Color spaces gray and lineargray appear to be the same.

[mfwitten]

There appears to be no difference between the colorspaces [non-linear] gray and lineargray.

Certainly, I don't understand what's going on, which might indicate a bug in ImageMagick or a need to improve the documentation.

Version:

$ magick -version
Version: ImageMagick 7.0.8-53 Q16 i686 2019-07-15 https://imagemagick.org
Copyright: © 1999-2019 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC HDRI Modules OpenCL OpenMP(4.5) 
Delegates (built-in): bzlib cairo fontconfig freetype gslib heic jbig jng jp2 jpeg lcms lqr ltdl lzma openexr pangocairo png ps raqm raw rsvg tiff webp wmf x xml zlib

In the following POSIX shell code, there is initially created at index 0 in the ImageMagick image sequence an sRGB color image with one orange pixel; from this sRGB image, the following 2 grayscale images are created next:

• A [non-linear] gray image (at index 1).
• A lineargray image (at index 2).

Then, from that same sRGB color image (at index 0), a linear RGB image is created (at index 3, which can be seen to differ in its green component due to the color-space conversion; from this RGB image, the following 2 grayscale images are created next:

• A [non-linear] gray image (at index 4).
• A lineargray image (at index 5).

Here is the code:

magick \
  -size 1x1 canvas:"orange"                 \
                                            \
  \( -clone 0 -colorspace gray \)           \
  \( -clone 0 -colorspace lineargray \)     \
                                            \
  \( -clone 0 -colorspace rgb \)            \
                                            \
  \( -clone 3 -colorspace gray \)           \
  \( -clone 3 -colorspace lineargray \)     \
                                            \
  txt:

This is the code's output on my system:

# ImageMagick pixel enumeration: 1,1,65535,srgb
0,0: (65535,42405,0)  #FFFFA5A50000  orange
# ImageMagick pixel enumeration: 1,1,65535,gray
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
# ImageMagick pixel enumeration: 1,1,65535,lineargray
0,0: (44262.7)  #ACE7ACE7ACE7  lineargray(67.5405%)
# ImageMagick pixel enumeration: 1,1,65535,rgb
0,0: (65535,24658.3,0)  #FFFF60520000  rgb(100%,37.6262%,0%)
# ImageMagick pixel enumeration: 1,1,65535,gray
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
# ImageMagick pixel enumeration: 1,1,65535,lineargray
0,0: (44262.7)  #ACE7ACE7ACE7  lineargray(67.5405%)

As you can see, the grayscale values are always the same!

In the following, the POSIX shell function make_grays produces 4 pairs of single-pixel images; each pair consists of a [non-linear] gray image and a lineargray image, such that the lineargray image is derived by converting the gray image:

gray()
{ 
  magick -size 1x1 canvas:"gray($1)" -write txt: -colorspace LinearGray txt:
}

make_grays()
{ 
  for v; do
    gray "$v"%
    printf '%s\n' --------------------
  done
}

Unexpectedly, the pixel value for each image in a pair is the same:

$ make_grays 1 45 55 99
# ImageMagick pixel enumeration: 1,1,65535,gray
0,0: (655.35)  #028F028F028F  gray(1%)
# ImageMagick pixel enumeration: 1,1,65535,lineargray
0,0: (655.35)  #028F028F028F  lineargray(1%)
--------------------
# ImageMagick pixel enumeration: 1,1,65535,gray
0,0: (29490.8)  #733373337333  gray(45%)
# ImageMagick pixel enumeration: 1,1,65535,lineargray
0,0: (29490.8)  #733373337333  lineargray(45%)
--------------------
# ImageMagick pixel enumeration: 1,1,65535,gray
0,0: (36044.2)  #8CCC8CCC8CCC  gray(55%)
# ImageMagick pixel enumeration: 1,1,65535,lineargray
0,0: (36044.2)  #8CCC8CCC8CCC  lineargray(55%)
--------------------
# ImageMagick pixel enumeration: 1,1,65535,gray
0,0: (64879.7)  #FD70FD70FD70  gray(99%)
# ImageMagick pixel enumeration: 1,1,65535,lineargray
0,0: (64879.7)  #FD70FD70FD70  lineargray(99%)
--------------------

