LZW decompression

[taram73658370]

Hi,

I am asking this question for preservation purposes. And understand this may be a complex issue.

I am wondering about data integrity of uncompressed TIFF files that were previously LZW files.

i.e. Rotation of files in Windows image viewer has applied LZW compression and I want to remove this compression for long-term preservation.

In theory, no data is removed with LZW compression so the algorithm can be reversed by opening the image in Photoshop and resaving it with no compression.

Will the data integrity be exactly the same as if these items were imaged originally using uncompressed methods? And does Adobe have any literature to support this?

Thanks,

Tara

[Mylenium]

I'm not clear what you mean by "integrity". Once you actually open the image and resave it, the original data structure is gone, anyway. The rest entirely depends on how the programs handle it and whether or not something goes wrong during the saving process. Otherwise none of them will insert extra pixel data, unless the image gets resampled or manipulated otherwise. Only the headers and markers in the binary data will change and deflating the image to an uncompressed format will simply re-insert all those redundant bits and bytes that the compression removed in the first place. Still, the actual data structure will have changed, if only slightly.

Mylenium

[taram73658370]

Very helpful response. Thank-you.

[Test Screen Name]

In theory and in practice LZW and ZIP compression keep things exactly. There is no reason at all to work uncompressed. Adobe does not need any literature, it’s an industry standard. Lempel–Ziv–Welch - Wikipedia

https://www.google.co.uk/amp/s/havecamerawilltravel.com/photographer/tiff-image-compression/amp/

[taram73658370]

Thanks for your responses.

I agree there is no reason to work uncompressed. The reason we work uncompression is that LZW code is still somewhat a proprietary product and in our industry (culture heritage preservation) open source coding formats are preferred for final images.

We were able to verify the information I needed through the published patent.

https://patentimages.storage.googleapis.com/2c/fc/25/2b29895a953f8f/US4558302.pdf

Claim 206. Which basically confirms image content in equal the same as image content out.

[Test Screen Name · Accepted solution]

The LZW patent expired a few years ago, and so open source LZW is available and does not offer IP issues. However, when preserving for future generations it's as well to be conservative.

Uncompressed does offer one advantage: if there is disk damage (a huge rarity today but who knows the future), in an uncompressed file there is a reasonable change that the data will just have small damage. A compressed file of any kind is much more likely to lose much or all of the image. Curiously, movie formats are made to recover from this.

[taram73658370]

Good to know. Thank-you 🙂