composite operation create a gray outline with the latest 0.30.2

Possible bug

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

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

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.

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

  System:
    OS: macOS 12.2.1
    CPU: (8) arm64 Apple M1
    Memory: 131.50 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.7.0 - ~/.nvm/versions/node/v16.7.0/bin/node
    Yarn: 1.22.15 - ~/.yarn/bin/yarn
    npm: 7.20.3 - ~/.nvm/versions/node/v16.7.0/bin/npm
  npmPackages:
    sharp: 0.30.2 => 0.30.2 

What are the steps to reproduce?

take a transparent png, apply a composite with tiles to the background, output to jpeg or webp, a gray outline is visble (obvious on white images like attached)

inputFile

inputTile

example code 1:

let image = sharp(inputFile);
image.composite([{ input: inputTile, tile: true, blend: 'dest-over', gravity: 'northwest' }]);
image.resize({ width: 400, fit: 'outside' });
image.jpeg({ quality: 100 });
OR
image.webp({ quality: 100 });

example code 2:

let image = sharp(inputFile);
image.resize({ width: 400, fit: 'outside' });
image = sharp(await image.png().toBuffer());
image.composite([{ input: inputTile, tile: true, blend: 'dest-over', gravity: 'northwest' }]);
image.jpeg({ quality: 100 });
OR
image.webp({ quality: 100 });

output with 0.30.2

output with 0.30.1

[lovell]

Thanks John, I think I’ve found the problem, there was a extra unpremultiply taking place in sharp after composite that affects some of the dest-* blend modes.

Rather annoyingly the unit tests did pick this up at the time but I incorrectly assumed the “new” behaviour was more accurate. https://github.com/lovell/sharp/commit/c62002554b5938c43c14fefafad7353ae4d720cf#diff-7a6322e019754e2e5f843a65aa8cfb3635d77a757b88ef4c6a9450a022e5678f

[lovell]

I’ve reverted the test fixtures to their v0.30.1 state, as well as removing the unpremultiply operation when compositing, in commit https://github.com/lovell/sharp/commit/23033e2050d9ef099ecb02dde0f56f9f23e8d30d

This will be in v0.30.3, thanks for reporting.