Isn't this just a duplicate of bug #136861 ?

Created attachment 25852 [details]
Numbered markers on an image with values display

Hmm, essentially yes. I missed that one. I don't know what the "zone
system" or "exposure latitude" is (contrast?). He suggests the
existing colorpicker be extended, I suggested a separate related tool.
He elaborates two points I merely referred to as "akin to Photoshop",
firstly the sample points are numbered, though he talks of floating
pop-ups. I only suggested having them referenced in the Info palette
by the sample number. 
Also, when applying something like Levels, all the color values in the
Info palette show before and after values separated by a slash in the
format current/new as in 3/5, other times they show as normal. I see
no point in showing XY coordinates as he suggests. In Photoshop, the
markers are only visible when either you are using the eyedropper or
color sampler tool, or using an Adjustment dialog like levels or
curves. Other times they are not visible, but still display samples in
the Info palette. The Color Sampler tool can also be used to move the
points when over them, the cursor changes accordingly (diagonal arrow
with marker). The contextual menu is used to Delete a point, and alter
what values are shown in the Info palette, eg RGB, Web(Hex), HSV or
Greyscale. The contextual menu over the Info window similarly could,
as in Photoshop, change the color model being displayed, rather than a
drop-down menu as in The Gimp.

His idea of a floating popup over the samples could be refined. Having
it a bit more like a sticky tooltip with no widgets is better.
Similarly, you can have it only show when the color samper tool is
selected or when any of the adjustment tools like Levels/Curves are
being used.
His reference to a colorblind person being able to color correct is
perhaps a reference to the excellent book (the pre-eminent pre-press
color correction guide) by Dan Margulis called Professional Photoshop.
He made a point of training a colorblind person to prove the point of
being able to correct by the numbers. Although many parts deal with
CMYK, Lab, and pre-press, many sections are still relevant to any GIMP
user interested in color correction and image restoration.

I'll make two attachments, one a mockup of the marker and "tooltip"
dialog showing an image being corrected with the curves, and showing
before/after values, as well as the info palette from Photoshop
showing the before/after display with the numbered markers. Not that
Gimp's info is identical, just didn't make a mockup. 

Created attachment 25856 [details]
Photoshop info palette showing before/after samples of markers

Forgot to mention, like the Color Picker, the Color Sampler Options
has a Sample Average slider and Sample Merged checkbox. 

*** Bug 136861 has been marked as a duplicate of this bug. ***

This is a quite easy addition. It needs:

- a persistent list of sample points which is part of the image
- code to load/save this points in the XCF
- display drawing code similar to guides/grid
- a tool to create/delete/mode sample points (this
  could be a color picker mode or a separate tool)
- PDB accessors for the new functionality

All these changes are quite local and could be done for 2.2

*** Bug 152713 has been marked as a duplicate of this bug. ***

See bug #152713 for a book recommendation where these sample points
are used to do color correction.

We won't add new tools or new GimpImage features that need to be saved
in the XCF before 2.2. Bumping to FUTURE.

I implore you to reconsider putting it off to after 2.2.  Saving the picked
coordinates in the XCF is a nice feature, but the usefulness of that aspect
doesn't seem compelling to me (I don't think the picked points are saved in
photoshop).  Not having the basic functionality of monitoring selected
points/regions while adjusting curves/levels makes it really difficult to adjust
color casts in digital photos, a process I'm finding really important.  Having
to re-pick the monitor regions the next time I want to work on the photo doesn't
seem nearly as much of a problem as not having the feature at all.  Pretty please? 

A whole lot of features are going to be postponed to the next development cycle.
The idea of having a short development cycle is not to let people wait too long
for the new features. By putting too much stuff onto the feature list, all we
end up with is everyone waiting even longer for the nice new stuff that is in
CVS. We are way behind our schedule already and we are basically feature frozen
for 2.2.

I have now put together an implementation of this by creating a list of
GimpSamplePoints attached to each image in much the same way as the list of
guides; the code for dealing with them is very closely modeled on the guide code.  