Here is a more extensive list, indicating that there is no difference between [non-linear] gray and lineargray:

$ make_grays $(seq 1 100) | grep '^[-0-9]'
0,0: (655.35)  #028F028F028F  gray(1%)
0,0: (655.35)  #028F028F028F  lineargray(1%)
--------------------
0,0: (1310.7)  #051F051F051F  gray(2%)
0,0: (1310.7)  #051F051F051F  lineargray(2%)
--------------------
0,0: (1966.05)  #07AE07AE07AE  gray(3%)
0,0: (1966.05)  #07AE07AE07AE  lineargray(3%)
--------------------
0,0: (2621.4)  #0A3D0A3D0A3D  gray(4%)
0,0: (2621.4)  #0A3D0A3D0A3D  lineargray(4%)
--------------------
0,0: (3276.75)  #0CCD0CCD0CCD  gray(5%)
0,0: (3276.75)  #0CCD0CCD0CCD  lineargray(5%)
--------------------
0,0: (3932.1)  #0F5C0F5C0F5C  gray(6%)
0,0: (3932.1)  #0F5C0F5C0F5C  lineargray(6%)
--------------------
0,0: (4587.45)  #11EB11EB11EB  gray(7%)
0,0: (4587.45)  #11EB11EB11EB  lineargray(7%)
--------------------
0,0: (5242.8)  #147B147B147B  gray(8%)
0,0: (5242.8)  #147B147B147B  lineargray(8%)
--------------------
0,0: (5898.15)  #170A170A170A  gray(9%)
0,0: (5898.15)  #170A170A170A  lineargray(9%)
--------------------
0,0: (6553.5)  #199A199A199A  gray(10%)
0,0: (6553.5)  #199A199A199A  lineargray(10%)
--------------------
0,0: (7208.85)  #1C291C291C29  gray(11%)
0,0: (7208.85)  #1C291C291C29  lineargray(11%)
--------------------
0,0: (7864.2)  #1EB81EB81EB8  gray(12%)
0,0: (7864.2)  #1EB81EB81EB8  lineargray(12%)
--------------------
0,0: (8519.55)  #214821482148  gray(13%)
0,0: (8519.55)  #214821482148  lineargray(13%)
--------------------
0,0: (9174.9)  #23D723D723D7  gray(14%)
0,0: (9174.9)  #23D723D723D7  lineargray(14%)
--------------------
0,0: (9830.25)  #266626662666  gray(15%)
0,0: (9830.25)  #266626662666  lineargray(15%)
--------------------
0,0: (10485.6)  #28F628F628F6  gray(16%)
0,0: (10485.6)  #28F628F628F6  lineargray(16%)
--------------------
0,0: (11141)  #2B852B852B85  gray(17%)
0,0: (11141)  #2B852B852B85  lineargray(17%)
--------------------
0,0: (11796.3)  #2E142E142E14  gray(18%)
0,0: (11796.3)  #2E142E142E14  lineargray(18%)
--------------------
0,0: (12451.6)  #30A430A430A4  gray(19%)
0,0: (12451.6)  #30A430A430A4  lineargray(19%)
--------------------
0,0: (13107)  #333333333333  gray(51)
0,0: (13107)  #333333333333  lineargray(51)
--------------------
0,0: (13762.4)  #35C235C235C2  gray(21%)
0,0: (13762.4)  #35C235C235C2  lineargray(21%)
--------------------
0,0: (14417.7)  #385238523852  gray(22%)
0,0: (14417.7)  #385238523852  lineargray(22%)
--------------------
0,0: (15073.1)  #3AE13AE13AE1  gray(23%)
0,0: (15073.1)  #3AE13AE13AE1  lineargray(23%)
--------------------
0,0: (15728.4)  #3D703D703D70  gray(24%)
0,0: (15728.4)  #3D703D703D70  lineargray(24%)
--------------------
