Lab Implementation

[stupid]

Hi.

Like to draw your attention to a possible bug (or at least an inconsistency of implementation) in Lab when translating between colour spaces. Fuller explanation here: viewtopic.php?f=1&t=16223&p=58811&sid=0 ... 641#p58796


Thanks.



Jo

[stupid]

Bump!


Thanks.



Jo

[magick]

We are aware of the problem but we do not have an ETA on the solution.

[stupid]

Groovy! Baited breath etc.


Thanks for the heads up.



Jo

[magick]

We can repoduce the problem you posted and have a patch in ImageMagick 6.6.2-0 Beta available by sometime tomorrow. Thanks.

[stupid]

Okay. I tested it, and you won't be surprised - it works!

Just for curiosity's sake; why are the L channel separations in PhotoShop, GIMP, etc. so much lighter in tone?

Anyway, thanks a lot - very speedy service. Cheers.



Jo

[magick]

We're using a D50 white point. Does Photoshop use D65?

[stupid]

Well, you've got me there. I've no access to PhotoShop (or GIMP), just their (converted) output files; but virtually everything to do with running anything on Windows has 6500K on it so you're likely right. My monitor calibration software says D65 with gamma 2.20.



Jo

[magick]

We did compare the output of gimp to that of ImageMagick and it returned an RMSE of 0.0121 which suggests the pixels are fairly close in value.

[stupid]

Standard deviations and their implications aren't really my bag but I get the general drift - the lower the number the better. My comparison was by eyeballing:

GIMP and original ImageMagick


New ImageMagick


Not complaining though; merely curious.



Jo

[magick]

It could be a simple gamma adjustment or converting to sRGB rather than RGB. We're not sure. ImageMagick does follow the published standards for Lab to XYZ, XYZ to sRGB, and back.

Post a URL to your original Lab-encoded image and we'll download and take a closer look at the problem.

[stupid]

Hi ho.

Original
http://img193.imageshack.us/img193/2889/img2641eg.jpg

ImageMagick: Separated L*a*b Lightness Channel
http://img143.imageshack.us/img143/4527 ... tnessc.jpg

GIMP: Decompose to L*a*b Lightness Channel
http://img641.imageshack.us/img641/1484 ... blight.jpg



Jo

[stupid]

Bump.

Any further thoughts on this?



Jo

[stupid]

Bump.

[labs]

Hello,

I've got the values (ImageMagick-6.7.5-7-Q16-windows.zip):
# ImageMagick pixel enumeration: 475,650,255,rgb
0,0: ( 51, 33, 12) #33210C rgb(51,33,12)
1,0: ( 48, 32, 10) #30200A rgb(48,32,10)
2,0: ( 60, 41, 14) #3C290E rgb(60,41,14)

# ImageMagick pixel enumeration: 475,650,255,lab
0,0: (112, 2, 19) #700213 lab(112,2,19)
1,0: (110, 1, 20) #6E0114 lab(110,1,20)
2,0: (123, 1, 20) #7B0114 lab(123,1,20)

# ImageMagick pixel enumeration: 475,650,255,xyz
0,0: ( 98,103, 71) #626747 xyz(98,103,71)
1,0: ( 95,100, 67) #5F6443 xyz(95,100,67)
2,0: (107,113, 80) #6B7150 xyz(107,113,80)


CCCEn.xls (http://ccc.orgfree.com/) for RGB 51, 33, 12 and D50 gives the values:
Lab 14.50 5.34 14.17
XYZ 2.0 1.8 0.6

It could be useful to have parameters to set X Y Z reference values.