4 channel LZW TIFF output returns a value of 3 for info.channels

[BryceCicada]

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I'm trying to create a 4-channel tiff from raw pixel data. Unfortunately, when this is written to disk, the image has only 3 channels. Here's snippet using sharp 0.24.0

import * as sharp from 'sharp'

async function main () {
  const width = 32
  const height = 32
  const channels = 4
  const info = await sharp(
    Buffer.from(new Array(width * height * channels).fill(0)),
    {
      raw: { width, height, channels }
    }
  ).toFile('test.tif')

  if (info.channels !== channels) {
    throw new Error(`Unexpected number of channels in output image. Expected ${channels}, but got ${info.channels}`)
  }
}

main()

Issue also occurs when saving jpgs (eg toFile('test.jpg')). Does not occur saving pngs.

I can't see anything obvious in the docs that excludes this possibility. Advice appreciated.

[BryceCicada]

If I set the tiff compression to 'lzw', then the output saves to disk as 4-channel tiff. The returned info still has info.channels === 3 though:

  const info = await sharp(
    Buffer.from(new Array(width * height * channels).fill(0)),
    {
      raw: { width, height, channels }
    }
  )
  .tiff({
    compression: 'lzw'
  })
  .toFile('test.tif')

[lovell]

The default TIFF compression setting is jpeg, which only supports 3 channels, so a 4th channel if any is considered alpha and flattened out.

As you've discovered, the lzw compression setting does support 4 channels, so the value of 3 returned in info.channels looks like a bug.

[BryceCicada]

Thanks for the advice about jpeg and 3 channels. Minor brain fail on that one.

Triage to bug for lzw image info appreciated.

[lovell]

Fixed and tests added in commit a2314c4. This will be in v0.24.1, thanks for reporting.

[lovell]

v0.24.1 now available.