0,0: (16383.8)  #400040004000  gray(25%)
0,0: (16383.8)  #400040004000  lineargray(25%)
--------------------
0,0: (17039.1)  #428F428F428F  gray(26%)
0,0: (17039.1)  #428F428F428F  lineargray(26%)
--------------------
0,0: (17694.5)  #451E451E451E  gray(27%)
0,0: (17694.5)  #451E451E451E  lineargray(27%)
--------------------
0,0: (18349.8)  #47AE47AE47AE  gray(28%)
0,0: (18349.8)  #47AE47AE47AE  lineargray(28%)
--------------------
0,0: (19005.2)  #4A3D4A3D4A3D  gray(29%)
0,0: (19005.2)  #4A3D4A3D4A3D  lineargray(29%)
--------------------
0,0: (19660.5)  #4CCD4CCD4CCD  gray(30%)
0,0: (19660.5)  #4CCD4CCD4CCD  lineargray(30%)
--------------------
0,0: (20315.9)  #4F5C4F5C4F5C  gray(31%)
0,0: (20315.9)  #4F5C4F5C4F5C  lineargray(31%)
--------------------
0,0: (20971.2)  #51EB51EB51EB  gray(32%)
0,0: (20971.2)  #51EB51EB51EB  lineargray(32%)
--------------------
0,0: (21626.5)  #547B547B547B  gray(33%)
0,0: (21626.5)  #547B547B547B  lineargray(33%)
--------------------
0,0: (22281.9)  #570A570A570A  gray(34%)
0,0: (22281.9)  #570A570A570A  lineargray(34%)
--------------------
0,0: (22937.2)  #599959995999  gray(35%)
0,0: (22937.2)  #599959995999  lineargray(35%)
--------------------
0,0: (23592.6)  #5C295C295C29  gray(36%)
0,0: (23592.6)  #5C295C295C29  lineargray(36%)
--------------------
0,0: (24248)  #5EB85EB85EB8  gray(37%)
0,0: (24248)  #5EB85EB85EB8  lineargray(37%)
--------------------
0,0: (24903.3)  #614761476147  gray(38%)
0,0: (24903.3)  #614761476147  lineargray(38%)
--------------------
0,0: (25558.7)  #63D763D763D7  gray(39%)
0,0: (25558.7)  #63D763D763D7  lineargray(39%)
--------------------
0,0: (26214)  #666666666666  gray(102)
0,0: (26214)  #666666666666  lineargray(102)
--------------------
0,0: (26869.4)  #68F568F568F5  gray(41%)
0,0: (26869.4)  #68F568F568F5  lineargray(41%)
--------------------
0,0: (27524.7)  #6B856B856B85  gray(42%)
0,0: (27524.7)  #6B856B856B85  lineargray(42%)
--------------------
0,0: (28180)  #6E146E146E14  gray(43%)
0,0: (28180)  #6E146E146E14  lineargray(43%)
--------------------
0,0: (28835.4)  #70A370A370A3  gray(44%)
0,0: (28835.4)  #70A370A370A3  lineargray(44%)
--------------------
0,0: (29490.8)  #733373337333  gray(45%)
0,0: (29490.8)  #733373337333  lineargray(45%)
--------------------
0,0: (30146.1)  #75C275C275C2  gray(46%)
0,0: (30146.1)  #75C275C275C2  lineargray(46%)
--------------------
0,0: (30801.5)  #785178517851  gray(47%)
0,0: (30801.5)  #785178517851  lineargray(47%)
--------------------
0,0: (31456.8)  #7AE17AE17AE1  gray(48%)
0,0: (31456.8)  #7AE17AE17AE1  lineargray(48%)
--------------------
0,0: (32112.2)  #7D707D707D70  gray(49%)
0,0: (32112.2)  #7D707D707D70  lineargray(49%)
--------------------
0,0: (32767.5)  #800080008000  gray(50%)
