need explicitly linear and nonlinear GRAY colorspaces

Post by **fmw42** » 2013-10-08T10:26:41-07:00

glennrp wrote:Yes, the PNG reader now sets image->gamma=1.0 when it reads a gAMA chunk with gamma==1.0.

The bug fix to make gamma=1 for the IM gamma (PNG gamma is already 1) seems to have been lost in the current beta.

see
viewtopic.php?f=3&t=24205#p103692

Dabrosny · Post by **Dabrosny** » 2013-10-08T11:09:57-07:00

glennrp wrote:Yes, the PNG reader now sets image->gamma=1.0 when it reads a gAMA chunk with gamma==1.0.

Okay, thanks!

Now I'd like to read this image back into IM, which now knows it's linear, and make it a 3-channel linear image (all channels initially equal of course), so that I can do some further linear processing that will change some of these color channels.

So I guess this will accomplish this, right?

Code: Select all

convert rose_luminance.png -colorspace RGB ....some processing... result.png

...or will it instead try to "linearize" the data with an sRGB-->RGB gamma expansion even though the gamma=1 indicates that it was already linear?

Post by **fmw42** » 2013-10-08T11:16:42-07:00

convert rose_luminance.png -colorspace RGB ....some processing... result.png

But my images were not properly linear since the IM gamma=0.4545

Also IM does not have single channel grayscale images. So separating the channels into linear grayscale channels will all be the same as the input and 3 channel gray.

Dabrosny · Post by **Dabrosny** » 2013-10-08T11:34:15-07:00

fmw42 wrote:
But my images were not properly linear since the IM gamma=0.4545

But what I'm asking is, once the fix is (back) in place, and I read into IM a 1-channel linear image properly identified as linear by gamma=1, can I explicitly expand it to a 3-channel linear RGB image via -colorspace so that I can do some further linear processing that will change some of these color channels.

Code: Select all

    convert rose_luminance.png -colorspace RGB ....some processing... result.png

...or will this instead try to "linearize" the data with an sRGB-->RGB gamma expansion even though the gamma=1 indicates that it was already linear?

Post by **fmw42** » 2013-10-08T14:10:22-07:00

Dabrosny wrote: But what I'm asking is, once the fix is (back) in place, and I read into IM a 1-channel linear image properly identified as linear by gamma=1, can I explicitly expand it to a 3-channel linear RGB image via -colorspace so that I can do some further linear processing that will change some of these color channels.
Code: Select all
    convert rose_luminance.png -colorspace RGB ....some processing... result.png
...or will this instead try to "linearize" the data with an sRGB-->RGB gamma expansion even though the gamma=1 indicates that it was already linear?

I am not sure, but I believe that it should not matter if you do

convert rose_luminance.png -colorspace RGB ....some processing... result.png

or

convert rose_luminance.png -set colorspace RGB ....some processing... result.png

or just leave it out

convert rose_luminance.png ....some processing... result.png

I would think that IM would know it is linear by the gamma=1 which any of the above would still maintain. Once the image is linear, IM should know and -colorspace RGB will not matter nor will -set colorspace RGB.

But I think it would be best to wait until the bug gets fixed so that I can be sure.

The only issue would be if you use some function or image format for the output that may generate non-linear results. That would be the case for example if you composite a linear image with a non-linear image.

Post by **fmw42** » 2013-10-08T21:32:07-07:00

The bug fix is in place. I had checked the verbose info with an older release and that misread the gamma value. So

imbh convert rose: -colorspace sRGB -grayscale rec709luminance rose_luminance.png
imbh convert rose_luminance.png -format "%[gamma]\n" info:
1

imbh convert rose_luminance.png -level 0x90% rose_luminance_level.png
imbh convert rose_luminance_level.png -format "%[gamma]\n" info:
1

imbh convert rose_luminance.png -resize 200% rose_luminance_resize.png
imbh convert rose_luminance_resize.png -format "%[gamma]\n" info:
1

So IM knows that the image is linear and processes it to preserve the gamma=1 linearity

Dabrosny · Post by **Dabrosny** » 2013-10-15T06:03:40-07:00

fmw42 wrote: I am not sure, but I believe that it should not matter if you do

convert rose_luminance.png -colorspace RGB ....some processing... result.png

or

convert rose_luminance.png -set colorspace RGB ....some processing... result.png

or just leave it out
...<snip/>...
I would think that IM would know it is linear by the gamma=1 which any of the above would still maintain. Once the image is linear, IM should know and -colorspace RGB will not matter nor will -set colorspace RGB.