The infrastructure for this is the important thing, but here are the details,
all of which easily can (and many should!) be changed:  in my implementation,
sample points are drawn as small "+" symbols.  They are created inside image map
tools, by ctrl-clicking on the image.  (If none yet exist, a new "Color Samples"
area of the image map tool dialog shows a message telling how to add them.) 
Each sample point gives rise to a small color-picker-like display at the bottom
of the image map dialog, in the "Color Samples" area, showing a swath of color
and the RGB pixel values.  The colors are taken from the projection.  I haven't
yet done anything to make the sample points be saved in XCF files.

I will attach below a diff for this change.  It is pretty large, since quite a
substantial number of files needed to be modified.

Created attachment 38226 [details] [review]
patch implementing "sample points", touches many files

That's a nice start but I strongly disagree with the addition of more GUI
elements to the imagemap tool dialogs. We have tried very hard to make these
dialogs compact and I would prefer to see color information being added to the
color picker window (which is supposed to become a dockable anyway).

I also don't think we need to store the sample points in the XCF file. Can't see
any reason why that would be useful, but perhaps I am missing something?

Well, the image map tools are exactly where the information is useful, and this
can be done such that no extra space is consumed unless the user actually
creates sample points.  But I would be happy to put the image map tool part
aside for the moment if it means I can put the basic sample point code into cvs
before it bitrots -- the image map tool stuff all lives inside
gimpimagemaptool.[ch] and can easily be separated out and handled later.

Re xcf, I don't see any very strong reasons to store sample points in them at
this time.  I only brought it up because Mitch in his comments above seemed to
say that this is important.

Well, this is quite an important decision. If the color sample points are only
relevant to the imagemap tools, then they only need to exist in the imagemap
tools. They would not have to be part of the image structure, there wouldn't
have to be PDB commands for handling them, no need for global IDs, no
transformation support and so on. The more I think about this, the more sense
does this approach make. Perhaps Mitch can explain why he thinks that sample
points should be a permanent part of the image like guides?

Ok. First some comments about the patch itself:

- It's almost perfect :)
- IMHO the sample points should always appear *after* guides and grid,
  in all structs menus, property installation code etc.
- If the functions are called foo_sample_points(), the files
  must be called gimpimage-sample-points.[ch], not -samplepoints.[ch]
- The image map tools should not be cluttered with sample points GUI,
- The manipluation of the sample points should live in GimpColorTool,
  not in GimpImageMap tool.
- They are members of the image, so the best thing to do IMHO is to
  create a sample points dockable that shows the active image's
  points and can be open all the time, regardless of the active tool

WRT to saving the points in XCF:

I think we should finally save them in XCF because I see no reason
why they would be less important than guides and grid. For some
people I'm sure they are much more important because they do lots
of color correction work.

First however, I suggest getting the basic infrastructure in CVS,
plus a sample point dialog that should perhaps be integrated in
the long-outstanding info window replacement dockable which is
on my TODO.

Raising priority to prevent bit-rot...

Ok, so please commit this to CVS with the changes that Mitch suggested but for
now w/o any GUI.

I think we should finally move the color and cursor information out of the Info
window and make it a dockable. Additional color sample points can then happily
live there. Does that make sense?

That absolutely makes sense and is on my TODO for a long time.
Will look into that after the (busy) weekend.

Okay, I made the following changes:
1) Changed gimpimage-samplepoints.[ch] to gimpimage-sample-points.[ch]
2) Reverted gimpimagemaptool.[ch] back to its previous form, without any
sampling point code.

The other issues can be handled one thing at a time, without making or breaking
anything.

