[argyllcms] Question regarding White Point and spotread
- From: <robert@xxxxxxxxxxxxxxxxxx>
- To: <argyllcms@xxxxxxxxxxxxx>
- Date: Sat, 1 Nov 2014 21:31:38 -0000
I wonder if you would be kind enough to clarify something for me?
I'm trying to do a spot color test from a document through to print, and
this is what I'm doing (using Photoshop and Argyll):
1. The spot color has a Lab value of 90, -14, 87 in Photoshop.
2. I convert the document to the print destination space (relative
colorimetric with BPC).
3. After conversion, the spot color is RGB 240, 247, 52 (or 0.941176,
0.968627, 0.203922). The Lab value from Photoshop is still 90, -14, 87, as
expected.
4. xicclu (rel. col.), with the RGB colors above through the profile
(forwards), gives Lab 90.33650, -14.520704, 87.115660. I assume that
Photoshop is effectively doing the same as xicclu but is rounding the
values.
5. I also tried fakeread (rel.col) which gives me exactly the same Lab
values as xicclu.
6. I print the image with no color management.
7. spotread gives me Lab values of 88.987, -13.637, 87.268. This is a
dE-Lab of about 1.6 compared to the xicclu reading.
The dE-Lab error seems too high as I have only just calibrated the printer
(iPF6400) and profiled the paper (Canson Baryta, so good paper).
colverify has an option to normalise each file's readings to white XYZ, but
xicclu, fakeread and spotread have no such adjustment.
I would have thought that the paper white would need to be taken into
account in comparing the spotread value to the image Lab value. Should the
paper white be measured and the spotread value normalised? If so, how
should this be done?
I appreciate your help.
Robert
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