Resizing images using createjs / easeljs

问题

I'd like to dynamically downsize some images on my canvas using createjs, and then store the smaller images to be displayed when zooming out of the canvas for performance reasons. Right now, I'm using the following code:

var bitmap = createjs.Bitmap('somefile.png');
// wait for bitmap to load (using preload.js etc.)

var oc = document.createElement('canvas');
var octx = oc.getContext('2d');
oc.width = bitmap.image.width*0.5;
oc.height = bitmap.image.height*0.5;
octx.drawImage(bitmap.image, 0, 0, oc.width, oc.height);

var dataUrl = oc.toDataURL('image/png'); // very expensive
var smallBitmap = new createjs.Bitmap(dataUrl);

This works, but:

1. The toDataURL operation is very expensive when converting to image/png and too slow to use in practice (and I can't convert to the faster image/jpeg due to the insufficient quality of the output for all settings I tried)

2. Surely there must be a way to downsize the image without having to resort to separate canvas code, and then do a conversion manually to draw onto the createjs Bitmap object??

I've also tried:

octx.drawImage(bitmap.image, 0, 0, oc.width, oc.height);
var smallBitmap = new createjs.Bitmap(oc);

But although very fast, this doesn't seem to actually work (and in any case I'm having to create a separate canvas element every time to facilitate this.)

I'm wondering if there is a way that I can use drawImage to draw a downsampled version of the bitmap into a createjs Bitmap instance directly without having to go via a separate canvas object or do a conversion to string?

If I understand correctly, internally this is how the createjs cache property works (i.e. uses drawImage internally to write into the DisplayObject) but I'm unable to figure out how use it myself.

回答1:

You have tagged this post with createjs and easeljs, but your examples show plain Canvas context usage for scaling.

You can use the scale parameter on Bitmap.cache() to get the result you want, then reuse the cacheCanvas as necessary.

// This will create a half-size cache (50%)
// But scale it back up for you when it displays on the stage
var bmp = new createjs.Bitmap(img);
bmp.cache(0, 0, img.width, img.height, 0.5);

// Pull out the generated cache and use it in a new Bitmap
// This will display at the new scaled size.
var bmp2 = new createjs.Bitmap(bmp.cacheCanvas);

// Un-cache the first one to reset it if you want
bmp.uncache();

Here is a fiddle to see it in action: http://jsfiddle.net/lannymcnie/ofdsyn7g/

Note that caching just uses another canvas with a drawImage to scale it down. I definitely would stay away from toDataURL, as it not performant at all.

来源：https://stackoverflow.com/questions/35977804/resizing-images-using-createjs-easeljs