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Band names for arrow exported images #9099

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Band names for arrow exported images #9099

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@wiredfool wiredfool commented Jul 19, 2025 •
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Addresses #8329 (comment) . see also #8329 (comment)
Changes proposed in this pull request:

  • Add the mode name as the schema name instead of "pixel"
  • Add image band metadata in the arrow schema
  • Tests for metadata interop
  • Client testing with arro3 and nanoarrow similar to the pyarrow tests

For multiband images, we issue:

ArrowSchema:
  Format: FixedLengthArray[4]
  Children: 
    - ArrowSchema:
       Format: uint8
       name: RGB
       metadata: {image: {bands: [R, G, B, X]}}

This metadata is accessible using pa.array(img).type.field(0).metadata

For single band images we emit:

ArrowSchema:
  Format: ...
  name: F
  metadata: {image: {bands: [F]}}

This metadata is not apparently accessible via pyarrow or arro3, but is accessible via nanoarrow.

The ultimate goal is to be able to have:

  • numpy/pandas/arrow users be able to do np.array(img) and be able to identify RGB channel names
  • height/width would be ideal

@rok

@wiredfool wiredfool marked this pull request as draft July 19, 2025 15:38
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rok commented Jul 20, 2025

@paleolimbot am I right to say array metadata was not really designed to pass application metadata? What are your thoughts on what's being done here?
@wiredfool has the issue here that he can't access metadata of children when in certain cases.
I'm suggesting usage of FixedShapeTensorArray, but I imagine working with arrays directly would simplify things here.

@wiredfool wiredfool force-pushed the pyarrow_band_names branch from e768bbf to 28c7645 Compare July 21, 2025 09:21
@wiredfool wiredfool force-pushed the pyarrow_band_names branch from 14ac76c to 7d2abbd Compare July 21, 2025 09:23
@wiredfool
Added flat image metadata tests
c07fe6e 
This metadata is available in nanoarrow, but not pyarrow or arro3
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Cool!

It's true that field metadata at anything other the top-level struct has a medium chance of getting propagated through various Arrow operations. If you used the two metadata fields ARROW:extension:name (== pillow.image) and ARROW:extension:metadata as the JSON you're proposing above at the Array level, I think you will have more success getting this to propagate correctly. For pyarrow you'd have to implement a minimal ExtensionType class that a (pyarrow) user would have to "register" for that type to show up nicely in pyarrow.

Feel free to ping on anything if you have a question!

@wiredfool wiredfool force-pushed the pyarrow_band_names branch from ce3c7c9 to 9e415c7 Compare July 21, 2025 15:28
@radarhere radarhere mentioned this pull request Jul 22, 2025
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@wiredfool wiredfool marked this pull request as ready for review October 10, 2025 14:40
radarhere
radarhere reviewed Oct 11, 2025
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radarhere
radarhere reviewed Oct 11, 2025
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Tests/test_arro3.py
Comment on lines +158 to +159
img2 = img.copy()
px = img2.load()
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Suggested change
img2 = img.copy()
px = img2.load()
px = img.load()

Or are you using copy() because you're testing that the data in img2 is still correct after img is no longer used?

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This is to ensure we haven't over freed the memory, as we're refcounting on the arrow usages.

@wiredfool @radarhere
Update Tests/test_arro3.py
52413cf 
Co-authored-by: Andrew Murray <[email protected]>
radarhere
radarhere reviewed Oct 11, 2025
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