Committed to HEAD:

2005-03-04  Bill Skaggs  <weskaggs@primate.ucdavis.edu>

	* app/core/gimpimage-sample-points.c
	* app/core/gimpimage-sample-points.h: new files

	* app/actions/view-actions.c
	* app/actions/view-commands.c
	* app/actions/view-commands.h
	* app/config/gimprc-blurbs.h
	* app/core/Makefile.am
	* app/core/core-enums.c
	* app/core/core-enums.h
	* app/core/core-types.h
	* app/core/gimp.c
	* app/core/gimp.h
	* app/core/gimpimage-crop.c
	* app/core/gimpimage-duplicate.c
	* app/core/gimpimage-flip.c
	* app/core/gimpimage-rotate.c
	* app/core/gimpimage-scale.c
	* app/core/gimpimage-undo-push.c
	* app/core/gimpimage-undo-push.h
	* app/core/gimpimage.c
	* app/core/gimpimage.h
	* app/display/gimpdisplayoptions.c
	* app/display/gimpdisplayoptions.h
	* app/display/gimpdisplayshell-appearance.c
	* app/display/gimpdisplayshell-appearance.h
	* app/display/gimpdisplayshell-callbacks.c
	* app/display/gimpdisplayshell-draw.c
	* app/display/gimpdisplayshell-draw.h
	* app/display/gimpdisplayshell-handlers.c
	* app/display/gimpdisplayshell.c
	* app/display/gimpdisplayshell.h
	* app/widgets/gimphelp-ids.h
	* menus/image-menu.xml.in: add support for a list of "sample
	points" in each image, coded and handled very similarly to
	guides, for use mainly in color correction.  See bug #137776.

2005-03-09  Michael Natterer  <mitch@gimp.org>

	More sample point stuff. Addresses bug #137776.

	* app/core/gimpimage-sample-points.c
	* app/core/gimpimage-undo-push.c: append, not prepend the sample
	paints to the image's list because their index matters. Update
	sample points when their index changes.

	* app/display/gimpcanvas.[ch]: added own sytles for the sample
	points.  Added gimp_canvas_draw_text() which uses a PangoLayout
	which is cached in the canvas.

	* app/display/gimpdisplayshell-draw.c
	(gimp_display_shell_draw_sample_point): draw the sample points
	more distinct from guides using the new canvas APIs above.

	* app/display/gimpdisplayshell-callbacks.c
	(gimp_display_shell_[hv]ruler_button_press): factored out all
	code to

	(gimp_display_shell_ruler_burron_press): which takes a boolean
	"horizontal" variable and allows to add sample points with
	<control>+drag.

	* app/tools/gimpcolortool.[ch]: implement adding, moving and
	removing of sample points in the same way as the move tool moves
	guides.

	* app/tools/gimpcolorpickertool.c
	(gimp_color_picker_tool_oper_update): chain up.

First version of the sample points dockable:

2005-04-03  Michael Natterer  <mitch@gimp.org>

	* app/core/gimpimage.[ch]: added new signals "sample-point-added"
	and "sample-point-removed" and public functions to emit them.

	* app/core/gimpimage-sample-points.c (gimp_image_add_sample_point)
	(gimp_image_remove_sample_point): emit them accordingly.

	* app/core/gimpimage-undo-push.c (undo_pop_image_sample_point):
	ditto.

	(undo_pop_image_guide)
	(undo_pop_image_sample_point): added comments why we add/remove
	stuff manually instead of using the GimpImage APIs.

	* app/widgets/Makefile.am
	* app/widgets/widgets-types.h
	* app/widgets/gimpcursorview.[ch]
	* app/widgets/gimpsamplepointeditor.[ch]: new widgets.
	GimpCursorView is a replacement for the info window's "Cursor"
	page, GimpSamplePointEditor is a view on an image's sample points.
	The sample point editor does nothing yet except keeping a 2x2 grid
	of GimpColorFrames. Addresses bug #137776.

	* app/dialogs/dialogs.c
	* app/dialogs/dialogs-constructors.[ch]: register the new widgets
	as dockable dialogs.

	* app/actions/dialogs-actions.c (dialogs_dockable_actions)
	* menus/dialogs-menuitems.xml: added actions and menu items for
	the new dialogs.

	* app/display/gimpdisplayshell-cursor.c
	(gimp_display_shell_update_cursor)
	(gimp_display_shell_clear_cursor): update the new cursor view.

	* app/widgets/gimphelp-ids.h: help IDs for the new dialogs.

	* app/widgets/widgets-enums.[ch] (enum GimpColorFrameMode):
	changed description "Pixel values" to "Pixel" because the former
	was too long.

