HTML5 Canvas平移，放缩，旋转演示

HTML5 Canvas中提供了实现图形平移，旋转，放缩的API。

平移(translate)

平移坐标translate(x, y)意思是把(0,0)坐标平移到(x, y)，原来的(0,0)坐标则变成(-x, -y)

图示如下：

任何原来的坐标点p(ox, oy)在translate之后的坐标点为p(ox-x, oy-y),其中点(x, y)为平移

点坐标translate(x, y)。

代码演示：

// translate is move the startpoint to centera and back to top left corner

function renderText(width, height, context) {

    context.translate(width/ 2, height / 2); // 中心点坐标为(0, 0)

    context.font="18px Arial";

    context.fillStyle="blue";

    context.fillText("Please Press <Esc> to Exit Game",5,50);

    context.translate(-width/2, -height/2); // 平移恢复(0,0)坐标为左上角

    context.fillText("I'm Back to Top",5,50);

}

放缩(Scale)

Scale(a, b)意思是将对象沿着XY轴分别放缩至a*x,  b*y大小。效果如图示：

// translation the rectangle.

function drawPath(context) {

    context.translate(200,200);

    context.scale(2,2);// Scale twice size of original shape

    context.strokeStyle= "green";

    context.beginPath();

    context.moveTo(0,40);

    context.lineTo(80,40);

    context.lineTo(40,80);

    context.closePath();

    context.stroke();

}

旋转(rotate)

旋转角度rotate(Math.PI/8)

旋转前的坐标p(x, y)在对应的旋转后的坐标P(rx, ry)为

Rx = x * cos(-angle) - y * sin(-angle);

Ry = y * cos(-angle) + x * sin(-angle);

旋转90度可以简化为：

Rx = y;

Ry = -x;

Canvas中旋转默认的方向为顺时针方向。演示代码如下：

// new point.x = x * cos(-angle) -y * sin(-angle),

// new point.y = y * cos(-angle) +x * sin(-angle)

function renderRotateText(context) {

    context.font="24px Arial";

    context.fillStyle="red";

    context.fillText("i'm here!!!",5,50);

    // rotate -90 degreee

    // context.rotate(-Math.PI/2);

    // context.fillStyle="blue";

    // context.fillText("i'm here!!!", -400,30);

    // rotate 90 degreee

    context.rotate(Math.PI/2);

    context.fillStyle="blue";

    context.fillText("i'm here!!!",350,-420);

    console.log(Math.sin(Math.PI/2));

    // rotae 90 degree and draw 10 lines

    context.fillStyle="green";

    for(var i=0; i<4; i++) {

        var x = (i+1)*20;

        var y = (i+1)*60;

        var newX = y;

        var newY = -x; 

        context.fillRect(newX,newY, 200, 6);

    }

}

通常做法是旋转与平移一起使用，先将坐标(0,0)平移到中心位置

translate (width/2, height/2)然后再使用rotate(Math.PI/2)完成旋转

代码示例如下：

function saveAndRestoreContext(context) {

    context.save();

    context.translate(200,200);

    context.rotate(Math.PI/2);

    context.fillStyle="black";

    context.fillText("2D Context Rotate And Translate", 10, 10);

    context.restore();

    context.fillText("2D Context Rotate And Translate", 10, 10);

}

 全部JavaScript代码：

		var tempContext = null; // global variable 2d context
		window.onload = function() {
			var canvas = document.getElementById("target");
			canvas.width = 450;
			canvas.height = 450;
			
			if (!canvas.getContext) {
			    console.log("Canvas not supported. Please install a HTML5 compatible browser.");
			    return;
			}
			
			// get 2D context of canvas and draw image
			tempContext = canvas.getContext("2d");
			// renderText(canvas.width, canvas.height, tempContext);
			saveAndRestoreContext(tempContext);
			// drawPath(tempContext);
		}
		
		// translate is move the start point to centera and back to top left corner
		function renderText(width, height, context) {
			context.translate(width / 2, height / 2);
			context.font="18px Arial";
			context.fillStyle="blue";
			context.fillText("Please Press <Esc> to Exit Game",5,50);
			context.translate(-width / 2, -height / 2);
			context.fillText("I'm Back to Top",5,50);
		}
		
		// translation the rectangle.
		function drawPath(context) {
			context.translate(200, 200);
			context.scale(2,2); // Scale twice size of original shape
			context.strokeStyle = "green";
			context.beginPath();
			context.moveTo(0, 40);
			context.lineTo(80, 40);
			context.lineTo(40, 80);
			context.closePath();
			context.stroke();
		}
		
		// new point.x = x * cos(-angle) - y * sin(-angle), 
		// new point.y = y * cos(-angle) + x * sin(-angle)
		function renderRotateText(context) {
			context.font="24px Arial";
			context.fillStyle="red";
			context.fillText("i'm here!!!",5,50);
			
			// rotate -90 degreee
			// context.rotate(-Math.PI/2);
			// context.fillStyle="blue";
			// context.fillText("i'm here!!!", -400,30);
			
			// rotate 90 degreee
			context.rotate(Math.PI/2);
			context.fillStyle="blue";
			context.fillText("i'm here!!!", 350,-420);
			
			console.log(Math.sin(Math.PI/2));
			
			// rotae 90 degree and draw 10 lines
			context.fillStyle="green";
			for(var i=0; i<4; i++) {
				var x = (i+1)*20;
				var y = (i+1)*60;
				var newX = y;
				var newY = -x;	
				context.fillRect(newX, newY, 200, 6);
			}
		}
		
		function saveAndRestoreContext(context) {
			context.save();
			context.translate(200,200);
			context.rotate(Math.PI/2);
			context.fillStyle="black";
			context.fillText("2D Context Rotate And Translate", 10, 10);
			context.restore();
			context.fillText("2D Context Rotate And Translate", 10, 10);
		}