 0, 0);
            canopyShadow.addColorStop(0, &#39;rgba(0, 0, 0, 0.5)&#39;);
            canopyShadow.addColorStop(0.2, &#39;rgba(0, 0, 0, 0.0)&#39;);
            context.fillStyle = canopyShadow;
            context.fillRect(-5, -50, 10, 50);
 
            createCanopyPath(context);
 
            context.lineWidth = 4;
            context.lineJoin = &#39;round&#39;;
            context.strokeStyle = &#39;#663300&#39;;
            context.stroke();
 
            context.fillStyle = &#39;#339900&#39;;
            context.fill();
        }
 
        function drawTrails() {
            var canvas = document.getElementById(&#39;trails&#39;);
            var context = canvas.getContext(&#39;2d&#39;);
            // 在 X=130, Y=250的位置绘制第一棵树
            context.save();
            context.translate(130, 250);
            drawTree(context);
            context.restore();
            // 在 X=260, Y=500 位置绘制第二棵树
            context.save();
            context.translate(260, 500);
 
            // 将第二棵树的宽高分别放大至原来的2倍
            context.scale(2, 2);
            drawTree(context);
            context.restore();
 
            context.save();
            context.translate(-10, 350);
            context.beginPath();
            context.moveTo(0, 0);
            context.quadraticCurveTo(170, -50, 260, -190);
            context.quadraticCurveTo(310, -250, 410,-250);
            context.strokeStyle = context.createPattern(gravel, &#39;repeat&#39;);
            context.lineWidth = 20;
            context.stroke();
            context.restore();
        }
  </script>
</html>

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二. 旋轉context.rotate(angle), 參數angle 以弧度為單位

看程式碼

<!DOCTYPE html>
<html>
<meta charset="UTF-8">
  <title>旋转示例</title>
  <canvas id="trails" style="border: 1px solid;"  width="400" height="600"> </canvas>
  <script>
        var gravel = new Image();
        gravel.src = "gravel.jpg";
        gravel.onload = function () {
            drawTrails();
        }
 
        function createCanopyPath(context) {
            context.beginPath();
            context.moveTo(-25, -50);
            context.lineTo(-10, -80);
            context.lineTo(-20, -80);
            context.lineTo(-5, -110);
            context.lineTo(-15, -110);
 
            context.lineTo(0, -140);
 
            context.lineTo(15, -110);
            context.lineTo(5, -110);
            context.lineTo(20, -80);
            context.lineTo(10, -80);
            context.lineTo(25, -50);
            context.closePath();
        }
 
        // 将绘制树的方法 放到 drawTree()
        function drawTree(context) {
            var trunkGradient = context.createLinearGradient(-5, -50, 5, -50);
            trunkGradient.addColorStop(0, &#39;#663300&#39;);
            trunkGradient.addColorStop(0.4, &#39;#996600&#39;);
            trunkGradient.addColorStop(1, &#39;#552200&#39;);
            context.fillStyle = trunkGradient;
            context.fillRect(-5, -50, 10, 50);
 
            var canopyShadow = context.createLinearGradient(0, -50, 0, 0);
            canopyShadow.addColorStop(0, &#39;rgba(0, 0, 0, 0.5)&#39;);
            canopyShadow.addColorStop(0.2, &#39;rgba(0, 0, 0, 0.0)&#39;);
            context.fillStyle = canopyShadow;
            context.fillRect(-5, -50, 10, 50);
 
            createCanopyPath(context);
 
            context.lineWidth = 4;
            context.lineJoin = &#39;round&#39;;
            context.strokeStyle = &#39;#663300&#39;;
            context.stroke();
 
            context.fillStyle = &#39;#339900&#39;;
            context.fill();
        }
 
        function drawTrails() {
            var canvas = document.getElementById(&#39;trails&#39;);
            var context = canvas.getContext(&#39;2d&#39;);
            // 在 X=130, Y=250的位置绘制第一棵树
            context.save();
            context.translate(100, 150);
            context.rotate(1.57);    //旋转角度以弧度为单位
            drawTree(context);
            context.restore();
 
            context.save();
            context.translate(-10, 450);
            context.beginPath();
            context.moveTo(0, 0);
            context.quadraticCurveTo(170, -50, 260, -190);
            context.quadraticCurveTo(310, -250, 410,-250);
            context.strokeStyle = context.createPattern(gravel, &#39;repeat&#39;);
            context.lineWidth = 20;
            context.stroke();
            context.restore();
        }
 
