Re: [Gimp-developer] GIMP's ICM simulation must be improved as PhotoShop

[Elle Stone <l elle stone gmail com>]

On 9/23/12, ndummy001 yahoo co jp <ndummy001 yahoo co jp> wrote:
> GIMP(win) has some color management features on settings dialog. But it's
> not enough( e.g. black point calibration )
> Developers must implement these if you all truly thinking that GIMP is
> important.

I agree with what you say about the color management settings dialog.
Neither the Gimp Display settings nor the Softproof settings provide
the user with the very important choice of whether to use or not to
use black point compensation.

Also, Gimp display rendering intent (and probably the softproof
rendering intent, but I haven't checked) gives different results for
"relative colorimetric" and "perceptual", even when the destination
profile is a matrix profile (the more usual case for monitor
profiles).

The problem is, matrix profiles don't support perceptual intent:
perceptual intent requires a lookup table and matrix profiles don't
have lookup tables. As matrix profiles don't support perceptual
intent, having "perceptual intent" and "relative colorimetric" give
different results means a coding decision was made at some point in
the past.

In the lcms.c code, I found the following lines:

  if (config)
    intent = config->display_intent;
  else
    intent = GIMP_COLOR_RENDERING_INTENT_PERCEPTUAL;

  bpc = (intent == GIMP_COLOR_RENDERING_INTENT_RELATIVE_COLORIMETRIC);

which means that in Gimp, for your display profile, "relative
colorimetric intent" is hard-coded to give you "relative colorimetric
intent with black point compensation," and "perceptual intent" gives
you "without black point compensation".

Which gives you a partial workaround for the missing user option of
being able to choose "with or without black point compensation". If
you are using a matrix profile as your monitor profile, you really can
choose whether or not to use black point compensation, albeit in a
totally non-intuitive way. But if you are using a lut display profile
with a perceptual intent table and a relative colorimetric intent
table, you are stuck with "without black point compensation" if you
choose "perceptual intent" and you are stuck with "with black point
compensation" if you choose "relative colorimetric intent".

(As an aside, if you are using the V2 sRGB matrix profile as your
monitor profile, along with any standard RGB working space (sRGB,
BetaRGB, ClayRGB, Widegamut, etc), you won't see any difference
between "relative" (which in Gimp is with black point compensation)
and "perceptual" (which in Gimp is without black point compensation),
because sRGB and all the other standard working spaces all have zero
black points.)

Fixing the missing black point compensation option involves editing
"lcms.c", "preferences-dialog.c" and probably a few other files. I
would cheerfully attempt the task myself, but I think the "gui"
portion is beyond my coding skills, so I would need help from someone
more versed in the Gimp code.

On 9/23/12, scl <scl gplus gmail com> wrote:
> On 23.09.12 at 09:28 a.m. ndummy001 yahoo co jp wrote:
>
> The settings dialog is not the only part where you find color management
> in GIMP. Have you already looked at the Image/Mode/Assign Color
> Profile... and Image/Mode/Convert to Color Profile... menu items?
> Particularly the latter contains the desired black point compensation.

The Image/Mode/Convert menu items does offer the option of using or
not using black point compensation. But proper color management
requires that the same options be available for the monitor profile
and for softproofing.

> Also check View/Display Filters..., move there the 'Color proof' filter
> to the right list and click it. It give you a proof view of the image
> and its options also contain Black point compensation.
>

The Gimp Color Proof Display Filter seems to offer everything anyone
would want in the way of softproofing and display options.
Unfortunately, as far as I can tell from comparing the results to
other image editors (Cinepaint, Krita), the Gimp Color Proof Display
Filter doesn't work properly.

Kind regards,
Elle Stone


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