extend() background colour changes after composite()

[rexxars]

Are you using the latest version? Is the version currently in use as reported by npm ls sharp the same as the latest version as reported by npm view sharp dist-tags.latest?

Yes.

What are the steps to reproduce?

// Create an empty 150x150 image with fully transparent background
sharp({create: {height: 150, width: 150, channels: 4, background: '#fff0'}})
  // Add a square to the center of the image, with colour #bf1942 and 0.8 alpha
  .composite([
    {
      input: {
        create: {
          width: 140,
          height: 140,
          channels: 4,
          background: '#bf1942cc',
        },
      },
    },
  ])
  // Extend the image, adding a border with the same colour and alpha as the square
  .extend({
    background: '#bf1942cc',
    bottom: 10,
    left: 10,
    top: 10,
    right: 10,
  })
  .png()
  .toFile('out.png')

What is the expected behaviour?

I would think that the square in the center and the extension to the image would have the same colour.

What is the observed behaviour?

The square and the extension has differing colours.

If I specify premultiplied: true to the first step of the composite operation, they seem to have the same colour, but not the one I am expecting (#bf1942).

If I do not run the composite() operation, the extension has the correct colour.

What is the output of running npx envinfo --binaries --system?

  System:
    OS: macOS 11.3
    CPU: (12) x64 Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
    Memory: 36.26 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.15.4 - ~/.nvm/versions/node/v14.15.4/bin/node
    Yarn: 1.22.10 - ~/.nvm/versions/node/v14.15.4/bin/yarn
    npm: 7.17.0 - ~/.nvm/versions/node/v14.15.4/bin/npm
    Watchman: 2021.06.07.00 - /usr/local/bin/watchman

[lovell]

It looks like the extend background is not premultiplied, plus it looks like resize({ fit: 'contain' } and affine() are similarly affected.

To fix this, I think we'll need to pass shouldPremultiplyAlpha as an additional parameter to ApplyAlpha that optionally premultiplies the background colour.

sharp/src/common.cc

Lines 773 to 776 in 3402656

	/*
	Apply the alpha channel to a given colour
	*/
	std::tuple<VImage, std::vector<double>> ApplyAlpha(VImage image, std::vector<double> colour) {

[lovell]

Commit 60adc11 fixes this and adds a test that would previously have failed. Thanks for reporting!

[rexxars]

Fantastic - thanks for the amazing work you're doing with sharp and libvips, @lovell ❤️

[lovell]

v0.29.1 now available.