Wrong colors in composite with blend:'dest-in' + resize since v0.30.2

[tophf]

• Running npm install sharp completes without error.
• Running node -e "require('sharp')" completes without error.
• I am using the latest version of sharp as reported by npm view sharp dist-tags.latest.

What is the output of running npx envinfo --binaries --system --npmPackages=sharp --npmGlobalPackages=sharp?

  System:
    OS: Windows 10 10.0.19044
    CPU: (8) x64 Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz
    Memory: 8.27 GB / 31.95 GB
  Binaries:
    Node: 16.7.0 - C:\app\dev\nodejs\node.EXE
    Yarn: 1.22.17 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 7.20.3 - C:\app\dev\nodejs\npm.CMD
  npmPackages:
    sharp: ^0.30.7 => 0.30.7

What are the steps to reproduce?

1. Resize a semi-transparent image using the code below
2. The result is an overexposed image

What is the expected behaviour?

The image is half-transparent as shown in the table below.

Please provide a minimal, standalone code sample, without other dependencies, that demonstrates this problem

require('sharp')('icon.png').composite([{
  input: Buffer.from([255, 255, 255, 256 * 0.5]),
  raw: { width: 1, height: 1, channels: 4 },
  tile: true,
  blend: 'dest-in',
}]).resize({ width: 64 }).toFile('icon-t.png');

Type	Source	Expected	Observed
Image

Looking at v0.30.1...v0.30.2 I guess the bug was introduced in c620025 released in 0.30.2. Further investigation showed it was partly fixed - for non-resized composite - in 23033e2 released in 0.30.3.

[lovell]

Thanks for reporting this, the changes to composite you discovered did cause this. Commit a618ce7 now ensures the (implicit) unpremultiply operation occurs before composite and adds a test that would previously have failed. This will be included in v0.31.0.

[lovell]

v0.31.0 now available with the fix, thanks for reporting this.