I convert a RAW file using, say, C1 and exporting the TIFF embedded with Prophoto profile to open in PS CS2 set with a working Prophoto RGB space. I check to make sure highlights and shadows are not clipped by viewing the histogram. Next I convert that file in PS CS2 to sRGB space for web. Immediately I notice the highlights banging up the 255 wall and clipping. I understand that the smaller sRGB color space may not be able to map the wider range of colors tones from Prophoto, but is there a way to set PS CS2 to prevent the conversion from pushing the highlights and shadows outwards?

Thanks for any inputs.

Henry

I would suggest checking which rendering intent you are using in your colour space conversion. If you are using saturation, relative and absolute colorimteric intents then the out-of-gamut colours are clipped. To preserve the colour ratios you need to use perceptual rendering intent to 'fit' all of the colours within the target colour space.

Rendering Intents

This is probably why you are seeing clipped highlights in your histogram.

I'm set on Perceptual and the highlights clip. I've also tried the other intents needless to say they clip the highlights. This was what puzzled me.

Unfortunately, with matrix based profiles such as ProPhotoRGB and sRGB, there is only one rendering intent and that is relative colorimetric. Out of gamut colors are clipped. The lookup tables necessary for perceptual rendering are not present in the above matrix based profiles. Photoshop gives no indication of this problem: it lets you choose perceptual rendering, but ignores the option.

Bill

You can see which things will be clipped by using Photoshop's soft proof feature and choosing sRGB as the destination space. Then turn on the gamut warning option.

The clipping that you're seeing is normal. sRGB is a relatively small space and most natural imagery will contain some colors that exceed the space. The only way to preserve the color detail is to (selectively) desaturate the offending colors until they fit within the space. (Alternatively, you can try to shift the hue or lightness settings.)

This is generally known as output-dependent image tuning: you are fine-tuning the image based on the output considerations, in this case sRGB for web display. (You would be doing a similar process for printing, except using the printer profile and the corresponding soft proof as a guide.)

Thanks - that explains much (i.e. why I have never seen a difference between perceptual and relative col intents).

Is there any such a thing as a non-matrix based sRGB profile that can be used by Photoshop to convert from ProPhoto to sRGB using perceptual rendering intent?

Many out of gamut colors are clipped with Perceptual too, there's no way around this! You've got a lot of out of gamut colors, hence they can't fit within the new color space. But with RelCol only the out of gamut colors get clipped to the boundaries of the new color space, in gamut colors are left alone. With Perceptual, other colors can be mapped into the new gamut but those out of gamut colors will still get clipped somewhere.

They are coming (slowly) as Version 4 ICC profiles.

Perceptual rendering must provide, if required, a tonal mapping to smoothly compress shadows and highlights in such a way that tones can be expanded again without undue loss of detail.
In other words the transform must be invertible.

Of course that is the ICC spec....

Looks like there is nothing I can do for the moment then.

Henry