如何在HTML5 canvas，canvas.toDataURL（）中保存大小为a * a的图像，并稍后以不同的大小a * a / 2a * 2a绘制它？

Question

How to save a single image of size a * a using HTML5 canvas, canvas.toDataURL() and later draw it in different sizes, a * a / 2a * 2a without distortion?

OR

I have a HTML5 / JavaScript web application. There is an item catalogue, in which when I add a new item, I add an image of the item using HTML5 canvas:

<canvas id="canvasnew" height="100" style="border:1px dotted" width="100"></canvas>

This 100 * 100 image is converted to $('#canvasnew')[0].toDataURL() and saved in my database. And the image is later drawn into another canvas like this:

var ctx = $(this.el).find('#canvas')[0].getContext('2d');
var img = new Image;
img.onload = function () {
    ctx.drawImage(img, 0, 0);
};
img.src = this.model.toJSON().ImageUrl;`

this.model.toJSON().ImageUrl gets the image (size 100 * 100 )saved in dataUrl form. Now, I need to draw this same image into one more canvas of size 400 * 400 without distortion. How do I do this?

Answer 1

Try this:

ctx.drawImage(img, 0, 0, 400, 400);

drawImage accepts up to 9 parameters, the first 5 being :

1. The image,
2. x position,
3. y position,
4. width,
5. height.

So: drawImage(img, x, y, width, height);

Stretching a image like that will, however, always result in some sort of distortion, due to "new" pixels having to be calculated / interpolated. Some interpolation algorithms are more efficient, or result in better images than others, but so far the canvas has limited support for different algorithms. ( Take a look here for some more information )