0,0: (32767.5)  #800080008000  lineargray(50%)
--------------------
0,0: (33422.8)  #828F828F828F  gray(51%)
0,0: (33422.8)  #828F828F828F  lineargray(51%)
--------------------
0,0: (34078.2)  #851E851E851E  gray(52%)
0,0: (34078.2)  #851E851E851E  lineargray(52%)
--------------------
0,0: (34733.6)  #87AE87AE87AE  gray(53%)
0,0: (34733.6)  #87AE87AE87AE  lineargray(53%)
--------------------
0,0: (35388.9)  #8A3D8A3D8A3D  gray(54%)
0,0: (35388.9)  #8A3D8A3D8A3D  lineargray(54%)
--------------------
0,0: (36044.2)  #8CCC8CCC8CCC  gray(55%)
0,0: (36044.2)  #8CCC8CCC8CCC  lineargray(55%)
--------------------
0,0: (36699.6)  #8F5C8F5C8F5C  gray(56%)
0,0: (36699.6)  #8F5C8F5C8F5C  lineargray(56%)
--------------------
0,0: (37355)  #91EB91EB91EB  gray(57%)
0,0: (37355)  #91EB91EB91EB  lineargray(57%)
--------------------
0,0: (38010.3)  #947A947A947A  gray(58%)
0,0: (38010.3)  #947A947A947A  lineargray(58%)
--------------------
0,0: (38665.7)  #970A970A970A  gray(59%)
0,0: (38665.7)  #970A970A970A  lineargray(59%)
--------------------
0,0: (39321)  #999999999999  gray(153)
0,0: (39321)  #999999999999  lineargray(153)
--------------------
0,0: (39976.3)  #9C289C289C28  gray(61%)
0,0: (39976.3)  #9C289C289C28  lineargray(61%)
--------------------
0,0: (40631.7)  #9EB89EB89EB8  gray(62%)
0,0: (40631.7)  #9EB89EB89EB8  lineargray(62%)
--------------------
0,0: (41287.1)  #A147A147A147  gray(63%)
0,0: (41287.1)  #A147A147A147  lineargray(63%)
--------------------
0,0: (41942.4)  #A3D6A3D6A3D6  gray(64%)
0,0: (41942.4)  #A3D6A3D6A3D6  lineargray(64%)
--------------------
0,0: (42597.8)  #A666A666A666  gray(65%)
0,0: (42597.8)  #A666A666A666  lineargray(65%)
--------------------
0,0: (43253.1)  #A8F5A8F5A8F5  gray(66%)
0,0: (43253.1)  #A8F5A8F5A8F5  lineargray(66%)
--------------------
0,0: (43908.5)  #AB84AB84AB84  gray(67%)
0,0: (43908.5)  #AB84AB84AB84  lineargray(67%)
--------------------
0,0: (44563.8)  #AE14AE14AE14  gray(68%)
0,0: (44563.8)  #AE14AE14AE14  lineargray(68%)
--------------------
0,0: (45219.2)  #B0A3B0A3B0A3  gray(69%)
0,0: (45219.2)  #B0A3B0A3B0A3  lineargray(69%)
--------------------
0,0: (45874.5)  #B333B333B333  gray(70%)
0,0: (45874.5)  #B333B333B333  lineargray(70%)
--------------------
0,0: (46529.8)  #B5C2B5C2B5C2  gray(71%)
0,0: (46529.8)  #B5C2B5C2B5C2  lineargray(71%)
--------------------
0,0: (47185.2)  #B851B851B851  gray(72%)
0,0: (47185.2)  #B851B851B851  lineargray(72%)
--------------------
0,0: (47840.6)  #BAE1BAE1BAE1  gray(73%)
0,0: (47840.6)  #BAE1BAE1BAE1  lineargray(73%)
--------------------
0,0: (48495.9)  #BD70BD70BD70  gray(74%)
0,0: (48495.9)  #BD70BD70BD70  lineargray(74%)
--------------------
0,0: (49151.2)  #BFFFBFFFBFFF  gray(75%)
0,0: (49151.2)  #BFFFBFFFBFFF  lineargray(75%)
