Grey scale image decoder color type : InterpretationWithBits(RGBPalette, [8])

[aguennecjacq]

Hi,
Maybe this is just me being new to rust, but i've had an issue reading the color type from a greyscale 8bits tif image.
Here is the image I've had some issues with : Barbara.tif
When trying to read the color type of the image from the decoder I get the following error:
UnsupportedError(InterpretationWithBits(RGBPalette, [8])).

This also leads to an incorrect reading in the image crate image open function: image::open() ->
Unsupported(UnsupportedError { format: Exact(Tiff), kind: GenericFeature("RGBPalette with [8] bits per sample is unsupported") }).

[Jondeen]

Just to chime in, although somewhat unrelated, this image is not technically greyscale. Instead, it utilizes the ColorMap feature specified in the TIFF format, which employs a lookup table (e.g., x => RGB(r, g, b)). While the data storage type is 8-bit, the lookup table provides 16bit RGB colors.

Here's the tiffinfo output for Barbara.tif:

      $ tiffinfo -r Barbara.tif
      TIFFReadDirectory: Warning, Unknown field with tag 37724 (0x935c) encountered.
      TIFF Directory at offset 0x8 (8)
        Subfile Type: (0 = 0x0)
        Image Width: 512 Image Length: 512
        Resolution: 72, 72 pixels/inch
-->     Bits/Sample: 8
        Compression Scheme: None
-->     Photometric Interpretation: palette color (RGB from colormap)
        Samples/Pixel: 1
        Rows/Strip: 512
        Planar Configuration: single image plane
-->     Color Map: (present)
        Photoshop Data: <present>, 5314 bytes
        Tag 37724: 0x41,0x64,0x6f,0x62,0x65,0x20,0x50,0x68,0x6f,0x74,0x6f,0x73,0x68,0x6f,0x70,0x20,0x44,0x6f,0x63,0x75,0x6d,0x65,0x6e,0x74,0x20,0x44,0x61,0x74,0x61,0x20,0x42,0x6c,0x6f,0x63,0x6b,0x0

Barbara.tif contains a lookup table that does indeed generate greyscale colors, but they are techincally represented as 16bit RGB:

Reds: (0, 257, ... , 65278, 65535)
Greens: (0, 257, ... , 65278, 65535)
Blues: (0, 257, ... , 65278, 65535)

Finally, why its not working for you:
Looking into the source of the Rust package, ColorMaps are not yet implemented:

image-tiff/src/tags.rs

Line 79 in a67f505

ColorMap = 320, // TODO add support

Some TIFF libraries collapse the color resolution if the color map is consistently grey scale, and some retain it at its full resolution.