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

Typedefs

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.