How to scale images on a html5 canvas with better interpolation?

Question

First of all: what am I trying to do?

I have an application to view images. It uses the canvas element to render the image. You can zoom in, you can zoom out, and yo

Answer 1

You need to "step down" several times. Instead of scaling from a very large image to a very small, you need to re-scale it to intermediary sizes.

Consider an image you want to draw at 1/6 scale. You could do this:

var w = 1280;
var h = 853;

ctx.drawImage(img, 0, 0, w/6, h/6);   

Or you could draw it to an in-memory canvas at 1/2 scale, then 1/2 scale again, then 1/2 scale again. The result is a 1/6 scale image, but we use three steps:

var can2 = document.createElement('canvas');
can2.width = w/2;
can2.height = w/2;
var ctx2 = can2.getContext('2d');

ctx2.drawImage(img, 0, 0, w/2, h/2);
ctx2.drawImage(can2, 0, 0, w/2, h/2, 0, 0, w/4, h/4);
ctx2.drawImage(can2, 0, 0, w/4, h/4, 0, 0, w/6, h/6);

Then you can draw that back to your original context:

ctx.drawImage(can2, 0, 0, w/6, h/6, 0, 200, w/6, h/6);

You can see the difference live, here:

var can = document.getElementById('canvas1');
var ctx = can.getContext('2d');

var img = new Image();
var w = 1280;
var h = 853;
img.onload = function() {
    // step it down only once to 1/6 size:
    ctx.drawImage(img, 0, 0, w/6, h/6);   
    
    // Step it down several times
    var can2 = document.createElement('canvas');
    can2.width = w/2;
    can2.height = w/2;
    var ctx2 = can2.getContext('2d');
    
    // Draw it at 1/2 size 3 times (step down three times)
    
    ctx2.drawImage(img, 0, 0, w/2, h/2);
    ctx2.drawImage(can2, 0, 0, w/2, h/2, 0, 0, w/4, h/4);
    ctx2.drawImage(can2, 0, 0, w/4, h/4, 0, 0, w/6, h/6);
    ctx.drawImage(can2, 0, 0, w/6, h/6, 0, 200, w/6, h/6);
}



img.src = 'http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Equus_quagga_%28Namutoni%2C_2012%29.jpg/1280px-Equus_quagga_%28Namutoni%2C_2012%29.jpg'

canvas {
    border: 1px solid gray;
}

<canvas id="canvas1" width="400" height="400"></canvas>

View same snippet on jsfiddle.