README.md

Color Dropper

Demo
Sandbox

Recommended IDE Setup

VS Code + Svelte.

Possible improvements

1. Don't scale images in drawImage().
   Cache various image sizes on an offscreen canvas when loading instead of constantly scaling them in drawImage().

Current technical issues

In some cases, the ColorDropper component does not work properly with Workers because of a function serialization problem.
When we register a new worker via the useWorker property, we need to serialize and deserialize data for transferring between threads.
This is where the problem with render functions and their closures comes up.
Serialization works correctly only for functions without closures.

For example, this works correctly, because the render function does not have a closure with external variables:

<ColorDropper useWorker>
  <Layer
    render={({ context }) => {
      const bounds = { x0: 160, y0: 160, x1: 480, y1: 480 };
      let { x0, y0, x1, y1 } = bounds;
      context.globalAlpha = 0.9;
      context.fillStyle = 'tomato';
      context.fillRect(x0, y0, x1 - x0, y1 - y0);
      context.globalAlpha = 1;
    }}
  />
</ColorDropper>

But this does not work, because the render function has a doSomething function and variable in the closure:

<ColorDropper useWorker>
  <Layer
    render={({ context }) => {
      doSomething(variable);
    }}
  />
</ColorDropper>

Examples

Without Worker:

let imageSource: CanvasImageSource | null = null;

onMount(async () => {
  const image = new Image();
  image.src = 'images/your-image.jpg';

  image.onload = async () => {
    const imageBitmap = await createImageBitmap(image);
    imageSource = imageBitmap;
  };
});

<ColorDropper useProxyCanvas {imageSource}>
  <Layer
    render={({ context, width, height, imageSource }) => {
      if (!imageSource) return;
      context.drawImage(imageSource, 0, 0, width, height);
    }}
  />
</ColorDropper>

const colors = ['tomato', 'goldenrod', 'mediumturquoise'];

<ColorDropper useProxyCanvas>
  {#each colors as color, i (color)}
    {@const c = (i + 1) * 85}
    <Layer
      render={({ context }) => {
        const bounds = { x0: c, y0: c, x1: c + 338, y1: c + 338 };
        const { x0, y0, x1, y1 } = bounds;
        context.globalAlpha = 0.9;
        context.fillStyle = color;
        context.fillRect(x0, y0, x1 - x0, y1 - y0);
        context.globalAlpha = 1;
      }}
    />
  {/each}
</ColorDropper>

With Worker:

let imageSource: CanvasImageSource | null = null;

onMount(async () => {
  const image = new Image();
  image.src = 'images/your-image.jpg';

  image.onload = async () => {
    const imageBitmap = await createImageBitmap(image);
    imageSource = imageBitmap;
  };
});

<ColorDropper useWorker {imageSource}>
  <Layer
    render={({ context, width, height, imageSource }) => {
      if (!imageSource) return;
      context.drawImage(imageSource, 0, 0, width, height);
    }}
  />
</ColorDropper>

<ColorDropper useWorker>
  <Layer
    render={({ context }) => {
      const bounds = { x0: 160, y0: 160, x1: 480, y1: 480 };
      let { x0, y0, x1, y1 } = bounds;
      context.globalAlpha = 0.9;
      context.fillStyle = 'tomato';
      context.fillRect(x0, y0, x1 - x0, y1 - y0);
      context.globalAlpha = 1;
    }}
  />
</ColorDropper>