Enhancement: expose vips_relational operation

[Cristy94]

I want to process my images such that only pixels that are a given color (eg: RGB(0,0,100)) will appear in the final image as black, the rest should be white.

Is there any way to add a custom filter/function that is called for each pixel before it is added to the output?

[lovell]

Hello, the vips_relational equal function should be able to do this but it is not yet exposed in sharp. PRs always welcome.

In the meantime, it might be possible for you to achieve something similar with a combination of the existing boolean and threshold operations. Perhaps you could experiment and report back here.

[Cristy94]

Thanks for your response! As of now I solved my problem using another library, JIMP, https://www.npmjs.com/package/jimp#low-level-manipulation-

They expose a function that enables you to set the value of each pixel individually inside a callback function. Although it is no way near as efficient as sharp, it was good enough for my needs (one time conversion of images).

[lovell]

If you're having to write your own logic, perhaps try raw to access raw pixel data, something like (untested, assumes 3 channel input):

sharp(input)
  .raw()
  .toBuffer((data, info) => {
    for (let i = 0; i < info.length; i += 3) {
      const r = data[i];
      const g = data[i + 1];
      const b = data[i + 2];
      // your logic to process a pixel
    }
  });