问题
Guess the title of the post may need editing, but for now I don't know where the problems are. I have read pages and answers to similar questions, here and elsewhere. One Stack Overflow answer is especially close, but I don't understand it.
I want a function, to draw polygons on canvas at desired coordinates and to fill them with some background image loaded from a file (large enough that no tiling is needed). Triangles would be fine for a test. Apparently I should use drawImage and clip, and to give the polygon a border, I can resuse the same path for the clip and the stroke. Also apparently I should keep the order of
- define path
- save
- clip
- drawImage
- restore
- stroke.
Also read somewhere that it is enough to load the image once. (If uou want me to quote sources for all these assumptions, I will look for where I saw them. Most of them on Stack Overflow)
The HTML is an otherwise empty
<body onload = "main ();"></body>
First approach, pretending that the browser will wait for the picture to load:
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 ();
}
That draws all three triangles but no clipped images.
Second try, with drawImage inside image.onload:
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 ();
}
}
This one does draw the clipped image, but only one triangle, the last one. Just commenting out save and restore doesn't help.
So, I don't understand loading images, saving, restoring and probably a million other things. Where be the bugs?
回答1:
I see you already understand the basics of clipping:
save context, define path, clip, drawImage, restore context.
you can stroke after restore if you want the stroke to slightly overlap the clipped image.
you can stroke before clipping if you don't want the stroke to overlap the clipped image.
Here's example code and a Demo: 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>
来源：https://stackoverflow.com/questions/25701798/a-reusable-function-to-clip-images-into-polygons-using-html5-canvas