Pillow TiffImagePlugin doesn't scale 0-255 16bit palette

What did you do?

Read in a tiff that is a uint8 raster with a RGB colortable of 256 values.

What did you expect to happen?

When using tmp = im.convert() the image would be converted to a 256 RGB image.

What actually happened?

The image was all black as the color table was read incorrectly.

What versions of Pillow and Python are you using?

Python 3.4. Pillow 3.2.0.

self.tag_v2
<PIL.TiffImagePlugin.ImageFileDirectory_v2 object at 0x0000000009046D68>
self.tag_v2[320]
(0, 0, 1, 1, 2, 3, 5, 7, 9, 10, 11, 11, 12, 11, 11, 11, 10, 9, 9, 8, 8, 8, 7, 7, 6, 5, 5, 4, 4, 3, 3, 3, 4, 4, 3, 3, 3, 2, 2, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 3, 3, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 21, 22, 24, 26, 27, 29, 30, 31, 33, 34, 36, 37, 38, 40, 41, 42, 44, 45, 46, 48, 49, 50, 52, 53, 54, 56, 57, 59, 60, 61, 63, 64, 66, 67, 69, 70, 72, 73, 75, 77, 78, 80, 82, 83, 85, 87, 88, 90, 92, 94, 95, 97, 99, 101, 102, 104, 106, 108, 109, 111, 113, 115, 116, 118, 119, 121, 122, 124, 126, 128, 130, 132, 133, 135, 137, 139, 141, 143, 144, 146, 148, 150, 152, 154, 155, 157, 159, 161, 162, 164, 166, 168, 169, 171, 173, 174, 176, 177, 179, 181, 182, 184, 185, 187, 188, 190, 191, 193, 194, 195, 197, 198, 200, 201, 202, 204, 205, 206, 207, 209, 210, 211, 212, 214, 215, 216, 217, 218, 219, 220, 222, 223, 224, 225, 226, 227, 228, 229, 229, 230, 231, 232, 233, 234, 234, 235, 236, 236, 237, 238, 238, 239, 239, 240, 241, 241, 242, 242, 242, 243, 243, 244, 244, 244, 245, 245, 245, 246, 246, 246, 247, 247, 247, 248, 248, 248, 248, 249, 249, 249, 250, 250, 250, 251, 251, 252, 252, 253, 253, 253, 254, 255, 0, 1, 2, 4, 5, 6, 6, 7, 8, 9, 10, 11, 12, 13, 14, 14, 15, 16, 17, 17, 18, 18, 19, 20, 20, 21, 21, 22, 23, 23, 24, 25, 25, 26, 27, 28, 28, 29, 30, 31, 31, 32, 33, 34, 34, 35, 36, 37, 38, 39, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 179, 180, 181, 182, 183, 184, 185, 186, 187, 189, 190, 191, 192, 193, 194, 196, 197, 198, 199, 200, 202, 203, 204, 205, 206, 208, 209, 210, 211, 213, 214, 215, 216, 218, 219, 220, 221, 222, 224, 225, 226, 227, 228, 230, 231, 232, 233, 234, 235, 237, 238, 239, 240, 241, 242, 243, 245, 246, 247, 248, 249, 250, 251, 252, 253, 255, 0, 3, 8, 13, 18, 22, 26, 29, 32, 34, 37, 40, 42, 45, 48, 50, 53, 56, 58, 61, 63, 66, 68, 71, 73, 75, 78, 80, 82, 84, 86, 88, 89, 91, 93, 95, 97, 98, 100, 102, 104, 106, 108, 109, 111, 113, 114, 116, 117, 118, 120, 121, 122, 124, 125, 126, 127, 127, 128, 129, 129, 130, 130, 131, 132, 132, 133, 133, 134, 134, 135, 135, 136, 136, 136, 137, 137, 137, 137, 138, 138, 138, 138, 138, 138, 138, 138, 138, 138, 138, 138, 137, 137, 137, 137, 136, 136, 136, 135, 135, 135, 134, 134, 133, 133, 132, 132, 131, 131, 130, 129, 129, 128, 127, 127, 126, 125, 125, 124, 123, 123, 122, 121, 121, 120, 119, 119, 118, 119, 118, 117, 116, 114, 113, 112, 110, 109, 107, 106, 104, 102, 100, 99, 97, 95, 93, 91, 90, 88, 86, 84, 83, 81, 79, 78, 76, 75, 73, 72, 71, 69, 68, 67, 66, 66, 65, 64, 64, 63, 63, 63, 63, 62, 62, 62, 62, 63, 63, 63, 63, 63, 64, 64, 64, 65, 65, 66, 66, 67, 67, 68, 69, 70, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 81, 82, 84, 85, 87, 89, 91, 93, 96, 98, 101, 103, 106, 109, 112, 115, 118, 122, 125, 128, 132, 136, 139, 143, 147, 151, 155, 158, 162, 166, 170, 174, 178, 182, 187, 191, 195, 199, 203, 207, 210, 214, 218, 222, 226, 230, 233, 237, 241, 244, 248, 251, 254)

On line 1264 in TiffIMagePlugin.py.
palette = [o8(b // 256) for b in self.tag_v2[COLORMAP]]
self.tag_v2[COLORMAP] colormap listed above. 
palette
[b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', b'\x00', 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the colormap is not being parsed correctly.

