recomb: bands in must equal matrix width, greyscale image

[CharlieHess]

• Are you using the latest version? Is the version currently in use as reported by npm ls sharp the same as the latest version as reported by npm view sharp dist-tags.latest?
  Yes

• What are the steps to reproduce?

sharp().recomb([
  [0.3588, 0.7044, 0.1368],
  [0.299, 0.587, 0.114],
  [0.2392, 0.4696, 0.0912],
])

On the image below (taken from TMDb's API). You'll get an error:

recomb: bands in must equal matrix width

• What is the expected behaviour?
  The recomb operation succeeds instead of throwing

• Are you able to provide a minimal, standalone code sample, without other dependencies, that demonstrates this problem?
  Code snippet above, combined with a pipe, is all you need

• Are you able to provide a sample image that helps explain the problem?
  The image below, taken from TMDb's backdrop_image for Malcolm & Marie: https://image.tmdb.org/t/p/w1280/iOrjCeIyRb4vDY3vbPSaEBCA1D3.jpg

• What is the output of running npx envinfo --binaries --system?

  System:
    OS: macOS 11.0.1
    CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
    Memory: 568.23 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 12.18.1 - ~/.nvm/versions/node/v12.18.1/bin/node
    Yarn: 1.22.4 - ~/.yarn/bin/yarn
    npm: 6.14.5 - ~/.nvm/versions/node/v12.18.1/bin/npm

[lovell]

Hi, this sample input is a single-channel greyscale image. It looks like the recomb operation is correctly converting to sRGB but there's a bug where it uses the wrong channel count, in this example image.bands() will be 1, from the single-channel image.

sharp/src/operations.cc

Lines 152 to 154 in df7b8ba

	return image
	.colourspace(VIPS_INTERPRETATION_sRGB)
	.recomb(image.bands() == 3

[CharlieHess]

Ah, good info @lovell thanks. In the meantime I've worked around it by checking .metadata().channels and skipping this step in my pipeline for greyscale images.

[lovell]

Commit f6f16b9 adds a test for this, as well as fixing it. This will be in v0.27.2, thanks for reporting.

[lovell]

v0.27.2 now available with the fix.