Can you convert between sRGB or Adobe RGB and Lab without quality loss?
Asked 7/6/2014
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If I convert an image from sRGB or Adobe RGB into Lab color space and later convert it back, will the image remain unchanged? Since Lab is a larger color space, I assume colors from sRGB or Adobe RGB should fit inside it without clipping. Is the round trip effectively lossless, and does bit depth affect the result?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Because, as you say, L*a*b* is a superset larger than either sRGB or Adobe RGB, you're right, this is safe — if you have enough bit depth to prevent color mapping errors. In practice, this means that if you are working in 16 bits per channel, there is no loss. If you are working in 8 bits per channel, there may be some, as you are spreading your "crayons" quite thin (see the post linked above).
I say "in practice", because there is a very small chance that even when working in 16 bits, if you go back to 8 bit for results, you may find that a value which was represented as (say) "127" gets mapped to something which, due to rounding, comes back as "128". This would only happen in borderline conditions, and it's incredibly unlikely that anyone could ever tell the difference even under the closest inspection. And, since you're most likely doing something to the image in the meantime (working in that Lab* space), any potential minute errors are going to be totally lost in even the most minor of "real" adjustments.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
12y ago
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Mostly yes in practice, but not perfectly lossless in all cases. Because Lab is larger than sRGB or Adobe RGB, converting into Lab should not clip colors from those spaces. If you convert back to the original RGB space without making other changes, the main issue is precision: the conversion can introduce rounding or mapping errors.
With high precision data—such as 16-bit per channel or floating point with sufficient precision—the round trip is generally safe and any differences are usually too small to see. With 8-bit data, small shifts are more likely because there are fewer tonal steps available.
So the practical answer is:
- no clipping just from going RGB → Lab → RGB
- possible tiny numeric differences from rounding
- 16-bit or floating point makes those differences negligible
- 8-bit is more likely to show small errors if you repeat conversions or do heavy edits
In short: visually, a Lab round trip is usually safe if you use enough bit depth, but it is not mathematically guaranteed to be perfectly lossless.
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