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v0.31.1: palette-based RGBA PNG output can add transparent pixels

See original GitHub issue

Possible bug

After upgrading to 0.31.1, compress is changing image color. We are sending a png buffer to sharp to change it’s quality to 95% and it was working fine. But now it’s changing adding these black color Image attached below

Is this a possible bug in a feature of sharp, unrelated to installation?

yes

Running npm install sharp completes without error.
Running node -e "require('sharp')" completes without error.

If you cannot confirm both of these, please open an installation issue instead.

Are you using the latest version of sharp?

I am using the latest version of sharp as reported by npm view sharp dist-tags.latest.

If you cannot confirm this, please upgrade to the latest version and try again before opening an issue.

If you are using another package which depends on a version of sharp that is not the latest, please open an issue against that package instead.

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

  System:
    OS: Linux 5.15 Ubuntu 20.04.4 LTS (Focal Fossa)
    CPU: (24) x64 12th Gen Intel(R) Core(TM) i9-12900K
    Memory: 88.28 GB / 125.61 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 16.16.0 - ~/.volta/tools/image/node/16.16.0/bin/node
    Yarn: 3.2.3 - ~/.volta/tools/image/yarn/1.22.19/bin/yarn
    npm: 8.11.0 - ~/.volta/tools/image/node/16.16.0/bin/npm
  npmPackages:
    sharp: ^0.31.1 => 0.31.1

What are the steps to reproduce?

provide a png to sharp

const finalImage = await sharp(renderedImage).png({ quality: 94 }).toBuffer();

What is the expected behaviour?

It should work like before without adding those black colors

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

// canvas is @napi-rs/canvas and it's output is working fine. The issue occurs only when passed to shard.
const renderedImage = canvas.png();
const finalImage = await sharp(renderedImage).png({ quality: 94 }).toBuffer();