But I think it would be best to wait until the bug gets fixed so that I can be sure.

Okay, can you try those two with the beta? (even without "...some processing..." for starters).

I hope you're right that the resulting png file is the same ({edit:} the data values themselves as well as gamma and colorspace) -- that would certainly make sense.

Post by **fmw42** » 2013-10-15T10:02:58-07:00

imbh convert rose: -colorspace sRGB -grayscale rec709luminance rose_luminance.png

Code: Select all

Image: rose_luminance.png
Geometry: 70x46+0+0
Page geometry: 70x46+0+0
Class: PseudoClass
Colorspace: Gray
Type: Grayscale
Depth: 8/1-bit
Alpha: False
Channels: gray
Rendering intent: Undefined
Gamma: 1
Colors: 200
Gray:


see http://www.imagemagick.org/discourse-server/viewtopic.php?f=2&t=24256
min: 5 (0.0196078)
max: 255 (1)
mean: 49.9761 (0.195985)
standard deviation: 53.9087 (0.211407)

imbh convert rose_luminance.png -colorspace RGB rose_luminance_1.png

Code: Select all

Image: rose_luminance_1.png
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  Type: Grayscale
  Endianess: Undefined
  Colorspace: Gray
  Depth: 8/1-bit
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      standard deviation: 52.26 (0.204941)
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       158: (158,158,158) #9E9E9E gray(158)
       159: (159,159,159) #9F9F9F gray(159)
       160: (160,160,160) #A0A0A0 gray(160)
       161: (161,161,161) #A1A1A1 gray(161)
       162: (162,162,162) #A2A2A2 gray(162)
       163: (163,163,163) #A3A3A3 gray(163)
       164: (164,164,164) #A4A4A4 gray(164)
       165: (165,165,165) #A5A5A5 gray(165)
       166: (166,166,166) #A6A6A6 gray(166)
       167: (167,167,167) #A7A7A7 gray(167)
       168: (168,168,168) #A8A8A8 gray(168)
       169: (169,169,169) #A9A9A9 gray(169)
       170: (170,170,170) #AAAAAA gray(170)
       171: (171,171,171) #ABABAB gray(171)
       172: (172,172,172) #ACACAC gray(172)
       173: (173,173,173) #ADADAD gray(173)
       174: (174,174,174) #AEAEAE gray(174)
       175: (175,175,175) #AFAFAF gray(175)
       176: (176,176,176) #B0B0B0 gray(176)
       177: (177,177,177) #B1B1B1 gray(177)
       178: (178,178,178) #B2B2B2 gray(178)
       179: (179,179,179) #B3B3B3 gray(179)
       180: (180,180,180) #B4B4B4 gray(180)
       181: (181,181,181) #B5B5B5 gray(181)
       182: (182,182,182) #B6B6B6 gray(182)
       183: (183,183,183) #B7B7B7 gray(183)
       184: (184,184,184) #B8B8B8 gray(184)
       185: (185,185,185) #B9B9B9 gray(185)
       186: (186,186,186) #BABABA gray(186)
       187: (187,187,187) #BBBBBB gray(187)
       188: (188,188,188) #BCBCBC gray(188)
       189: (189,189,189) #BDBDBD gray(189)
       190: (190,190,190) #BEBEBE gray(190)
       191: (191,191,191) #BFBFBF gray(191)
       192: (192,192,192) #C0C0C0 gray(192)
       193: (193,193,193) #C1C1C1 gray(193)
       194: (194,194,194) #C2C2C2 gray(194)
       195: (195,195,195) #C3C3C3 gray(195)
       196: (196,196,196) #C4C4C4 gray(196)
       197: (197,197,197) #C5C5C5 gray(197)
       198: (198,198,198) #C6C6C6 gray(198)
       199: (199,199,199) #C7C7C7 gray(199)
       200: (200,200,200) #C8C8C8 gray(200)
       201: (201,201,201) #C9C9C9 gray(201)
       202: (202,202,202) #CACACA gray(202)
       203: (203,203,203) #CBCBCB gray(203)
       204: (204,204,204) #CCCCCC gray(204)
