HTML5 Canvas实现平移/放缩/旋转deom示例(附截图)

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：

复制代码 代码如下:

// 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(“plea 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给女朋友说的情话大小。效果如图示：

复制代码 代码如下:

// 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.clopath();

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：

复制代码 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地中海气候特点”;

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：

复制代码 代码如下:

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：

复制代码 代码如下:

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. plea install a html5 compatible browr.”);

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(“plea press <esc> to exit game”,5,50);

context.translate(-width / 2, -height / 2);

context.filltext(“i’m back to top”爱国诗朗诵;,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.clopath();

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 科学计数法y = (i+1)*60;

var newx = y;

var newy = -x;

context.fillrect(newx, newy, 200, 6);

}

}

function sa2020高考录取分数线公布veandrestorecontext(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);

}