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Author Topic: Will the real histogram please stand up  (Read 1981 times)
bill t.
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« on: June 01, 2013, 11:53:25 PM »
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The first attached jpeg shows a newly loaded image to which I have added a flat Curves control.  Note the nice histogram marching up from a few black pixels.  At Level 1 I have a Count of 553 pixels.

Now I do nothing more than click the triangled "!" to see a "more accurate" histogram, if I understand Adobe's help pages.

Now look at what that does in the second attached jpeg.  YIKES!  A whole different histogram!  Now at Level 1 it says I have 183,320 pixels!  Without making any adjustments.

But if I go back and click on either the unmodified Curves layer or the image layer, I'm back to the histogram presentation in the first attached jpeg, where once again I am told I have 553 pixels at level 1 (which I take to mean RGB 1:1:1).

One of those histograms has got to be wrong.  The discrepancy at level 1 is is 553 pixels versus 183,320 pixels, a factor of 331.  IMHO that's not "more accurate" but rather "wildly different."

Comments?  What am I missing here?  Pretty much any random image will show the same effect, where clicking the "!" will reveal hugely more pixels clipped at either end of the histogram.  I'm thinking I'm just gonna have to pretend that "!" is not there.
« Last Edit: June 02, 2013, 12:00:27 AM by bill t. » Logged
hjulenissen
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« Reply #1 on: June 02, 2013, 12:15:12 AM »
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Be warned: I don't know Photoshop.

Your images seems to suggest that one image is based on 190000 pixels, while the other is based on 12 MP? Are your number of clipped pixels normalized to the total number of pixels in each instance?

Scaling overshoot can alter the number of clipped samples along highly modulated edges.

-h
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bill t.
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« Reply #2 on: June 02, 2013, 12:20:35 AM »
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You are misreading the data under the histogram, which is difficult to capture with the snipping tool.  Both screen captures were taken back to back from the same input file.
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FranciscoDisilvestro
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« Reply #3 on: June 02, 2013, 02:51:44 AM »
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The default configuration for Photoshop is to build the histogram from a representitave sampling of pixels from a image cache. This is for performace reasons.

The triangle is a "Cached data" warning . When you click on it, the histogram is calculated with the actual image and supposedly considering all pixels, so it is more accurate.

You can adjust the cache level in Edit -> Preferences -> Performance.

Regards
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BartvanderWolf
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« Reply #4 on: June 02, 2013, 03:03:54 AM »
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The default configuration for Photoshop is to build the histogram from a representitave sampling of pixels from a image cache. This is for performance reasons.

The triangle is a "Cached data" warning . When you click on it, the histogram is calculated with the actual image and supposedly considering all pixels, so it is more accurate.

You can adjust the cache level in Edit -> Preferences -> Performance.

That's correct, and it is signaled in the screen shots that Bill posted. One histogram is based on a "Cache Level 4" (see lower right-hand side), and the other on a "Cache Level 1", using 64 times as many pixels to sample from.

It's explained here.

Cheers,
Bart
« Last Edit: June 02, 2013, 03:06:43 AM by BartvanderWolf » Logged
bill t.
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« Reply #5 on: June 02, 2013, 01:14:26 PM »
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Perhaps I'm being unreasonable in wanting to see only one version of the histogram for any particular phase of image development.

I have to conclude that the phenomenon whereby Photoshop can display two conflicting histograms for the same image, in the absence of any modifications, is damned misleading!  Photoshop loads up an image, shows me a nice histogram, but as soon as I click the old "!" I see a horribly clipped histogram!  But then I go to make an adjustment, and without doing anything except clicking on the adjustment layer I'm back to the un-clipped version or the histogram.  Gimme a break!

I assume it's a performance related compromise.  Very likely a fossil of the days when we had 16mHz processors.  But to my simple mind it seems that an overly large and no longer needed compromise is being accepted here.  Hey, I want to use that histogram to control clipping!

PS.  Just noticed that "PhotoLine" does not share this two-faced phenomenon.  Its histogram always fully tracks the current state of the image in exquisite detail and never speaks with a forked tongue.  That's how it should be, right?

(And BTW, the gamma slider on PhotoLine's Curves adjustment panel is awesome!)

*******************************************************************************

Oh Duh!  I just realized that I've been using a Quantum Image Editor.  I can never know both the histogram and the appearance of my image at the same time!  That's as it should be.  Sorry for the fuss.  
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digitaldog
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« Reply #6 on: June 02, 2013, 01:18:37 PM »
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Perhaps I'm being unreasonable in wanting to see only one version of the histogram for any particular phase of image development.
I have to conclude that the phenomenon whereby Photoshop can display two conflicting histograms for the same image, in the absence of any modifications, is damned misleading!

