imwrite: TIFF header TIFFTAG_SAMPLEFORMAT not written

asked 2018-06-29 20:42:57 -0600

jonathan
36 ●2 ●5

Writing an OpenCV Mat to a TIFF file using imwrite's default settings, I noticed that a standard TIFF tag, TIFFTAG_SAMPLEFORMAT, is not written.

Other header tags, TIFFTAG_IMAGEWIDTH, TIFFTAG_IMAGELENGTH, TIFFTAG_BITSPERSAMPLE, and TIFFTAG_SAMPLESPERPIXEL, are correctly set by OpenCV.

The OpenCV Mat I'm writing has type CV_8UC3, so it seems as though TIFFTAG_SAMPLEFORMAT should be set to SAMPLEFORMAT_UINT.

Why is this tag missing? Is there a simple method to have OpenCV generate this tag as well? I'd like to avoid rewriting this convenient one-liner with a lot of libtiff code, just to get a standard tag.

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please remember, that opencv is a computer-vision library, not photoshop.

berak ( 2018-06-30 00:20:02 -0600 )edit

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answered 2018-06-29 21:44:37 -0600

hellloooooo
36 ●2

updated 2018-06-29 21:45:12 -0600

Taking a quick look at the imgcodes module code, looks like it only writes out the TIFFTAG_SAMPLEFORMAT for 32-bit floating point sample types. Otherwise it's not set, in which case an external Tiff reader implementation would interpret the sample format as value 1 (sample format = unsigned number) if it follows the standard.

If you know some C++ and don't mind hacking the source a little, you can probably add a couple lines in this function in the source: https://github.com/opencv/opencv/blob... to get it to write out the TIFFTAG_SAMPLEFORMAT based on the data type that's being used.

However I suggest creating a feature request on OpenCV's Github (https://github.com/opencv/opencv/issues) if you believe this is a standard tag and should be standard.

As a sidenote, isn't TIFFTAG_SAMPLEFORMAT is an extension tag and not a baseline one? I'm not a Tiff genius but it seems like this tag should be standard, although it's not a baseline Tiff tag.

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Aha. Thank you, that's the key right there: the TIFF spec specifies a default, so OpenCV is intelligently omitting SampleFormat. In my case, the consuming application is in error by requiring the tag.

Looks like floating-point support was added to the TIFF standard late (TIFF 6.0), so the tag should continue to be an extension, and OpenCV's doing it right.

jonathan ( 2018-07-02 14:30:19 -0600 )edit

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imwrite: TIFF header TIFFTAG_SAMPLEFORMAT not written

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