  </script>
</html>

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以上兩個範例，需注意之處：

總是在原點執行圖形和路徑的變換操作範例中示範了為什麼要在原點執行圖形和路徑的變換操作, 執行完後再統一平移. 因為縮放scale 和旋轉rotate 等變換操作都是針對原點進行的.
如果對一個不在原點的圖形進行旋轉，那麼圖形繞著原點旋轉而不是在原地旋轉。類似地，如果進行縮放操作時沒有將圖形放置到適當的座標上，那麼所有座標都會被同時縮放, 新的座標可能會超出canvas 範圍。

三。相關代碼：

<!DOCTYPE html>
<html>
<meta charset="UTF-8">
  <title>变换阴影</title>
  <canvas id="trails" style="border: 1px solid;"  width="400" height="600"> </canvas>
  <script>
        var gravel = new Image();
        gravel.src = "gravel.jpg";
        gravel.onload = function () {
            drawTrails();
        }
 
        function createCanopyPath(context) {
            context.beginPath();
            context.moveTo(-25, -50);
            context.lineTo(-10, -80);
            context.lineTo(-20, -80);
            context.lineTo(-5, -110);
            context.lineTo(-15, -110);
 
            context.lineTo(0, -140);
            context.lineTo(15, -110);
            context.lineTo(5, -110);
            context.lineTo(20, -80);
            context.lineTo(10, -80);
            context.lineTo(25, -50);
            context.closePath();
        }
 
        function drawTree(context) {
            context.save();
            // X值随着Y值的增加而增加，借助拉伸变换，可以创建一棵用作阴影的倾斜的树，应用了
            // 变换以后，所有坐标都与矩阵相乘
            context.transform(1, 0, -0.5, 1,  0, 0);
 
            // 在Y轴方向，将阴影的高度压缩为原来的60%
            context.scale(1, 0.6);
 
            // 使用透明度为20%的黑色填充树干
            context.fillStyle = &#39;rgba(0, 0, 0, 0.2)&#39;;
            context.fillRect(-5, -50, 10, 50);
 
            // 使用已有的阴影效果重新绘制
            createCanopyPath(context);
            context.fill();
            context.restore();
 
            var trunkGradient = context.createLinearGradient(-5, -50, 5, -50);
            trunkGradient.addColorStop(0, &#39;#663300&#39;);
            trunkGradient.addColorStop(0.4, &#39;#996600&#39;);
            trunkGradient.addColorStop(1, &#39;#552200&#39;);
            context.fillStyle = trunkGradient;
            context.fillRect(-5, -50, 10, 50);
 
            var canopyShadow = context.createLinearGradient(0, -50, 0, 0);
            canopyShadow.addColorStop(0, &#39;rgba(0, 0, 0, 0.5)&#39;);
            canopyShadow.addColorStop(0.2, &#39;rgba(0, 0, 0, 0.0)&#39;);
            context.fillStyle = canopyShadow;
            context.fillRect(-5, -50, 10, 50);
 
            createCanopyPath(context);
 
            context.lineWidth = 4;
            context.lineJoin = &#39;round&#39;;
            context.strokeStyle = &#39;#663300&#39;;
            context.stroke();
 
            context.fillStyle = &#39;#339900&#39;;
            context.fill();
        }
        function drawTrails() {
            var canvas = document.getElementById(&#39;trails&#39;);
            var context = canvas.getContext(&#39;2d&#39;);
            context.save();
            context.translate(130, 250);
            drawTree(context);
            context.restore();
 
            context.save();
            context.translate(260, 500);
            context.scale(2, 2);
            drawTree(context);
            context.restore();
 
            context.save();
            context.translate(-10, 350);
            context.beginPath();
            context.moveTo(0, 0);
            context.quadraticCurveTo(170, -50, 260, -190);
            context.quadraticCurveTo(310, -250, 410,-250);
            context.strokeStyle = context.createPattern(gravel, &#39;repeat&#39;);
            context.lineWidth = 20;
            context.stroke();
            context.restore();
        }
  </script>
</html>