TIFF output, bitdepth of 1 with LZW compression, should ignore predictor

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.
• 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 10.0.22621
CPU: (6) x64 Intel(R) Core(TM) i5-8600K CPU @ 3.60GHz
Memory: 20.28 GB / 63.95 GB
Binaries:
Node: 19.2.0 - C:\Program Files\nodejs\node.EXE
npm: 9.1.3 - C:\Program Files\nodejs\npm.CMD

(tried also node 16.x and npm 8.x)

What are the steps to reproduce?

image.toColorspace('b-w').toFormat('tiff', { bitdepth: 1 }) // works, exports a 1-bit tiff file
image.toColorspace('b-w').toFormat('tiff', { bitdepth: 1, compression: 'lzw' }) // works, but exports a 24-bit file.

What is the expected behaviour?

the second row will export a 1-bit (or 2 or 4) bit TIFF file.

Thanks for all ! I like the package and it works good and fast.

Start by running the two image.toColorspace('b-w').toFormat('tiff', ...) examples from the issue and compare their output bit depth. Trace the TIFF conversion path used by sharp for bitdepth 1 with LZW compression, then verify that the resulting 1-, 2-, or 4-bit TIFF behavior matches the expected result.