[argyllcms] Re: Verifying profile quality of LUT-based scanner and printer profiles

Milton Taylor wrote:

I will rebuild the profile, but other than seeing what effect it has on dE (I would expect it might go up slightly?), I'm not really sure how to verify if it's any better. 

In terms of your "rainbow" test chart, it may give a smoother
result.

You're right...what I should have said was that I would expect the manufacturer to have ensured a smooth result through the continuum of raw device RGB values, and this it does seem to do. However the greys are not neutral enough and the colours are probably out somewhat compared to sRGB. 

The manufacturers will certainly try, but they don't have any particular
magic in acheiveing this.

I will regenerate the profile for Perceptual and see what the result is then. 

It should help somewhat.

(or you need to convert using a device link with the right intent).

I use Photoshop so I'm not sure how this works with device links? 

Photoshop doesn't (currently) support transforming through device
links. Other software does, or it can be done manually using
Argyll/imdi/cctiff

Interestingly, I did a soft proof of Canon's own icm profile for this printer, and the rainbow test shows reasonably well on that one...still gamut clipping of sRGB range, but only slight banding visible on the rainbow really. Even at Rel Col intent. 

If the printer driver is operating in RGB mode (which is often the
case for MSWindows), then they may well be operating in fairly perceptual
type mode all the time, depending on how they've setup the printers
RGB to CMYK conversion.

I tried that. There is some effect caused by the display definitely (I'm using rather bad LCD...don't ask...)...but generally it didn't look too bad. How does this work though? i.e. I'm using an input profile as if it was an output profile? 

If all you're doing is wanting to check smoothness, then as long as
the display's characteristic is inherently smooth, and the input profile
you are testing is linked to a display profile that is inherently smooth
(such as a matrix/shaper profile), then the smoothness characteristics
of the input profile can be examined to some degree.

You can attempt to do so for a printer that emulates an RGB device,
and if it's emulation was very very good, and it compressed the gamut
sufficiently to emulate an additive device (leading to a poor gamut),
the matrix might fit reasonably well. But if the emulated RGB
at all reflects the behaviour of the underlying CMYK printer and
its gamut, the matrix will be a very poor fit to the device
(ie. 10+ delta E errors is the sort of thing I'd expect).


I'd like to try that just for the hell of it, but the profile.exe won't let me select -as to create an output profile? (I get an error message) 

You could try this for an RGB device by editing the .ti3 file and replace 
the
DEVICE_CLASS value "OUTPUT" with "DISPLAY".

Graeme Gill.

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