       205: (205,205,205) #CDCDCD gray(205)
       206: (206,206,206) #CECECE gray(206)
       207: (207,207,207) #CFCFCF gray(207)
       208: (208,208,208) #D0D0D0 gray(208)
       209: (209,209,209) #D1D1D1 gray(209)
       210: (210,210,210) #D2D2D2 gray(210)
       211: (211,211,211) #D3D3D3 gray(211)
       212: (212,212,212) #D4D4D4 gray(212)
       213: (213,213,213) #D5D5D5 gray(213)
       214: (214,214,214) #D6D6D6 gray(214)
       215: (215,215,215) #D7D7D7 gray(215)
       216: (216,216,216) #D8D8D8 gray(216)
       217: (217,217,217) #D9D9D9 gray(217)
       218: (218,218,218) #DADADA gray(218)
       219: (219,219,219) #DBDBDB gray(219)
       220: (220,220,220) #DCDCDC gray(220)
       221: (221,221,221) #DDDDDD gray(221)
       222: (222,222,222) #DEDEDE gray(222)
       223: (223,223,223) #DFDFDF gray(223)
       224: (224,224,224) #E0E0E0 gray(224)
       225: (225,225,225) #E1E1E1 gray(225)
       226: (226,226,226) #E2E2E2 gray(226)
       227: (227,227,227) #E3E3E3 gray(227)
       228: (228,228,228) #E4E4E4 gray(228)
       229: (229,229,229) #E5E5E5 gray(229)
       230: (230,230,230) #E6E6E6 gray(230)
       231: (231,231,231) #E7E7E7 gray(231)
       232: (232,232,232) #E8E8E8 gray(232)
       233: (233,233,233) #E9E9E9 gray(233)
       234: (234,234,234) #EAEAEA gray(234)
       235: (235,235,235) #EBEBEB gray(235)
       236: (236,236,236) #ECECEC gray(236)
       237: (237,237,237) #EDEDED gray(237)
       238: (238,238,238) #EEEEEE gray(238)
       239: (239,239,239) #EFEFEF gray(239)
       240: (240,240,240) #F0F0F0 gray(240)
       241: (241,241,241) #F1F1F1 gray(241)
       242: (242,242,242) #F2F2F2 gray(242)
       243: (243,243,243) #F3F3F3 gray(243)
       244: (244,244,244) #F4F4F4 gray(244)
       245: (245,245,245) #F5F5F5 gray(245)
       246: (246,246,246) #F6F6F6 gray(246)
       247: (247,247,247) #F7F7F7 gray(247)
       248: (248,248,248) #F8F8F8 gray(248)
       249: (249,249,249) #F9F9F9 gray(249)
       250: (250,250,250) #FAFAFA gray(250)
       251: (251,251,251) #FBFBFB gray(251)
       252: (252,252,252) #FCFCFC gray(252)
       253: (253,253,253) #FDFDFD gray(253)
       254: (254,254,254) #FEFEFE gray(254)
       255: (255,255,255) #FFFFFF gray(255)
  Rendering intent: Undefined
  Gamma: 1
  Background color: gray(255)
  Border color: gray(223)
  Matte color: gray(189)
  Transparent color: gray(0)
  Interlace: None
  Intensity: Rec709Luminance
  Compose: Over
  Page geometry: 70x46+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: Undefined
  Properties:
    date:create: 2013-10-15T09:49:07-07:00
    date:modify: 2013-10-15T09:49:07-07:00
    png:bKGD: chunk was found (see Background color, above)
    png:gAMA: gamma=1 (See Gamma, above)
    png:IHDR.bit-depth-orig: 8
    png:IHDR.bit_depth: 8
    png:IHDR.color-type-orig: 0
    png:IHDR.color_type: 0 (Grayscale)
    png:IHDR.interlace_method: 0 (Not interlaced)
    png:IHDR.width,height: 70, 46
    png:text: 2 tEXt/zTXt/iTXt chunks were found
    signature: 6c40361a5da66526ad84d455db2743f96ce3d551608fc16d9f7ef75030e83eac
  Artifacts:
    filename: rose_luminance_1.png
    verbose: true
  Tainted: True
  Filesize: 1.88KB
  Number pixels: 3.22K
  Pixels per second: 3.22EB
  User time: 0.010u
  Elapsed time: 0:01.000
  Version: ImageMagick 6.8.7-1 2013-10-08 Q16 http://www.imagemagick.org

