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 Author Topic: RGB values of a 18% Gray Card?  (Read 22898 times)
Min
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 « on: April 14, 2008, 12:11:13 PM » Reply

Any one know? I dont think this will be a straight forward answer...

thanks

min
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Jonathan Wienke
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This has been discussed a few times in recent threads. Do a search.
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Min
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Peter_DL
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IIRC - this 18% gray refers to a white of 90% reflectance.

So the normalized reflectance for this perceived mid gray is 18/90= 0.2

In a linear gamma space this makes an RGB of 0.2 x 255 = 51

In a 2.2 gamma space such as Adobe RGB it is (51/255)^(1/2.2) x 255 = 123

Peter

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digitaldog
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The other question would be, why would it a useful number?
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Andrew Rodney
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http://digitaldog.net/
Jonathan Wienke
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Read this thread, paying close attention to the end of post #4. You'll learn what the value is for various color spaces, and also why worrying about isn't very useful.
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bjanes
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The other question would be, why would it a useful number?
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The middle gray can be important for calibration and proofing purposes as explained in some detail in this post [a href=\"http://www.cdiny.com/ArticlesWhitePapers/ISO%20Standards%20for%20Museum%20Imaging_cdi_v1.0.pdf]ISO Standards for Museum Imaging[/url]

Bill
 « Last Edit: April 14, 2008, 01:57:55 PM by bjanes » Logged
wolfnowl
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The middle gray can be important for calibration and proofing purposes as explained in some detail in this post ISO Standards for Museum Imaging.
[a href=\"index.php?act=findpost&pid=189479\"][{POST_SNAPBACK}][/a]

Thanks,
Mike.
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Jonathan Wienke
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bjanes
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Thanks,
Mike.
[a href=\"index.php?act=findpost&pid=189480\"][{POST_SNAPBACK}][/a]

Bill
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Jonathan Wienke
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bjanes
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[a href=\"index.php?act=findpost&pid=189490\"][{POST_SNAPBACK}][/a]

The link works now with Firefox. A few minutes ago, the link's server appeared to be responding very slowly, so try again.

Bill
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digitaldog
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The middle gray can be important for calibration and proofing purposes as explained in some detail in this post ISO Standards for Museum Imaging

Ah but the issue is, its based on the encoding color space, the values can change.

Also, there's been some debate about using a scale like this, instead of say how Lightroom presents the values using a scale of 0-100%. 50% gray is 50% gray, using a scale of zero to 255 is based on the encoding color space which can range from 1.0 to 2.2 (or further if for some reason, someone's using some exotic color space). Yes, a 50% gray in a 1.8 gamma space is different than 50% gray in a 2.2 gamma space but the nice thing is, its always presented to the user as 50%.
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Andrew Rodney
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http://digitaldog.net/
Peter_DL
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Ah but the issue is, its based on the encoding color space, the values can change.
[a href=\"index.php?act=findpost&pid=189494\"][{POST_SNAPBACK}][/a]
A bit of math is simply part of the game.

Peter

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Colorwave
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The link just worked for me in Safari.  I have only skimmed it so far, but find the contents to be quite informative.  I notice that near the end they discuss the differences in reading color values from Photoshop and Lightroom and how they vary.  It seems that we have come a very long way from digital color's dark ages, but in terms of absolutes and color standards, we still have a ways to go.
-Ron H.
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bjanes
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Ah but the issue is, its based on the encoding color space, the values can change.
[a href=\"index.php?act=findpost&pid=189494\"][{POST_SNAPBACK}][/a]

Yes, that is one of the main points in the article. The author suggests using L* to remove these ambiguities.

