What is the difference of HISTOGRAM-VALUE, LUMINANCE, RGB?

Software GIMP
1

Hi,
In the document of gimp-drawable-curve-spline in the procedure browser there are the following parameters.
HISTOGRAM-VALUE (0), HISTOGRAM-RED (1), HISTOGRAM-GREEN (2), HISTOGRAM-BLUE (3), HISTOGRAM-ALPHA (4), HISTOGRAM-LUMINANCE (5), HISTOGRAM-RGB (6)

But what is the difference between HISTOGRAM-VALUE, HISTOGRAM-LUMINANCE, HISTOGRAM-RGB?

I suppose HISTOGRAM-LUMINANCE may be a greyscale value based on weighted R, G, B value, and the others, one may be mean value of RGB, and the other may be max(R, G, B). But which is which?

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2

“Value” likely comes from HSV.

On Luminance and other intensities, please refer to:

There is quite a lot of confusion regarding intensities and colour models, so I hope this reference does not contain any misinformation…

1 Like
3

AFAIK “value” is max of R/G/B, luminance is a weighted addition of R/G/B, and RGB is an error (only usable with the interactive plugin).

1 Like
4

Thanks a lot!

5

Thank you!

6

Value is max(R,G,B)

Luminance is weighted sum, approx (0.3R+0.6G+0.1B).

RGB is three separate curves R,G,B - not an error.

7

RGB is three separate curves R,G,B - not an error.

It’s an error when using the API, that can only return data for one channel.

  • HISTOGRAM_VALUE works
>>> pdb.gimp_drawable_histogram(layer,HISTOGRAM_VALUE, 0., 1.)
(0.0, 0.0, 0.0, 8754.952965072822, 8754.952965072822, 1.0)
  • HISTOGRAM_RGB fails
>>> pdb.gimp_drawable_histogram(layer,HISTOGRAM_RGB, 0., 1.)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
NameError: name 'HISTOGRAM_RGB' is not defined
  • There are indeed only six HISTOGRAM_* constants defined
>>> [h for h in dir() if 'HISTOGRAM' in h]
['HISTOGRAM_ALPHA', 'HISTOGRAM_BLUE', 'HISTOGRAM_GREEN', 'HISTOGRAM_LUMINANCE', 'HISTOGRAM_RED', 'HISTOGRAM_VALUE']
  • Fun thing, you can use "6" as a channel value and get something (but still data for a single channel)
>>> pdb.gimp_drawable_histogram(layer,6, 0., 1.)
(0.0, 0.0, 0.0, 26264.858895218465, 26264.858895218465, 1.0)

Using '6` on a very plain layer returns a pixel count 3 times the pixel count of the layer so it could be a histogram done on all three channels combined.

The 3.0 API still only documents 6 values.

8

Sorry, I was talking about the GUI, not the coding thereof.

9

I have tested gimp-drawable-curves-spline with these parameters. gimp-drawable-curves-spline accepts HISTOGRAM-VALUE, -RED, -GREEN, -BLUE, ｰALPHA, but it rejects HISTOGRAM-LUMINANCE, and -RGB. It generates error message “global name ‘HISTOGRAM_RGB’ is not defined” etc.