More progress:

2005-04-18  Michael Natterer  <mitch@gimp.org>

	* app/widgets/gimpsamplepointeditor.[ch]: Connect to the
	GimpImage::update-sample-point and GimpProjection::update signals
	and idle-pick colors at the sample points' coordinates.
	Addresses bug #137776.

Most of the infrastructure needed seems to be in place by now. What remains to
be done is to review the user interface and to check if this feature can be
integrated better. The current way of using it is not easily discoverable. It
would be nice if someone could write down

 - how the feature is currently implemented from a users point of view
 - what is good or bad about this
 - what's missing, what should be changed

One of the minor things that needs to be improved is the stock icon. Another
thing that might be worth adding is a way to specify whether to sample-merge the
picked colors.

For the records, this commit adds a context menu to the sample
point dockable which for now just contains "Sample Merged":

2005-07-09  Michael Natterer  <mitch@gimp.org>

	* app/widgets/gimpsamplepointeditor.[ch]: added "sample-merged"
	property and API. Pass it to gimp_image_pick_color().

	* app/actions/Makefile.am
	* app/actions/actions.c
	* app/actions/sample-point-editor-actions.[ch]
	* app/actions/sample-point-editor-commands.[ch]: actions and
	callbacks for the sample point editor's menu.

	* app/widgets/gimphelp-ids.h: its help IDs.

	* app/menus/menus.c
	* menus/Makefile.am
	* menus/sample-point-editor-menu.xml: the sample point editor menu.

Image scaling was forgotten:

2005-08-07  Michael Natterer  <mitch@gimp.org>

	* app/core/gimpimage-resize.c (gimp_image_resize_with_layers):
	take care of sample points. Addresses bug #137776.

Sven asked how the feature is currently used from a user's point so here goes..
This is a mock up how it could work in Gimp..

I'm including a link because i'm not sure if there is a size limit on 
attachements in Bugzilla.. hope the link works..

<a href="http://imageshack.us"><img 
src="http://img463.imageshack.us/img463/4585/sampletool15mx.png" border="0" 
alt="Image Hosted by ImageShack.us" /></a>

This image shows the cursor over the existing Color Picker tool.
The tool info below shows Color Picker as well..

Since Color Picker and Color Sampler are in a way similar, it makes sense
to group them together to save real estate... so no new tool need to be
displayed in the crowded tool window, instead it is hidden together
with Color Picker.. This may or may not be possible to implement with Gtk, but
that's how it is used currently..

Now see what happens as the left mouse button is pressed once and hold...

<a href="http://imageshack.us"><img 
src="http://img461.imageshack.us/img461/2421/sampletool24xa.png" border="0" 
alt="Image Hosted by ImageShack.us" /></a>

The second image, shows the new Color Sampler tool being revealed, while
the left mouse button is pressed down... now to select it, simply slide the
mouse over to the right icon..

When the left mouse button is released, this is what happens...

<a href="http://imageshack.us"><img 
src="http://img462.imageshack.us/img462/4198/sampletool39fi.png" border="0" 
alt="Image Hosted by ImageShack.us" /></a>

Notice the tools info window below... It changes to the Sample Point Tools 
Info...

Here my suggestion is to incorporate the Cursor Info with the new Color Sample 
Tool info... so the user can see the X and Y co-ordinates together at
the same time as the RGB/CMYK/etc values..
(The X/Y coordinates are important to the color blind person who is trying
to "color correct by the numbers")

Now the user has finished selecting his tool, it is the new Color Sampler Tool,
and the Info window has the 4 sample points grey out because no point is 
selected yet..

Now the user goes over to an open image - one that badly needs color correction
because some cheap digital camera did not do white balance properly, in this
case the photo is spoilt by a blue tinit.

<a href="http://imageshack.us"><img 
src="http://img362.imageshack.us/img362/5788/horse2nt.png" border="0" 
alt="Image Hosted by ImageShack.us" /></a>

Notice the cursor has changed to the Color Sampler icon, and notice the 
position of the cursor is being placed over an area which the user believed
correctly represents the white point in the photo at 268,199
This co-ordinate is again useful for the color blind person..

