javascript – 使用html5画布将图像剪辑成多边形的可重用函数
作者：互联网
猜猜帖子的标题可能需要编辑,但是现在我不知道问题出在哪里.我在这里和其他地方阅读了类似问题的页面和答案. One Stack Overflow答案特别接近,但我不明白.
我想要一个函数,在画布上以所需的坐标绘制多边形,并用从文件加载的一些背景图像填充它们(足够大,不需要平铺).三角形可以用于测试.显然我应该使用drawImage和clip,并为多边形赋予边框,我可以为剪辑和笔划重用相同的路径.显然我应该保持秩序
- define path
- save
- clip
- drawImage
- restore
- stroke.
还可以在某处读取一次加载图像就足够了. (如果你想让我引用所有这些假设的来源,我会寻找我看到它们的位置.大部分都在Stack Overflow上)
HTML是空的
<body onl oad = "main ();"></body>
第一种方法,假装浏览器将等待图片加载：
var ctx, img;
var image_path = 'bg.jpg';
function main () {
var CANVAS_SIZE = 600;
var view_field_cnv = document.createElement ('canvas');
view_field_cnv.width = CANVAS_SIZE;
view_field_cnv.height = CANVAS_SIZE;
view_field_cnv.style.border = "1px solid";
document.body.appendChild (view_field_cnv);
ctx = view_field_cnv.getContext ('2d');
img = document.createElement ('img');
img.src = image_path;
place_triangle (0, 0);
place_triangle (300, 300);
place_triangle (500, 500);
place_triangle (0, 0);
}
function place_triangle (x, y) {
console.log (x, y);
ctx.beginPath ();
ctx.moveTo (x + 10, y);
ctx.lineTo (x + 110, y);
ctx.lineTo (x + 60, y + 40);
ctx.closePath ();
img = document.createElement ('img');
img.src = image_path;
ctx.save ();
ctx.clip ();
ctx.drawImage (img, x, y);
ctx.restore ();
ctx.stroke ();
}
绘制所有三个三角形但没有剪切图像.
第二次尝试,在image.onload中使用drawImage：
var ctx;
var image_path = 'bg.jpg';
function main () {
var CANVAS_SIZE = 600;
var view_field_cnv = document.createElement ('canvas');
view_field_cnv.width = CANVAS_SIZE;
view_field_cnv.height = CANVAS_SIZE;
view_field_cnv.style.border = "1px solid";
document.body.appendChild (view_field_cnv);
ctx = view_field_cnv.getContext ('2d');
place_triangle (0, 0);
place_triangle (300, 300);
place_triangle (500, 500);
place_triangle (0, 0);
}
function place_triangle (x, y) {
console.log (x, y);
var img;
ctx.beginPath ();
ctx.moveTo (x + 10, y);
ctx.lineTo (x + 110, y);
ctx.lineTo (x + 60, y + 40);
ctx.closePath ();
img = document.createElement ('img');
img.src = image_path;
img.onload = function () {
ctx.save ();
ctx.clip ();
ctx.drawImage (img, x, y);
ctx.restore ();
ctx.stroke ();
}
}
这个绘制图像,但只绘制一个三角形,最后一个.只是注释掉保存和恢复没有帮助.
所以,我不明白加载图像,保存,恢复以及可能还有一百万个其他东西.错误在哪里？
解决方法:
我看到你已经理解了剪辑的基础知识：
>保存上下文,定义路径,剪辑,drawImage,还原上下文.
>如果您希望笔划与剪切的图像略微重叠,则可以在恢复后进行描边.
>如果您不希望笔划与剪切的图像重叠,则可以在剪裁前进行描边.
这是示例代码和演示：http://jsfiddle.net/m1erickson/p0fup425/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// image loader
// put the paths to your images in imageURLs[]
var imageURLs=[];
// push all your image urls!
imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/multple/norwayFlag.jpg");
imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/multple/swedishFlag.jpg");
// the loaded images will be placed in images[]
var imgs=[];
var imagesOK=0;
loadAllImages(start);
function loadAllImages(callback){
for (var i=0; i<imageURLs.length; i++) {
var img = new Image();
imgs.push(img);
img.onload = function(){
imagesOK++;
if (imagesOK>=imageURLs.length ) {
callback();
}
};
img.onerror=function(){alert("image load failed");}
img.crossOrigin="anonymous";
img.src = imageURLs[i];
}
}
function start(){
// the imgs[] array now holds fully loaded images
// the imgs[] are in the same order as imageURLs[]
// clip image#1
clippingPath([10,70,50,10,90,70],imgs[0],10,10);
// clip image#2
clippingPath([10,170,50,110,90,170],imgs[1],10,110);
// append the original images for demo purposes
document.body.appendChild(imgs[0]);
document.body.appendChild(imgs[1]);
}
function clippingPath(pathPoints,img,x,y){
// save the unclipped context
ctx.save();
// define the path that will be clipped to
ctx.beginPath();
ctx.moveTo(pathPoints[0],pathPoints[1]);
// this demo has a known number of polygon points
// but include a loop of "lineTo's" if you have a variable number of points
ctx.lineTo(pathPoints[2],pathPoints[3]);
ctx.lineTo(pathPoints[4],pathPoints[5]);
ctx.closePath();
// stroke the path
// half of the stroke is outside the path
// the outside part of the stroke will survive the clipping that follows
ctx.lineWidth=2;
ctx.stroke();
// make the current path a clipping path
ctx.clip();
// draw the image which will be clipped except in the clipping path
ctx.drawImage(img,x,y);
// restore the unclipped context (==undo the clipping path)
ctx.restore();
}
}); // end $(function(){});
</script>
</head>
<body>
<p>Images clipped inside triangular canvas paths</p>
<canvas id="canvas" width=150 height=200></canvas>
<p>Original Images</p>
</body>
</html>
标签：javascript,canvas,onload,drawimage,clip 来源： https://codeday.me/bug/20190609/1207553.html