--------------------
0,0: (49806.6)  #C28FC28FC28F  gray(76%)
0,0: (49806.6)  #C28FC28FC28F  lineargray(76%)
--------------------
0,0: (50462)  #C51EC51EC51E  gray(77%)
0,0: (50462)  #C51EC51EC51E  lineargray(77%)
--------------------
0,0: (51117.3)  #C7ADC7ADC7AD  gray(78%)
0,0: (51117.3)  #C7ADC7ADC7AD  lineargray(78%)
--------------------
0,0: (51772.7)  #CA3DCA3DCA3D  gray(79%)
0,0: (51772.7)  #CA3DCA3DCA3D  lineargray(79%)
--------------------
0,0: (52428)  #CCCCCCCCCCCC  gray(204)
0,0: (52428)  #CCCCCCCCCCCC  lineargray(204)
--------------------
0,0: (53083.3)  #CF5BCF5BCF5B  gray(81%)
0,0: (53083.3)  #CF5BCF5BCF5B  lineargray(81%)
--------------------
0,0: (53738.7)  #D1EBD1EBD1EB  gray(82%)
0,0: (53738.7)  #D1EBD1EBD1EB  lineargray(82%)
--------------------
0,0: (54394.1)  #D47AD47AD47A  gray(83%)
0,0: (54394.1)  #D47AD47AD47A  lineargray(83%)
--------------------
0,0: (55049.4)  #D709D709D709  gray(84%)
0,0: (55049.4)  #D709D709D709  lineargray(84%)
--------------------
0,0: (55704.8)  #D999D999D999  gray(85%)
0,0: (55704.8)  #D999D999D999  lineargray(85%)
--------------------
0,0: (56360.1)  #DC28DC28DC28  gray(86%)
0,0: (56360.1)  #DC28DC28DC28  lineargray(86%)
--------------------
0,0: (57015.5)  #DEB7DEB7DEB7  gray(87%)
0,0: (57015.5)  #DEB7DEB7DEB7  lineargray(87%)
--------------------
0,0: (57670.8)  #E147E147E147  gray(88%)
0,0: (57670.8)  #E147E147E147  lineargray(88%)
--------------------
0,0: (58326.2)  #E3D6E3D6E3D6  gray(89%)
0,0: (58326.2)  #E3D6E3D6E3D6  lineargray(89%)
--------------------
0,0: (58981.5)  #E666E666E666  gray(90%)
0,0: (58981.5)  #E666E666E666  lineargray(90%)
--------------------
0,0: (59636.8)  #E8F5E8F5E8F5  gray(91%)
0,0: (59636.8)  #E8F5E8F5E8F5  lineargray(91%)
--------------------
0,0: (60292.2)  #EB84EB84EB84  gray(92%)
0,0: (60292.2)  #EB84EB84EB84  lineargray(92%)
--------------------
0,0: (60947.6)  #EE14EE14EE14  gray(93%)
0,0: (60947.6)  #EE14EE14EE14  lineargray(93%)
--------------------
0,0: (61602.9)  #F0A3F0A3F0A3  gray(94%)
0,0: (61602.9)  #F0A3F0A3F0A3  lineargray(94%)
--------------------
0,0: (62258.2)  #F332F332F332  gray(95%)
0,0: (62258.2)  #F332F332F332  lineargray(95%)
--------------------
0,0: (62913.6)  #F5C2F5C2F5C2  gray(96%)
0,0: (62913.6)  #F5C2F5C2F5C2  lineargray(96%)
--------------------
0,0: (63569)  #F851F851F851  gray(97%)
0,0: (63569)  #F851F851F851  lineargray(97%)
--------------------
0,0: (64224.3)  #FAE0FAE0FAE0  gray(98%)
0,0: (64224.3)  #FAE0FAE0FAE0  lineargray(98%)
--------------------
0,0: (64879.7)  #FD70FD70FD70  gray(99%)
0,0: (64879.7)  #FD70FD70FD70  lineargray(99%)
--------------------
0,0: (65535)  #FFFFFFFFFFFF  gray(255)
0,0: (65535)  #FFFFFFFFFFFF  lineargray(255)
--------------------

