Managing out-of-gamut colours and banding

[EmDashMan]

Hi,
Excuse the two-topics-in-one-post scenario, but I have both problems with one print :-)

I've got a nice shot of a seagull silhouetted against a sunset. The bright yellow immediately behind the seagull is out of gamut, and over to the left in the deeper red colours I'm getting banding.

Interestingly, when viewing a soft proof on my profiled monitor, the banding doesn't show up. But on my laptop screen next to it (standard LCD profile) it does.

So:

1. What's the best technique for getting out-of-gamut colours to print? I tried a Hue/Saturation/Value adjustment layer, and reduced the saturation in the offending portion of the image, but on the print that area now has a slightly pasty look to it (especially if you catch the area in the light).

2. How can I reduce the banding? My image is quite a tight crop from a larger file. I applied Nik Software's noise reduction already, so adding noise (one technique I've read about)) seems the wrong way to go :-) Are there other techniques?

Hopefully I've managed to attach a small jpeg version of the image, so you can see what I'm working with.



Martin Ley

[EmDashMan]

Obviously, the picture looks very weird for you guys as the correct profile is not applied :-)

[herbert]

Hi Martin,

You could convert the image using perceptual rendering and not relative colormetric:

Relative Colormetric - Converts colours to the nearest matching equivalent colour in the new colour space. Any out-of-gamut colours will be clipped. This causes saturation in your case. E.g square peg into a round hole by cutting off the corners.

Perceptual rendering - Converts the entire space to the new colour space, preserving the graduations between tones but altering the separations. Any out-of-gamut colours will be moved inside the new space. E.g. square peg into a round hole by making the peg smaller.

However perceptual rendering may mute the brilliance of your image as the mapping moves all the colours a bit closer together than if you were to use relative colormetric. This is partially offset by using clever mapping to compress the extreme tonal ranges more than the central ranges.

After you convert the colour space you can always boost the image saturation if it has been lost.

I cannot comment on the banding problem. Your image is too small to see anything.

Alex

[Photon Hacker]

1. What's the best technique for getting out-of-gamut colours to print? I tried a Hue/Saturation/Value adjustment layer, and reduced the saturation in the offending portion of the image, but on the print that area now has a slightly pasty look to it (especially if you catch the area in the light).

What?. Printing colors outside the system gamut?. Not possible, by definition. Think about it for a moment!.

2. How can I reduce the banding? My image is quite a tight crop from a larger file. I applied Nik Software's noise reduction already, so adding noise (one technique I've read about)) seems the wrong way to go :-) Are there other techniques?

Probably you mean dithering: the process of adding specific noise before quantization (Or re quantization) to decorrelate quantization error to the signal. The problem as you described it regards a reduction in color gamut extension, not depth. Dithering won't, indeed, address the problem. However, note it may make sense to perform noise reduction on an image and then dither it before requantization in some cases (When the remaining noise amplitude is lower than quantum). High sensitivity noise may work as dither, but dithering don't implies high-sensitivity-like appareance.

So this problem is analog to that of a bigger dynamic range in the image than that of the display system (Again, think for a moment about it. It don't takes more that a bit of intuition to know!). If you want to avoid hard clipping then adjust the saturation curve in a way that the output range fits the display system color gamut and color variations are preserved according to your artistic sense.

Good luck!.

[EmDashMan]

What?. Printing colors outside the system gamut?. Not possible, by definition. Think about it for a moment!.
Good luck!.

Um, well obviously out-of-gamut colours won't print, by definition. I guess my question was phrased rather clumsily. Rather, what's the best way to adjust the image so that it brings out-of-gamut colours back to something that's printable, without mangling the image too much?

Thanks to Herbert, who posted a simple suggestion.

I'm afraid, Photon Hacker, I'm not up to translating your reply on quantization into something I can understand or use. I'm out of my depth (or should that be colour gamut extension? :-)

Martin