HTML5 Canvas中提供了实现图形平移，旋转，放缩的API。
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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
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 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); }