It is, but it's done for performance issues. If you want more accuracy, you click on the symbol. That said, just what in either Histogram needs to be viewed so precisely that it tells you something you're not seeing displayed or the Info Palette (with appropriate sampling which is critical) doesn't tell you?
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Andrew Rodney
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bill t.
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« Reply #7 on: June 02, 2013, 01:49:38 PM »
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That said, just what in either Histogram needs to be viewed so precisely that it tells you something you're not seeing displayed or the Info Palette (with appropriate sampling which is critical) doesn't tell you?

1.  Any statistical information displayed by an image editing program should be dead-nuts accurate, or it should not be displayed at all.  Kinda-sorta accurate is unacceptable!  There is no possible excuse for data ambiguity in any form.  Period!  Over and out!  Do I have to say we're looking at a glaring case of slovenly program design here?  Seems like, although I know deep down Adobe may be stuck with this old dinosaur from days of yore.  If PhotoLine can get it right, so can PS.  Let's not forget this... Smiley 

2.  I have found that looking at the shape and distribution of my histogram is one of the best possible predictors of tonal weight and distribution in my prints.  I may be alone there, but that statistical approach has served me well.

3.  I rely on my histograms to tell me how much clipping I'm getting.  Yes I can spend several minutes poking around with the Arrow and looking at RGB values, and I can fiddle with Threshold, but I find the histogram so much faster in that regard especially while I'm actively making adjustments.
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digitaldog
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« Reply #8 on: June 02, 2013, 02:29:22 PM »
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1.  Any statistical information displayed by an image editing program should be dead-nuts accurate, or it should not be displayed at all.  Kinda-sorta accurate is unacceptable! 

That's a valid point but no matter what level of accuracy, someone can say it's not precise enough. Photoshop provides Lab values with one decimal point precision which for a lot of work is fine. ColorThink using more precision numerically expressing Lab values.

Quote
2.  I have found that looking at the shape and distribution of my histogram is one of the best possible predictors of tonal weight and distribution in my prints.  I may be alone there, but that statistical approach has served me well.

I don't know if you're alone but it isn't anything I've ever found useful. A Histogram for me is useful to see single, dual or all three channels have clipped. I can see that other ways without needing to look at the histogram too. You address this in #3 and I agree. Short of that, the height of the histogram, what's happening in between the end points is of no use to me. I'll start soft proofing and if the final is the print, at some point you have to make one and look at it. Far more valuable than a Histogram IMHO.

Now if you were to suggest that there be a preference whereby Photoshop always takes extra time to update the histogram, I have no problem with that. You might end up turning that off and ending up with the current behavior, then simply clicking on the triangle and looking at the update.
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Andrew Rodney
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FranciscoDisilvestro
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« Reply #9 on: June 02, 2013, 03:26:58 PM »
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Now if you were to suggest that there be a preference whereby Photoshop always takes extra time to update the histogram, I have no problem with that. You might end up turning that off and ending up with the current behavior, then simply clicking on the triangle and looking at the update.

But it is already there! Just go to Edit -> Preferences -> Performance and set the Cache level to 1 in the History & Cache section in the upper hand right. Now you will always see the correct histogram. If your performance is adversely affected you can always go back.

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bill t.
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« Reply #10 on: June 02, 2013, 03:49:24 PM »
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But it is already there! Just go to Edit -> Preferences -> Performance and set the Cache level to 1 in the History & Cache section in the upper hand right. Now you will always see the correct histogram. If your performance is adversely affected you can always go back.

I changed "Cache Level" to 1, rebooted PS, but I still see the same dueling histogram issue with all the images I try.  Tried a few other variations as well, none fixed the issue.
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FranciscoDisilvestro
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« Reply #11 on: June 02, 2013, 05:15:16 PM »
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I changed "Cache Level" to 1, rebooted PS, but I still see the same dueling histogram issue with all the images I try.  Tried a few other variations as well, none fixed the issue.

I was puzzled by your reply because mine works as expected, but then found this thread from the adobe forums, so it seems to be a bug in some versions of Photoshop where it does not respect the configured cache level.
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BartvanderWolf
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« Reply #12 on: June 02, 2013, 05:33:45 PM »
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I was puzzled by your reply because mine works as expected, but then found this thread from the adobe forums, so it seems to be a bug in some versions of Photoshop where it does not respect the configured cache level.

Hi,

Good catch! So we now only have to wait till the Photoshop CS6 version gets a maintenance update. This is assuming that Adobe indeed honors it's pledge to update CS6 for bug fixes ...

Cheers,
Bart
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FranciscoDisilvestro
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« Reply #13 on: June 21, 2013, 08:52:56 PM »
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Just an update on this issue.

I tested the cache levels / histogram issue with the latest Photoshop (CC) and it works fine. If I select cache levels = 1, then the histogram is always correct (updated)

Photoshop CC 64 bits
Windows 7 64 bits

I'm curious if other users keep having the issue of some versions not respecting the configuration for cache levels.

Regards
« Last Edit: June 21, 2013, 08:58:02 PM by FranciscoDisilvestro » Logged

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