Photoshop filters and gamut

[T774]

Hi everyone!

I was writing a shortish tutorial about color management, and I got to the point where I warn people about using 8 bits as a bit depth for very wide color spaces (like ProphotoRGB), as this could lead to posterization, and I realized something.
Is it possible to step outside of the gamut of a color space by using a filter in Photoshop? For example, if I have an image of a flower with nice primary colors in the sRGB color space, and tell Photoshop that I want to increase the saturation by +50, will I end up with colors outside of the sRGB space? And If I do, what happens to the out-of-gamut colors? Will they behave according to the rendering intent set in the color settings dialog (just as if I was converting from a larger color space), or will they go lost?

Anybody with knowledge of the inner workings of Photoshop have the answer?

[pfigen]

It's not physically possible to force any saturation beyond the gamut of the color space you're already in. You would have to convert the file to a larger space, increase saturation and then convert back to your smaller space, but there is little point in doing that. Lab is a great space for experimenting with that.

[T774]

It's not physically possible to force any saturation beyond the gamut of the color space you're already in. You would have to convert the file to a larger space, increase saturation and then convert back to your smaller space, but there is little point in doing that. Lab is a great space for experimenting with that.
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Well, come to think of it, you're quite right, since the top value in each channel (255 in 8-bit) is mapped to the most saturated colour for that channel in the color space I can't step out of gamut as there are no values to describe those colors with (ie.: you can't have a value of 276 in an 8-bit channel). So according to this, if you raise your saturation way up, you are actually clipping colours. (just like overexposure...)

Logical, thanks!

[Alaska]

What happens to the out of gamut colors will depend on your choice of rendering intent.    Perceptual compresses out of gamut colors into the target space.  Relative maps to the nearest in gamut equivalent in the target space.

Then you have the issue of paper and ink combinations.

See Martin Evening "Adobe Photoshop CS2 for Photographers" page 520 for more on rendering intents.

Jim

[digitaldog]

The working space sets the limit on the gamut (the color space). You can't exceed it by altering the numbers ala using some filter. You can change the scale of the numbers, which is what you do when you convert from one color space to another.

G255 is the most saturated green you can define in your 8bit file. That number is the same in sRGB or ProPhoto RGB yet, where that color lies within human gamut is different. The numbers are the same, the scale isn't. Just as 2mm and 2 inches are not the same, yet share the same number.

Oh, the reason you want to work in high bit with larger gamut spaces is because the space between the bits gets farther apart as the color space widens. That puts, can I say, more stress on 8-bit data.

[jbrembat]

Anybody with knowledge of the inner workings of Photoshop have the answer?

I don't know photoshop but.....

behave according to the rendering intent set in the color settings

Rendering intent applies to a conversion from a color space to a different color space. When you edit an image you are inside a color space... so clipping is the answer.

Jacopo