[英]Very High Resolution Images in HTML5 Canvas
該問題的末尾是一個SSCCE,它將在HTML5畫布上顯示24000x12000 Miller衛星圖像的投影。 它有幾個問題:
這是代碼。
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>The Earth</title>
<script type="text/javascript">
function draw() {
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var map = new Image();
map.src = "world.jpg";
map.onload = function() {
var width = window.innerWidth;
var height = window.innerHeight;
ctx.drawImage(map, 0, 0, width, height);
};
}
</script>
<style>
html, body {
width: 100%;
height: 100%;
margin: 0px;
}
#canvas {
width: 100%;
height: 100%;
position: absolute;
top: 0px;
left: 0px;
background-color: rgba(0, 0, 0, 0);
}
</style>
</head>
<body onload="draw();">
<canvas id="canvas"></canvas>
</body>
</html>
使用用於裁剪和縮放原始圖像的context.drawImage版本:
context.drawImage(
sourceImage,
clipSourceAtX, clipSourceAtY, sourceClipWidth, sourceClipHeight,
canvasX, canvasY, canvasDrawWidth, canvasDrawHeight
)
例如,假設您正在關注圖像上的坐標[x == 1000,y == 500]。
要以[1000,500]顯示圖像的640px x 512px部分,可以使用如下所示的drawImage:
context.drawImage(
// use "sourceImage"
sourceImage
// clip a 640x512 portion of the source image at left-top = [1000,500]
sourceImage,1000,500,640,512,
// draw the 640x512 clipped subimage at 0,0 on the canvas
0,0,640,512
);
演示: http : //jsfiddle.net/m1erickson/MtAEY/
這是有效的javascript代碼,格式正確。
function draw() {
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var map = new Image();
map.src = "world.jpg";
map.onload = function() {
var width = window.innerWidth;
var height = window.innerHeight;
canvas.width=width;
canvas.height=height;
var mapWidth=map.width;
var mapHeight=map.height;
var scale=scalePreserveAspectRatio(mapWidth,mapHeight,width,height);
ctx.mozImageSmoothingEnabled = false;
ctx.imageSmoothingEnabled = false;
ctx.webkitImageSmoothingEnabled = false;
ctx.drawImage(map, 0, 0, mapWidth, mapHeight, 0, 0, mapWidth*scale, mapHeight*scale);
};
}
function scalePreserveAspectRatio(imgW,imgH,maxW,maxH){
return(Math.min((maxW/imgW),(maxH/imgH)));
}
關鍵是兩行
canvas.width=width;
canvas.height=height;
在CSS中將它們簡單地設置為100%是行不通的,這100%顯然不是我假設的窗口內部尺寸的100%。 由於這些尺寸是動態的,因此必須通過JS而不是CSS進行設置。
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