Quote
Also, there's been some debate about using a scale like this, instead of say how Lightroom presents the values using a scale of 0-100%. 50% gray is 50% gray, using a scale of zero to 255 is based on the encoding color space which can range from 1.0 to 2.2 (or further if for some reason, someone's using some exotic color space). Yes, a 50% gray in a 1.8 gamma space is different than 50% gray in a 2.2 gamma space but the nice thing is, its always presented to the user as 50%.
[a href=\"index.php?act=findpost&pid=189494\"][{POST_SNAPBACK}][/a]

Your math is incorrect. Mid-gray (L* = 50) can be converted to gamma 1.8 or 2.2 values by use of Bruce Lindbloom's companding calculator. The normalized pixel values are 0.3907 for a gamma 1.8 space and 0.4635 for a gamma 2.2 space. The corresponding values in 8 bit are 99.62 and 118.2 respectively. Lightroom reports values in normalized notation for gamma 2.2.

If you create a mid-gray in Photoshop by filling an sRGB image with midgray (L* = 50), the sRGB pixel value in Photoshop is 119, close to the calculated value of 118.2. If you import this image into Lightroom, the pixel value is reported as 46.7 or 119 in 8 bit notation, not 50% as you claim.

If you read the paper, you will see documented inconsistencies between Photoshop and Lightroom.

Bill
 « Last Edit: April 14, 2008, 09:58:24 PM by bjanes » Logged
digitaldog
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Your math is incorrect. Mid-gray (L* = 50) can be converted to gamma 1.8 or 2.2 values by use of Bruce Lindbloom's companding calculator. The normalized pixel values are 0.3907 for a gamma 1.8 space and 0.4635 for a gamma 2.2 space.

You're missing the point and geeking up the experience. The math is necessary to convert but there's no need to do so with this simpler scale for end users. I don't need to know the values in 0-255 any more than I do 0-640000 odd values. I only need to know where highlight, shadow clip (0-100) and where middle gray falls (50%).

For years we've had photographers ask "what RGB values equal middle gray" and we have to jump through the math hoops due to the encoding space. Photoshop could do this for us! Its being done in Lightroom. 0-255 is fine for some old farts, its been around since day one of Photoshop. But that doesn't mean newer users need to worry about it (they don't) and in fact, we really NEED this scale in Photoshop for all the newer LR users who never see (nor need to see) 0-255.
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Andrew Rodney
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http://digitaldog.net/
bjanes
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You're missing the point and geeking up the experience. The math is necessary to convert but there's no need to do so with this simpler scale for end users. I don't need to know the values in 0-255 any more than I do 0-640000 odd values. I only need to know where highlight, shadow clip (0-100) and where middle gray falls (50%).

For years we've had photographers ask "what RGB values equal middle gray" and we have to jump through the math hoops due to the encoding space. Photoshop could do this for us! Its being done in Lightroom. 0-255 is fine for some old farts, its been around since day one of Photoshop. But that doesn't mean newer users need to worry about it (they don't) and in fact, we really NEED this scale in Photoshop for all the newer LR users who never see (nor need to see) 0-255.
[a href=\"index.php?act=findpost&pid=189657\"][{POST_SNAPBACK}][/a]

Well if you need information about the shadow clipping in terms of percent gray, your Lightroom readout will not be accurate. For L* = 5, the LR readout will be about 9%, not 5%. The Lightroom readout is not percent gray, but a normalized pixel value for gamma 2.2. Often the two are close (as for mid-gray) but they are not in the shadows.

Bill
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digitaldog
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Well if you need information about the shadow clipping in terms of percent gray, your Lightroom readout will not be accurate. For L* = 5, the LR readout will be about 9%, not 5%. The Lightroom readout is not percent gray, but a normalized pixel value for gamma 2.2. Often the two are close (as for mid-gray) but they are not in the shadows.

Bill
[a href=\"index.php?act=findpost&pid=189662\"][{POST_SNAPBACK}][/a]

Well considering that the data is in a linear encoding, that the numbers are using a 2.2 encoding, there's a disconnect here and L* is pretty immaterial. The numbers and histogram are based on a color space that is neither being used under the hood or what you'd end up with in Photoshop.  But that's a different issue, one that sometimes troubles me just a tad.
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Andrew Rodney
Author “Color Management for Photographers”
http://digitaldog.net/
Min
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