Very High Resolution Images in HTML5 Canvas
Question
At the end of this question is an SSCCE to display a 24000x12000 Miller projection of satellite imagery on an HTML5 canvas. It has a couple problems:
- Rather than showing the entire image in one screen as I desire only a small section of the upper left corner is displayed.
- The image suffers from extreme pixellation that should not appear at the scale displayed because the image is very high resolution
- The image used is available at Wikimedia . I renamed it "world.jpg".
- This is not a normal web application and the large image will not be downloaded during application execution so any advice to not require downloading such a large image is moot.
- The application will dynamically zoom into various map areas, thus the large image size.
- In the real code I use an external stylesheet and javascript file. I integrated them into the html solely for the SSCCE.
This is the code.
<!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>
2 answers
solution1
2 ACCPTED 2014-06-14 04:14:57
Use the version of context.drawImage that clips and scales the original image:
context.drawImage(
sourceImage,
clipSourceAtX, clipSourceAtY, sourceClipWidth, sourceClipHeight,
canvasX, canvasY, canvasDrawWidth, canvasDrawHeight
)
For example, assume that you are focusing on coordinate [x==1000,y==500] on the image.
To display the 640px x 512px portion of the image at [1000,500] you can use drawImage like this:
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
);
A demo: http://jsfiddle.net/m1erickson/MtAEY/
solution2
0 2014-06-16 20:27:33
Here's the working javascript code, formatted nicely.
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)));
}
The key is the two lines
canvas.width=width;
canvas.height=height;
Simply setting these to 100% in CSS doesn't work, the 100% is apparently not 100% of the inner dimensions of the window as I assumed. Since these dimensions are dynamic they must be set via JS not CSS.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.