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);
		}