[urban-warrior]

Named colors closely follows the SVG standard-- that is, named colors are interpreted as non-linear sRGB or gray. SVG expects rgb() to return values in the sRGB colorspace rather than linear RGB. For symmetry we interpret lineargray() as non-linear gray.

[mfwitten]

I'm still uncertain. Conversion from sRGB to RGB produces a difference (as expected):

$ magick canvas:orange \( +clone -colorspace RGB \) txt: | grep '^0'
0,0: (65535,42405,0)  #FFFFA5A50000  orange
0,0: (65535,24658.3,0)  #FFFF60520000  rgb(100%,37.6262%,0%)

However, I'll note that the color name/specification changed, too! According to you, it shouldn't have changed, and should be the following:

0,0: (65535,42405,0)  #FFFFA5A50000  orange
0,0: (65535,24658.3,0)  #FFFFA5A50000  orange

That is to say, only the channel values should be different; the color names should be the same.

Anyway, with regard to grays, according to the docs:

Note, ImageMagick assumes the sRGB colorspace if the image format does not indicate otherwise. For colorspace conversion, the gamma function is first removed to produce linear RGB.
[…]
Gray = 0.212656*R+0.715158*G+0.072186*B
[…]
LinearGray = 0.298839*R+0.586811*G+0.114350*B

(Please note that the documentation above is completely ambiguous, with a bias towards being wrong; in the formulas, the variables R, G, and B are non-linear, such that the coefficients of the formulas incorporate linearization.)

Therefore, the results of conversion to Gray should be different from the results of conversion to LinearGray, but they are exactly the same:

$ magick canvas:orange                    \
    \( -clone 0 -colorspace Gray \)       \
    \( -clone 0 -colorspace LinearGray \) \
    txt: | grep '^0'
0,0: (65535,42405,0)  #FFFFA5A50000  orange
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
0,0: (44262.7)  #ACE7ACE7ACE7  lineargray(67.5405%)

I expected something more like the following:

0,0: (65535,42405,0)  #FFFFA5A50000  orange
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
0,0: (44468.1)  #ACE7ACE7ACE7  gray(67.5405%)

which I got by doing these Python calculations:

>>> R,G,B = (65535,42405,0) # non-linear values
>>> Gray = 0.212656*R+0.715158*G+0.072186*B
>>> Gray
44262.68595
>>> Gray/65535
0.675405294117647
>>> LinearGray = 0.298839*R+0.586811*G+0.114350*B
>>> LinearGray
44468.13432
>>> LinearGray/65535
0.6785402352941176

and then the following ImageMagick commands (using -fx to bypass color names/specs):

$ magick canvas:'gray(0)' -fx 0.675405294117647  txt:
# ImageMagick pixel enumeration: 1,1,65535,gray
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
$ magick canvas:'lineargray(0)' -fx 0.6785402352941176 txt:
# ImageMagick pixel enumeration: 1,1,65535,lineargray
0,0: (44468.1)  #ADB4ADB4ADB4  lineargray(67.854%)

BUT NOTE:

The expected form was produced by manually replacing:

#ADB4ADB4ADB4  lineargray(67.854%)

with:

#ACE7ACE7ACE7  gray(67.5405%)

because you've stated that a color name is basically non-linear by definition; only the actual, stored channel value should be different.

Needless to say, I'm quite confused!

[urban-warrior]

Thanks for the problem report. We can reproduce it and will have a patch to fix it in GIT master branch @ https://github.com/ImageMagick/ImageMagick later today. The patch will be available in the beta releases of ImageMagick @ https://www.imagemagick.org/download/beta/ by sometime tomorrow.