imbh convert rose_luminance.png -set colorspace RGB rose_luminance_2.png

Code: Select all

Image: rose_luminance_2.png
Geometry: 70x46+0+0
Page geometry: 70x46+0+0
Class: PseudoClass
Colorspace: Gray
Type: Grayscale
Depth: 8/1-bit
Alpha: False
Channels: gray
Rendering intent: Undefined
Gamma: 1
Colors: 200
Gray:
min: 5 (0.0196078)
max: 255 (1)
mean: 49.9761 (0.195985)
standard deviation: 53.9087 (0.211407)

rose_luminance_2 is the same as rose_luminance. Both are colorspace gray, type grayscale and gamma=1 and the min,max,mean,std are all the same.

rose_luminance_1 is different. It has apparently undergone some change by the -colorspace RGB, which I did not expect, since it still results in gamma=1. The IM developers would have to explain.

Dabrosny · Post by **Dabrosny** » 2013-10-15T14:18:09-07:00

I believe it is treating the grayscale colorspace as being nonlinear similar to sRGB (regardless of the gamma value), so -colorspace RGB does an sRGB-to-RGB gamma expansion, thus the data has been "doubly-expanded".

This should be fixed.

A "Gray" colorspace image with gama=1 should be treated similarly to RGB when it is converted (explicitly or implicitly) to any other colorspace.

I.e. the result should be the same as if we started with an RGB (linear) image having three equal channels (equal to the single grayscale channel).

Post by **fmw42** » 2013-10-15T16:11:57-07:00

Yes, I agree. That is why I posted to the Developers forum to be sure before I report as a bug.

Dabrosny · Post by **Dabrosny** » 2013-10-15T17:46:22-07:00

I still think it might be cleaner to have a CieLuminance (linear grayscale) colorspace so that colorspace conversion for grayscale doesn't require a completely different internal identification mechanism (image->gamma) compared to all other colorspaces (where gamma has no effect).

Somebody said we shouldn't have a colorspace that isn't externally recognized, but in fact CIE Y is very well recognized and used. The representation in a PNG file would be exactly the same as it is today for linear RGB (and now the result of -grayscale rec709luminance), i.e. simply setting png chunk gAMA=1. But when reading in the PNG file with gamma=1, the IM colorspace would be set to CIELuminance internally so that colorspace conversion could be performed accurately without relying on image->gamma which is otherwise ignored (just propagated) for purposes of other colorspaces.

But so long as grayscale with gamma=1 can be consistently treated as if it were a linear Luminance colorspace (like linear RGB with R=G=B), that would certainly be much, much better than what we have today.

Post by **fmw42** » 2013-10-15T18:16:02-07:00

IM has -colorspace rec709luma which is the same as -grayscale rec709luma. I suppose IM could have -colorspace rec709luminance, but that is the same as -grayscale rec709luminance. At least that was the way it was supposed to be apart from a recent bug that changed its behavior and which in principle should be fixed in 6.8.7.1 when it is released.

Dabrosny · Post by **Dabrosny** » 2013-10-15T20:47:13-07:00

fmw42 wrote:I suppose IM could have -colorspace rec709luminance, but that is the same as -grayscale rec709luminance. At least that was the way it was supposed to be apart from a recent bug that changed its behavior and which in principle should be fixed in 6.8.7.1 when it is released.

I'm not just looking for a method to perform the conversion, but a colorspace that would tag the image internally (and at least in miff) as to what kind of grayscale data your image represents. If -colorspace rec709luminance would actually set a colorspace called rec709luminance (not just "gray"), that would be a step in the right direction.

(Aside from the name being strange -- the difference between rec709luminance and rec601luminance is just the coefficients you use to get from linear 709 primaries or 601 primaries to CIE Y. If calculated correctly from where ever you start, you should end up with the same linear CIE Luminance Y (as in CIE 1931 XYZ) in both cases as the final colorspace, so I'm not sure why they should be tagged as different colorspaces at that point)

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need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces

Re: need explicitly linear and nonlinear GRAY colorspaces