[Lcms-user] Re: sRGB color tranformation

[Marti M. <ma...@li...>]

Hi,

>It is a question about the sRGB color transformation.
>=20
>The steps I follow are
>
>(1) Measure the IT8 chart in XYZ with a color meter.
>(2) Scan the IT8 in RGB with a scanner.
>(3) Create a profile for color calibration based on the=20
>    measured XYZ and scanned RGB mentioned above.
>(4) Calibration the scanned IT8 image itself with the profile created=20
>    above and transform it to sRGB color space.


How are you creating the profile based on XYZ? Are you using any =
profiler program?
Profile building is somehow difficult, since it needs some cooking in =
the numbers.
Anyway, assuming that the profile is correct, and your transform chain =
is some
like:

    XYZ -> sRGB,

Then, assure to use absolute colorimetric intent  (if you are going to =
check absolute=20
numbers) and cmsFLAGS_NOTPRECALC (see a discussion on why to use this =
flag on=20
end of tutorial.txt

Note also that since IT8 is measured under D50, and sRGB is using D65, =
you will obtain
a lot of blue on reverse transform. (since D65 looks very bluish to an =
observer adapted=20
to D50). On direct transform you should obtain a yellow cast. Using =
perceptual intent=20
would correct these  white point translations.

>HP ScanJet 3400C claims supporting the sRGB, but its red channel
>is not lower than the other channels.

Yes... but plea note that RGB is device dependent... sRGB means that
R=3DG=3DB=3D255 will render a point on sRGB monitor with chromaticity of =
D65,=20
and a paper with chromaticity of D65 will be scanned as R=3DG=3DB=3D255.
A scanner that is working on, say for example, NTSC, will return =
different
RGB values for same piece of paper.

Equal R,G,B values doesn't mean nothing about specific color, is only a =
way to encode
a color in the target colorspace. For making such  judgaments about =
neutral, you need
to convert to any device independent color space, like XYZ or Lab.

Regards,
Marti Maria
The little cms project
http://www.littlecms.com
ma...@li...


  ----- Original Message -----=20
  From: Shiechen_Lee=20
  To: Marti Maria=20
  Sent: Thursday, June 14, 2001 11:55 PM
  Subject: sRGB color tranformation


  Hi,
  =20
     Sorry, I should have written it in English first. Thanks for your =
reply.=20
  =20
  It is a question about the sRGB color transformation.
  =20
  The steps I follow are
  (1) Measure the IT8 chart in XYZ with a color meter.
  (2) Scan the IT8 in RGB with a scanner.
  (3) Create a profile for color calibration based on the=20
      measured XYZ and scanned RGB mentioned above.
  (4) Calibration the scanned IT8 image itself with the profile created=20
      above and transform it to sRGB color space.
  =20
  The red channel of the 22 neutral blocks on the sRGB transformed=20
  IT8 image seems to be much lower than the green/blue channel.
  In fact, the red channel is still much lower if I transform the=20
  measured XYZ from step(1) in to sRGB color space directly.
  =20
  Of course I have the sense that the same value in R/G/B is not=20
  neutral gray in visual color at all. It is depend on the color space=20
  of the monitor phosphor.
  =20
  My question is whether the red channel is lower after=20
  your sRGB transformation? Or do I miss something?
  =20
  HP ScanJet 3400C claims supporting the sRGB, but its red channel
  is not lower than the other channels.
  =20
  Thanks. It will be very appreciatted for your help.=20


  Shiechen_Lee