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In article <lirhb30192s@news6.newsguy.com>, PeterN <peter.newnospam@verizon.net>wrote:

Eric Stevens
What are the two conversions?

nospam
rgb->lab->rgb

Eric Stevens
But isn't that exactly what the colour engine has to do?

It gets fed an RGB file from outside, then converts it to Lab for it's own processing and then converts it back to RGB for output to the monitor. It may also convert it to CMYK for output to a printer or saving it to a disc. http://www.eizo.com/global/library/management/cms/02e.jpg is the kind of diagram commonly used to explain this.

nospam
it doesn't.

from "color management for photographers: hands on techniques for photoshop users" by andrew rodney, page 53:

Editing in LAB: I have nothing against the LAB color model. However, there are a group of people who feel that editing in LAB is the only way to accomplish specific corrections, making it sound like a macho editing space. It is true, there are a few correction techniques that rely on a document being in LAB color space. The question becomes whether its worth taking the time or worse, producing image degradation to convert from a working space to LAB and back. Every time a conversion to LAB is produced, the rounding errors and severe gamut mismatch between the two spaces can account for data loss, known as quantization errors. The amount of data loss depends on the original gamut size and gamma of the working space.

...

Some users are under the impression that Photoshop does all its conversions to and from LAB, converting on-the-fly. this is untrue as it would greatly slow down performance. Instead, Photoshop uses LAB as a reference when conducting many operations. Photoshop is not actually converting pixel data between color spaces unless you, the user, actually ask for this. None of these issues should be interpreted as implying that a conversion from working space to LAB is bad. Just be aware of the issues involved with this kind of conversion and whenever possible, try to use similar techniques that can be conducted in the RGB working space.

PeterN
All of which is meaningless.simply put: the LAB color space is larger than RGB. Conversion ot RGB is loseless. Conversion from LAB to RGB may not be b looseness, but for pictorial photography purposes, you will not notice any loss. Try it 10 times and ost the results. Let the readers try it and see ifthey can see any difference.

nospam
it's not meaningless and there is a loss in *either* direction.

you might not care but others do.