[mfwitten]

There are still discrepancies as of this commit:

90ceb2688228a0be3d79714b3e985c57827eb50e

This is the behavior:

$ magick canvas:orange                            \
  \( -clone 0 -colorspace RGB                  \) \
  \( -clone 0 -colorspace Gray                 \) \
  \( -clone 0 -colorspace LinearGray           \) \
  \( -clone 0 -colorspace RGB -colorspace Gray \) \
  \( -clone 1 -colorspace Gray                 \) \
  \( +clone   -colorspace LinearGray           \) \
  \( +clone   -colorspace Gray                 \) \
  \( +clone   -colorspace LinearGray           \) \
  txt: | grep '^0'
0,0: (65535,42405,0)  #FFFFA5A50000  orange
0,0: (65535,24658.3,0)  #FFFF60520000  rgb(100%,37.6262%,0%)
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
0,0: (31571)  #7B537B537B53  lineargray(48.1743%)
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
0,0: (27115.7)  #69EC69EC69EC  lineargray(41.3759%)
0,0: (27115.7)  #69EC69EC69EC  gray(41.3759%)
0,0: (27115.7)  #69EC69EC69EC  lineargray(41.3759%)

Compare the results to these 2 sets of Python calculations (the formulas for which were taken from the docs, as already described):

>>> ############## Using NON-linear sRGB values
>>> R,G,B = (65535,42405,0) # canvas:orange
>>> Gray = 0.212656*R+0.715158*G+0.072186*B
>>> Gray
44262.68595
>>> LinearGray = 0.298839*R+0.586811*G+0.114350*B
>>> LinearGray
44468.13432
>>> ############## Using LINEAR RGB values
>>> R,G,B = (65535,24658.3,0) # canvas:orange -colorspace RGB
>>> Gray = 0.212656*R+0.715158*G+0.072186*B
>>> Gray
31570.9914714
>>> LinearGray = 0.298839*R+0.586811*G+0.114350*B
>>> LinearGray
34054.1755463

As you can see:

• To calculate the Gray value from sRGB values, ImageMagick takes the non-linear sRGB values and runs them through the formula for non-linear Gray, as expected (well, that is, if you expect R and G and B in the formula to be non-linear).

• Yet, to calculate the LinearGray value from sRGB values, ImageMagick now takes the linear RGB values and runs them through the same formula for non-linear Gray; the LinearGray formula is not used at all!

  I thought this might be the equivalent of -colorspace RGB -colorspace Gray (which I've seen recommended in the documentation as the traditional way to get a linearized gray), but this is not the case; as you can see from the output of ImageMagick, doing such a double conversion is equivalent to just plain -colorspace Gray on a non-linear sRGB image.

Furthermore, as seen in the output of ImageMagick:

• When converting back and forth between Gray and LinearGray, the result stabilizes on an entirely different value; there is not a conversion of the data, but rather a corruption of the data.

[mfwitten]

Perhaps put more concisely, I'd expect these command lines to produce the same output, but they don't:

$ magick canvas:orange -colorspace Gray -colorspace sRGB txt: | grep ^0
0,0: (44262.7,44262.7,44262.7)  #ACE7ACE7ACE7  srgb(67.5405%,67.5405%,67.5405%)
$ magick canvas:orange -colorspace LinearGray -colorspace sRGB txt: | grep ^0
0,0: (47395,47395,47395)  #B923B923B923  srgb(72.3202%,72.3202%,72.3202%)

[urban-warrior]

Thanks for the problem report. We can reproduce it and will have a patch to fix it in GIT master branch @ https://github.com/ImageMagick/ImageMagick later today. The patch will be available in the beta releases of ImageMagick @ https://www.imagemagick.org/download/beta/ by sometime tomorrow.

