[argyllcms] Re: Perceptual intent

  • From: <robert@xxxxxxxxxxxxxxxxxx>
  • To: <argyllcms@xxxxxxxxxxxxx>
  • Date: Wed, 17 Dec 2014 12:41:01 -0000

Graeme Gill wrote:

"As long as you are conscious that Argyll's perceptual conversions are
compressing from your source gamut to the printer, and that therefore to not
loose colorfulness, your images need to occupy the source gamut."

I don't see how the image can do anything but occupy the source gamut . I
assume you mean that the image needs to occupy the destination gamut?
However it's impossible to guarantee this, even if sRGB is used as the
source profile.  And if sRGB is used as the source profile then what's the
point of buying expensive printers with 12 inks?

Here are some figures.  

I took an image in Beta RGB with three gray patches, in Lab the values are: 
0,0,0; 7,0,0; 22, 0,0  

I then converted the image to the destination profile using Relcol and got
these values: 
3,0,0; 8,0,-3; 21,0,-4
I then converted the original image to the destination using Perceptual and
got these values: 
2,0,1; 13,1,-2; 28,1,-3.

As you can see, the perceptual mapping is lightening the grays considerably.
This can be seen in photos as a loss of contrast. The VERY dark blacks seem
OK, but otherwise dark colors are much lighter than they should be.

You can see a test image here:
The top image is a relative mapping to the destination while the bottom
image is a perceptual mapping to the destination.  Notice how much lighter
the grays are.  Also if you look at the photo you will see that the darks
are lighter in the perceptual mapping (look at the island in particular) and
there are also some much more saturated colours (look at the orange patches
for example)


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