Does converting color space degrade image?

[Satchidananda Ashram]

Let's say I convert an sRGB image to RGB1998, and then convert it back again later, does information get lost? Does the image degrade? In other words is converting an image to a different color space something that should not be overdone?

Thanks.

[DonLashier]

This is not an expert opinion (I'm sure a color expert will chime in) but this depends on the gamut of the color spaces involved and also probably whether you're working in 8 bits or 16 bits.

For instance, you definitely can lose information if you convert from a larger color space to a smaller then back (eg aRGB->sRGB->aRGB). Even the other way around may lose info due to rounding if you're working in 8 bits or if you made edits that pushed into the larger color space. I generally avoid color space conversion using the same space from RC->Photoshop and only converting for final output if necessary (eg to sRGB for web, CMYK for press).

- DL

[AndrewRodney]

Originally Posted by Satchidananda Ashram

Let's say I convert an sRGB image to RGB1998, and then convert it back again later, does information get lost? Does the image degrade? In other words is converting an image to a different color space something that should not be overdone?

Thanks.

Technically, yes. All color space conversions, for that matter any alteration to a value of a pixel (in any color space) will result in data loss due to rounding errors. But we all have to do this since the idea is to either edit the image to appear as we wish or to produce a color space ideal for printing or other ouptut.

IF you do this in high bit (more than 8-bits per color) you lose some data but have enormous overhead so it doesn't matter. But the data loss is there.

BTW, Photoshop does all its color space conversions using 21 bit precision. I'd still try to do all my work in more than 8-bits per color:

http://staging.digitalphotopro.com/t...-decision.html

[David_Buzzard]

I'm far from a colour maven, but I do have some rules of thumb. It's always better to shoot in the wider Adobe 1998 gamut, and if you're going to convert to sRGB, do it at the beginning of your work flow. I could see problems going from the narrower sRGB to Adobe RGB, but I don't have any personal experience (i.e. I wouldn't recommend doing it, and I don't do it myself). I'm sure there's loss involved with converting from Adode 1998 to sRGB at the end of the workflow, but I do it all the time, and haven't had any problems.

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[AndrewRodney]

Originally Posted by David_Buzzard

I could see problems going from the narrower sRGB to Adobe RGB, but I don't have any personal experience (i.e. I wouldn't recommend doing it, and I don't do it myself).

It buys you nothing. The ONLY reason I could see doing this is if you wanted to paste part of an sRGB document into an Adobe RGB (1998) document. If you want to preserve the color appearance, you must convert.

If you play with the Paste Mismatch options in Photoshop, you'll see this. When you copy and paste from sRGB into Adobe RGB (1998), you will get a warning. You're asked if you want to preserve color numbers (don't covert) or preserve color appearance (convert). You only have two options. Nearly all users want the image when pasted to appear as it originally did. You must preserve color appearance meaning you must convert the data.

Going from Adobe RGB (1998) (or ProPhoto to add to the mix) to sRGB is fine. You're moving from a larget to a smaller color gamut.

[Noel_Carboni]

It's always better to shoot in the wider Adobe 1998 gamut

Not meaning to offend, David, but this is a tremendous oversimplification, to the point where it's essentially wrong. Wider is not always better. Expose such that all levels in the image are within the sRGB gamut and you've got finer definition between subtle colors. This could be important, e.g. when a pastel sky is in the image.

Shoot raw and it doesn't matter, except that the embedded JPEG is in the color space it's more likely to be displayed in by an app that doesn't sport color management (e.g., Irfan View).

If you happen to have your system set up for end to end processing in sRGB, shooting in sRGB simplifies things greatly as there's never the chance that "Save As JPEG" will result in a file saved with a color space a browser or other app doesn't understand, or that a conversion will clip highlights or shadows.

How many people have read statements like "Use Adobe RGB because it has a wider gamut" and, without knowing why have done so, then suffered with images that don't look quite right because the color space isn't interpreted by whatever apps they're using. Granted, apps are getting smarter, but they're not there yet.

One must try to keep in mind there are very few simple "this way is best" answers. Canon wouldn't put sRGB on the camera if there was no good reason to have it.

-Noel

[David_Buzzard]

I like simple.

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