[mfwitten]

Alas, the original problem is back (Gray and LinearGray are indistinguishable):

$ magick                                \
  canvas:orange                         \
  \( -clone 0 -colorspace Gray       \) \
  \( -clone 0 -colorspace LinearGray \) \
  txt: | grep '^0'
0,0: (65535,42405,0)  #FFFFA5A50000  orange
0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
0,0: (44262.7)  #ACE7ACE7ACE7  lineargray(67.5405%)

That last line should have the following channel value (as calculated previously using the LinearGray formula with non-linear sRGB inputs R and G and B):

44468.13432

[urban-warrior]

Thanks for the problem report. We can reproduce it and will have a patch to fix it in GIT master branch @ https://github.com/ImageMagick/ImageMagick later today. The patch will be available in the beta releases of ImageMagick @ https://www.imagemagick.org/download/beta/ by sometime tomorrow.

[mfwitten]

• Now, conversion between Gray and LinearGray appears to cause the channel value to approach zero asymptotically:

  $ magick canvas:orange                \
    \( +clone -colorspace Gray       \) \
    \( +clone -colorspace LinearGray \) \
    \( +clone -colorspace Gray       \) \
    \( +clone -colorspace LinearGray \) \
    \( +clone -colorspace Gray       \) \
    \( +clone -colorspace LinearGray \) \
    \( +clone -colorspace Gray       \) \
    \( +clone -colorspace LinearGray \) \
    \( +clone -colorspace Gray       \) \
    \( +clone -colorspace LinearGray \) \
    \( +clone -colorspace Gray       \) \
    \( +clone -colorspace LinearGray \) \
    txt: | grep ^0
  0,0: (65535,42405,0)  #FFFFA5A50000  orange
  0,0: (44262.7)  #ACE7ACE7ACE7  gray(67.5405%)
  0,0: (27115.7)  #69EC69EC69EC  lineargray(41.3759%)
  0,0: (30762.1)  #782A782A782A  gray(46.94%)
  0,0: (12242.1)  #2FD22FD22FD2  lineargray(18.6803%)
  0,0: (16180.5)  #3F353F353F35  gray(24.6899%)
  0,0: (3253.38)  #0CB50CB50CB5  lineargray(4.96434%)
  0,0: (6002.42)  #177217721772  gray(9.1591%)
  0,0: (574.546)  #023F023F023F  lineargray(0.876701%)
  0,0: (1728.82)  #06C106C106C1  gray(2.638%)
  0,0: (133.809)  #008600860086  lineargray(0.20418%)
  0,0: (472.997)  #01D901D901D9  gray(0.721747%)
  0,0: (36.6097)  #002500250025  lineargray(0.0558628%)
  

• I'll note, too, that I have reservations about the existing calculation of Gray.

  As shown in the previous discussion, the initial Gray channel value (44262.7) seems to be derived from using non-linear values for R and G and B in the conversion formula; however, not only is the formula fundamentally linear in nature, but the literature I've looked at (including ImageMagick's own documentation, as pointed out) says that those inputs should be linearized (e.g., already gamma-expanded). In other words, currently, the calculation for Gray seems to be this (Python interactive interpreter):

  >>> R,G,B = (65535,42405,0) # canvas:orange (e.g., gamma-compressed sRGB values)
  >>> Gray = 0.212656*R+0.715158*G+0.072186*B
  >>> Gray
  44262.68595
  

  but it should be this (assuming these linearized values are themselves correct):

  >>> R,G,B = (65535,24658.3,0) # canvas:orange -colorspace RGB
  >>> Gray = 0.212656*R+0.715158*G+0.072186*B
  >>> Gray
  31570.9914714
  

  EDIT:

  I didn't finish the calculation; the value for the Python variable Gray presumably needs to be gamma-compressed via something similar to the following (this is what I guess based on what I've skimmed in the literature):

  >>> def gamma_compress(x):
  ...     n = x/65535.0 # normalized
  ...     if n < 0.0031308:
  ...         g = 12.92 * n
  ...     else:
  ...         g = 1.055*n**(1/2.4) - 0.055
  ...     return g*65535
  ...
  >>> gamma_compress(Gray)
  47395.00382781265
  

[mfwitten]

Please note the EDIT at the end of the last comment.

[urban-warrior]

Thanks for your careful analysis and follow-up. As you can tell, this has been a challenging fix. Try the latest commit and let us know what still breaks.