HTML5 Canvas平移，放縮，旋轉圖文程式碼詳情

HTML5 Canvas中提供了實作圖形平移，旋轉，放縮的API。

平移(translate)

#平移座標translate(x, y)意思是把(0,0)座標平移到(x, y)，原來的(0,0)座標則變成(-x, -y)

##圖示如下：

任何原來的座標點p(ox, oy)在translate之後的座標點為p(ox-x, oy-y),其中點(x , y)為平移

點座標translate(x, y)。

程式碼示範：

// 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&#39;m Back to Top",5,50);
}

# 

放縮(Scale)

Scale(a, b)意思是將物件沿著XY軸分別放縮至a*x,  b*y大小。效果如圖所示：

// 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();
}

旋轉(rotate)

旋轉角度rotate(Math.PI/8)

旋轉前的座標p(x, y)在對應的旋轉後的座標P(rx, ry)為

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

旋轉90度可以簡化為：

Rx = y;
Ry = -x;

##Canvas中旋轉預設的方向為順時針方向。示範程式碼如下：

// 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);
    }
}

#通常做法是旋轉與平移一起使用，先將座標

(0,0)平移到中心位置

translate (width/2, height/2)

然後再使用rotate(Math.PI/2)完成旋轉

#程式碼範例如下：

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程式碼：

		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&#39;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&#39;m here!!!",5,50);
			
			// rotate -90 degreee
			// context.rotate(-Math.PI/2);
			// context.fillStyle="blue";
			// context.fillText("i&#39;m here!!!", -400,30);
			
			// rotate 90 degreee
			context.rotate(Math.PI/2);
			context.fillStyle="blue";
			context.fillText("i&#39;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);
		}

   以上就是HTML5 Canvas平移，放縮，旋轉圖文代碼詳情的內容，更多相關內容請關注PHP中文網（www.php.cn）！