canvas-thumbnail-cache

Draw images into a canvas square grid for fast retrieval at a thumbnail size.

Installation

npm install canvas-thumbnail-cache

Usage

import CanvasThumbnailCache from "canvas-thumbnail-cache";
import createCanvasContext from "canvas-context";
import AsyncPreloader from "async-preloader";

const { canvas, context } = createCanvasContext("2d", {
  willReadFrequently: true,
});
document.body.appendChild(canvas);

const COUNT = 50;

const thumbnailsCache = new CanvasThumbnailCache({
  context,
  slotSize: 128,
});

const items = Array.from({ length: COUNT }, (_, index) => {
  let size = [(100 + index * 10) % 200, 200];
  if (index % 2 === 0) size.reverse();
  return {
    id: index,
    src: `https://picsum.photos/${size.join("/")}`,
    loader: "Image",
    body: "blob",
    options: {
      crossOrigin: "anonymous",
    },
  };
});

items.map(async (item) => {
  const image = await AsyncPreloader.loadItem(item);

  thumbnailsCache.add(item.id, image);
});

API

Classes

CanvasThumbnailCache

Typedefs

Slot : object

Options : object

CanvasThumbnailCache

Kind: global class

• CanvasThumbnailCache
  • new CanvasThumbnailCache([options])
  • .slotDrawSize ⇒ number
  • .reset()
  • .add(key, source) ⇒ Slot
  • .get(key) ⇒ Slot
  • .remove(key)

new CanvasThumbnailCache([options])

Creates an instance of CanvasThumbnailCache.

Param	Type	Default
[options]	Options	{}

canvasThumbnailCache.slotDrawSize ⇒ number

Retrieve the slot draw size (slot size without padding)

Kind: instance property of CanvasThumbnailCache

canvasThumbnailCache.reset()

Reset and clear the canvas size and empty the thumbnails cache.

Kind: instance method of CanvasThumbnailCache

canvasThumbnailCache.add(key, source) ⇒ Slot

Add an image (or anything that can be draw into a 2D canvas) to the cache and return its slot.

Kind: instance method of CanvasThumbnailCache

Param	Type	Description
key	string	Slots map key
source	CanvasImageSource	HTMLImageElement, SVGImageElement, HTMLVideoElement, HTMLCanvasElement, ImageBitmap, OffscreenCanvas

canvasThumbnailCache.get(key) ⇒ Slot

Get a slot

The slot can also be retrieved with get and the key passed when calling thumbnailsCache.add(key, source).

Kind: instance method of CanvasThumbnailCache

Param	Type
key	string

canvasThumbnailCache.remove(key)

Remove the specified image from the cache and clear its slot.

Kind: instance method of CanvasThumbnailCache

Param	Type
key	string

Slot : object

Kind: global typedef Properties

Name	Type	Description
x	number	Horizontal position in the grid.
y	number	Vertical position in the grid.

Options : object

Kind: global typedef Properties

Name	Type	Default	Description
[context]	CanvasRenderingContext2D	createCanvasContext("2d", { offscreen: true }).context	Canvas to render thumbnails too. Will try to get an offscreen canvas by default.
[size]	number	2	Size of the canvas at start: a square with sides of length slotSize * size.
[slotSize]	number	64	Size of the thumbnails. Will be drawn from center of the grid slot.
[padding]	number	0	Padding around the thumbnails, inside the slots.

License

MIT. See license file.