Oct 06 '16 20:10 isaacgerg

Can you upload a sample image?

Oct 10 '16 09:10 wiredfool

test.zip

Uploaded as zip as .tif not supported as github upload-able.

Oct 24 '16 15:10 isaacgerg

If you do Image.open(filename), you will get an all black image. However, if you open it with another viewer (windows photo viewer), you will see a blue and yellow pattern.

Oct 24 '16 20:10 isaacgerg

I think that we're parsing the colormap to spec.

From the tiff spec:

ColorMap Tag = 320 (140.H) Type = SHORT N = 3 * (2**BitsPerSample)

This field defines a Red-Green-Blue color map (often called a lookup table) for palette color images. In a palette-color image, a pixel value is used to index into an RGB-lookup table. For example, a palette-color pixel having a value of 0 would be displayed according to the 0th Red, Green, Blue triplet. In a TIFF ColorMap, all the Red values come first, followed by the Green values, then the Blue values. In the ColorMap, black is represented by 0,0,0 and white is represented by 65535, 65535, 65535.

Pillow doesn't have the ability to use a 16bit colormap, so it's converted to an 8 bit colormap on load.

The colormap in this image is represented by a max of 255/65535 which truncates to 0 and rounds to 1 as a uint8. We're truncating, which is a choice, but either is a reasonable interpretation of a 16bit short palette entry as a 8 bit number. At any rate, unless we stretched the colormap, we'd get a max of 0 or 1 for a color value. I've verified from the file that we're interpreting the colormap tag with the correct endianness.

If you want to reinterpret the palette, you can to something like: (at least in 2.7, 3.x would be bytes)

im.palette = ImagePalette.raw("RGB;L", str(bytearray(im.tag_v2[320])))
im.show()

Or, instead of using the tag directly, you could stretch by the max value or similar.

Nov 30 '16 11:11 wiredfool

I am a bit confused. Windows is able to read in and display the tiff correctly. Also, the palette I am passing should be uint8 and 255 entries. Are you saying that tiff palettes should be always 16bit? Does the attached image not meet the spec?

Nov 30 '16 20:11 isaacgerg

The palette in the file is little endian 16bit, so in hex in the file it's 0x00 00 00 00 01 00 01 00 02 00 ... This converts to a sequence of 16bit integers of 0 0 1 1 2..., as seen in tag 320. The available range of the numbers in 320 is 0->65k. Pillow is mapping that range of 0->65k to the available 0-255 by dividing by 256 (equivalent to taking the most significant byte). It appears that the windows viewer is mapping that 0->65k to 0->255 by taking the least significant byte.

The problem with tiff images is that it's a relatively old format, and very flexible, so nearly anything can be encoded in a tiff. Understanding it is still a challenge.

I suspect what the windows viewer is doing is taking the LSB if the max palette value is <256, and scaling if it's more. But without throwing other in spec tiffs at it, I couldn't tell.

Nov 30 '16 21:11 wiredfool

Thank you. I thought we were storing and 8bit palette. Will double check our code and then close/comment depending on the outcome.

I suspect what the windows viewer is doing is taking the LSB if the max palette value is <256, and scaling if it's more. But without throwing other in spec tiffs at it, I couldn't tell.

Would it make sense to do this in pillow?

Nov 30 '16 21:11 isaacgerg

Pillow multiplies the palette we're writing by 256 on save, so we're using the full space.

It might make sense to use an LSB palette if all of the elements are < 255. I don't think it's strictly correct, but it may be convenient.

Nov 30 '16 21:11 wiredfool

It looks like our tiff is displayed correctly in ubuntu's viewer. It must be checking if all the elements are less than 256. I'll leave it up to you guys to decide whether to "fix" this or close it. Thanks for all your help.

Dec 01 '16 17:12 isaacgerg

Testing the image, macOS Preview and GIMP read it as black. ImageMagick reads the image as yellow and blue.

Mar 10 '22 07:03 radarhere

Given that discussion has agreed that Pillow is interpreting the image according to the specification, and I've found that other software doesn't agree on what to do, I'm going to close this.

It would seem to me that if an image just happened to have a palette where all the elements where less than 255, adjusting for this image would break that one.

Nov 03 '22 10:11 radarhere