Now at this point, turn to look at the Tools info..

<a href="http://imageshack.us"><img 
src="http://img462.imageshack.us/img462/3103/sampletool45wz.png" border="0" 
alt="Image Hosted by ImageShack.us" /></a>

The tools info showed the X/Y correctly at 268,199
and the color under this point is 
R  157/157
G  175/175
B  195/195

  By clicking left button once at this point, the sample point 1 will be
captured as above..

Next the user will use the levels tools, or curves tools or some other
adjustment tool and work on the white point adjustment, while watching the
info window all the time.

The pair of numbers to the left should not change. That is the original color..
The numbers on the right marked XXX will change, as the user make color 
adjustments like this :

R  157/XXX
G  175/YYY
B  195/ZZZ

The object of making the white point adjustment is to adjust using 
levels/curves/etc until the numbers are near to 

R  157/195
G  175/195
B  195/195

i.e. XXX = YYY = ZZZ = highest RGB value, in this case 195.

Now, the user is done with the first sample point.

He will now select an area in the tinted photo which he believes truly
represents the black point.. and then make the second sample point
and repeat the above adjustment steps.. 

This time for black, he is looking to adjust
until XXX = YYY = ZZZ = lowest RGB value, which for example could be
26 or any other such number for the second sample point.

After the adjustment of black and white points, the white balance will be
corrected.

Notice that if the X/Y co-ordinates are shown, a color blind person can
do the job as long as he is given the correct sample point.. He just work
on the numbers !!

Notice in this example, only two sample points are needed, and after 
adjustment, the sample points are of no use to him any more, therefore,
it is really not necessary to keep it in the XCF data structure..

What is important to the user is to save a copy of the levels or curves 
preset..

Because if the camera has a white balance problem, it is likely to affect one
whole batch of photos not just one... And then, all he need to do, for 
subsequent series of photos is to re-apply the same saved levels, or curves
adjustment presets, and there is no need to find matching white and black
points anymore.

The fly-out item does not work under Windows (at least on the build I got from
Sourceforge).  Or I'm doing something wrong.  Holding down left-mouse on the
color picker does nothing at all...  :(

Could you please clarify?  Is your "does not work" referring to comment #28,
which is one person's idea of how things *ought* to work and does not correspond
to what is actually implemented in GIMP at the moment?  If not, could you please
tell us which version of GIMP you are using, what you expected to happen, and
what actually happened?

(In the current implementation, in the development version of GIMP, you create
color sample points by holding down the control key (with any tool active) and
dragging from a ruler into the image.  You see information about the colors of
the sample points by opening the "Sample Points" dialog.)

Very sorry to add my confusion to this thread.  I'm a moron.  I've been
desperately wishing for a) dockable cursor info; and b) color sample points. 
When I saw the GIMP 2.3.x changelog stating that (a) was done and (b) appeared
to be included as well I was elated.  I downloaded the 2.3.5 Windows binary from
Sourceforge and quickly saw the dockable cursor info and then the sample points
dialog.

The problem was I couldn't figure out how to get the sample points to work.  I
tried ctrl/shift/alt clicking to no avail.  Then I found this thread and
(wrongly) assumed that the current implementation had the fly-out toolbox icon
under the dropper tool (ref #28).  That was the reason for my post.

Dragging from the ruler works (of course).  :)  Thanks for clarifying!

In addition to lasm stated (which is good in my opinion) may I also suggest that the sample points be updated _without_ having the preview option enabled in curves?  On large images it can take some time to redraw, and without preview enabled the sample points don't update.  It would be very cool to work in non-preview mode and work the sample numbers to where you want them...and then enable the preview to see it.

I don't know if this is possible without redrawing the image, so take the suggestion for what it's worth.

Looking over what is here. I am going to resolve this bug report as FIXED.  There are some unimplemented suggestions in the last few comments, and it might be appropriate to open new bug reports for them if the commenters are still interested.  This report has gotten too long.