Luminous Landscape Forum

Raw & Post Processing, Printing => Colour Management => Topic started by: simplify on August 27, 2010, 08:38:52 PM

Title: Prophoto in 8bit?
Post by: simplify on August 27, 2010, 08:38:52 PM
I have heard some say not to use prophoto in 8bit.  If I am doing profiled 8bit scans and then converting to to prophoto rgb, am I better off converting to j Holmes ektaspace.  Prophoto is a bigger colorspace, but what are the issues with 8bit
Title: Re: Prophoto in 8bit?
Post by: NikoJorj on August 28, 2010, 10:58:03 AM
Banding.
Title: Re: Prophoto in 8bit?
Post by: Eric Myrvaagnes on August 28, 2010, 12:50:57 PM
Nicolas,

Isn't it the case that if you convert immediately to 16-bit prophoto, you shouldn't get any banding that isn't already there in the scan?

Eric
Title: Re: Prophoto in 8bit?
Post by: digitaldog on August 28, 2010, 01:23:47 PM
Actually, according to Kodak, and testing prior to its release by Bruce Fraser, its OK but not ideal to use 8-bit ProPhoto. IOW, if you have an option, use high bit. If you have ProPhoto RGB 8-bit, its not the end of the world. I kind of doubt Joe's space would be that much different (I don't know what the encoding TRC gamma is) but I think its a pretty wide gamut space. Since I know both Kodak and Bruce did test 8-bit ProPhoto and said its useable, I'd stick with that but ideally, you'd set the scanner for high bit capture.

Converting from 8-bit ProPhoto to high bit buys nothing here.
Title: Re: Prophoto in 8bit?
Post by: JeffKohn on August 28, 2010, 02:46:20 PM
Converting from 8-bit ProPhoto to high bit buys nothing here.
I agree 16-bit scanning would be ideal; and I also agree that moving to 16-bit after converting to ProPhoto gains you nothing.

But if the 8-bit file starts out in another color space like AdobeRGB, then moving to 16-bit first and then converting to ProPhoto makes sense, doesn't it? The problem with converting an 8-bit AdobeRGB file to 8-bit ProPhoto is that an 8-bit AdobeRGB value may shift just a little bit in ProPhoto RGB, because those 255 levels per channel are spread out further in the larger color space. But if you do the conversion in 16-bit, this problem would be greatly reduced if not eliminated right?
Title: Re: Prophoto in 8bit?
Post by: tho_mas on August 28, 2010, 03:09:07 PM
8bit?
sRGB is appropriate to work in 8bit.
AdobeRGB is somehow "okay" if you edit the files carefully.
Any larger gamut is far to big for 8bit... IMHO.
Title: Re: Prophoto in 8bit?
Post by: BartvanderWolf on August 28, 2010, 03:13:10 PM
I agree 16-bit scanning would be ideal; and I also agree that moving to 16-bit after converting to ProPhoto gains you nothing.

But if the 8-bit file starts out in another color space like AdobeRGB, then moving to 16-bit first and then converting to ProPhoto makes sense, doesn't it? The problem with converting an 8-bit AdobeRGB file to 8-bit ProPhoto is that an 8-bit AdobeRGB value may shift just a little bit in ProPhoto RGB, because those 255 levels per channel are spread out further in the larger color space. But if you do the conversion in 16-bit, this problem would be greatly reduced if not eliminated right?

Hi Jeff,

The problem will be reduced, not eliminated, because in the process of gamut (re-)mapping to more bits per data value the precision of mapping to values in between 8-bit values is higher. There are however coordinates that do not exactly map to unique new values, so some loss of distiction may occur. It would be better to scan in 16-bit/channel, preferrably directly into ProPhoto RGB should one wish to use that further down the workflow pipeline. Because e.g. Adobe RGB is a gamma 2.2 colorspace, and ProPhoto RGB a 1.8 gamma workspace, there is also a gamma remapping going to take place later in the pipeline, e.g. to display space and/or printer space. That's also posterization prone, and thus best done in 16-b/ch. The actual output can then be done in e.g. 8-bit space.

All conversions risk loss of data (due to rounding to similar integer numbers, and gamma adjustments), so they are best done at a high precision.

Cheers,
Bart
Title: Re: Prophoto in 8bit?
Post by: WombatHorror on August 30, 2010, 09:26:25 PM
I have heard some say not to use prophoto in 8bit.  If I am doing profiled 8bit scans and then converting to to prophoto rgb, am I better off converting to j Holmes ektaspace.  Prophoto is a bigger colorspace, but what are the issues with 8bit

what do you mean by and then converting to prophotorgb

what internal space do you have the scanner software using?

why are you scanning at 8bits?
Title: Re: Prophoto in 8bit?
Post by: WombatHorror on August 30, 2010, 09:28:23 PM
I agree 16-bit scanning would be ideal; and I also agree that moving to 16-bit after converting to ProPhoto gains you nothing.

But if the 8-bit file starts out in another color space like AdobeRGB, then moving to 16-bit first and then converting to ProPhoto makes sense, doesn't it? The problem with converting an 8-bit AdobeRGB file to 8-bit ProPhoto is that an 8-bit AdobeRGB value may shift just a little bit in ProPhoto RGB, because those 255 levels per channel are spread out further in the larger color space. But if you do the conversion in 16-bit, this problem would be greatly reduced if not eliminated right?

if you already had them in 8bit adobergb what could does it do to covert to prophoto at that point? the gamut has already been clipped to adobergb