Auto levels RGB output settings

In article ,
says…

Why do auto levels (and, indeed, auto curves) set output levels of individual R, G and B channels to start around 30?

Why not start at 0 for maximum dynamic range?

White and Black clips are set to 0%.

Is there some other setting which influences Auto operation which I may have set inadvertently?

The default clipping for Auto Levels clips 0.5% (which is a good value).

You can change this, in the (non-auto) Levels dialog, press the ALT key to change the Auto button to be Options. Then this Options will specify the default auto clipping. I assume this is what you mean above by 0%, but my 0.5% does in fact clip all three channels modestly and noticeably at both ends. I am guessing that you must have some nearly-but-not-quite zero data (pixel count) that is hard to see on the graph, but that qualifies to prevent more severe clipping. Maybe look at a different image to see if it shows better?

Searching the Photoshop Help for Auto Levels finds nothing, but searching for Auto can find it. I am referring to version 6.

—
Wayne
http://www.scantips.com "A few scanning tips"

On Sun, 26 Oct 2003 09:53:03 -0600, Wayne Fulton
wrote:

In article ,
says…

Why do auto levels (and, indeed, auto curves) set output levels of individual R, G and B channels to start around 30?

Why not start at 0 for maximum dynamic range?

White and Black clips are set to 0%.

Is there some other setting which influences Auto operation which I may have set inadvertently?

The default clipping for Auto Levels clips 0.5% (which is a good value).
You can change this, in the (non-auto) Levels dialog, press the ALT key to change the Auto button to be Options. Then this Options will specify the default auto clipping. I assume this is what you mean above by 0%,

Yes, that’s right. The reason I changed to 0% from the default 0.5% was to test if that’s what’s causing the shift.

but my 0.5% does in fact clip all three channels modestly and noticeably at both ends. I am guessing that you must have some nearly-but-not-quite zero data (pixel count) that is hard to see on the graph, but that qualifies to prevent more severe clipping. Maybe look at a different image to see if it shows better?

I have! I tried several quite different images and from different sources (film scanner, flatbed scanner, digital camera, etc) but there is a consistent clip of all 3 individual RGB *output* channels.

Indeed, as I click on Auto the whole histogram shrinks and shifts 30 clicks to the right. There is some slight and variable clipping on the right too. This is all on individual RGB output channels only! Master output is at full range as it should be.

Searching the Photoshop Help for Auto Levels finds nothing, but searching for Auto can find it. I am referring to version 6.

I also have 6 here.

BTW, this is not a big deal, because I don’t actually use Auto and set everything manually. But every now and then I’m just curious to see what PS "thinks" about an image and then I briefly click on Auto but the result I’m getting with this severe clipping is quite extreme.

Don.

"Don" wrote in message
[snip]

Indeed, as I click on Auto the whole histogram shrinks and shifts 30 clicks to the right. There is some slight and variable clipping on the right too. This is all on individual RGB output channels only! Master output is at full range as it should be.

The composite histogram is just that. IOW, if 2 channels are clipped and one is not, the composite will show an *unclipped* histogram. In any event, leave the auto-levels adjustment to the newbies…I’ve yet to see an image it didn’t damage.

[snip]

Do you have the histogram cache toggled in the prefs? If so, turn it off. If it is toggled, I’ve read the resulting histograms are built from the low rez screen shots, rather than the underlying image data, thus leading to inaccurate histograms.

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In article ,
says…

I have! I tried several quite different images and from different sources (film scanner, flatbed scanner, digital camera, etc) but there is a consistent clip of all 3 individual RGB *output* channels.

I assume you’re sure the clipping was not already present, as output from the scanner or camera. They often do this (and a little is often good for contrast).

I really dont see that here in Photoshop. I found an image that was originally already right at 0% clipping, then the Auto button on it with
0.0% has no additional effect (since it already is). 0.5% does clip it a
little, shifting it slightly, and causing the obvious taller data spikes right at the 0 and 255 ends. Any clipping will build or increase the height of those spikes at the ends (the count of pixels shifted to be zero value, ie, clipped).

—
Wayne
http://www.scantips.com "A few scanning tips"

in article , Don wrote on 26/10/03 10:32
am:

Why do auto levels (and, indeed, auto curves) set output levels of individual R, G and B channels to start around 30?

White and Black clips are set to 0%.

Is there some other setting which influences Auto operation which I may have set inadvertently?

Don

There are two settings, one in the Auto Color Corrections Options dialog box under Target Colors & Clipping (accessed by clicking on the Options button in Levels/Curves in Photoshop 7.0) and one in the Color Picker.

If you click on the black, gray or white color swatches in the ACCO dialog box, you can set the levels where the shadows, midtones and highlights are clipped. Alternatively, you can set them by double-clicking on the eyedroppers in Curves/Levels. I suspect you have R=G=B set to 30 (or thereabouts).

A good ball park setting for shadows is H0 S0 B5 (R=G=B 13), and for highlights H0 S0 B95 (R=G=B 242).

HTH.

–/ Shangara Singh.
:: Photoshop Glossary of Terms, Phrases & Acronyms
:: Photoshop 7.0 Essential Tips
:: http://www.photoshopace.com
:: Exam Aids for Photoshop, Illustrator & Dreamweaver :: http://www.examaids.com

On Sun, 26 Oct 2003 14:17:00 -0800, "Flycaster" wrote:

Do you have the histogram cache toggled in the prefs? If so, turn it off. If it is toggled, I’ve read the resulting histograms are built from the low rez screen shots, rather than the underlying image data, thus leading to inaccurate histograms.

I just checked, and it’s off. A very good tip, though!

FX: clicks "Keep Message"… 😉

Thanks very much!

Don.

On Sun, 26 Oct 2003 16:21:01 -0600, Wayne Fulton
wrote:

In article ,
says…

I have! I tried several quite different images and from different sources (film scanner, flatbed scanner, digital camera, etc) but there is a consistent clip of all 3 individual RGB *output* channels.

I assume you’re sure the clipping was not already present, as output from the scanner or camera. They often do this (and a little is often good for contrast).

No, it wasn’t present and that’s what confuses me. That’s also why I think it may be some "hidden" setting somewhere which instructs Auto to do this.

Oh well, I was just curious why it happens. If it continues to bug me I may reset all settings to default values and see what happens…

Thanks!

Don.

P.S. Never mind! As you can see in next message Shangara cracked it!

On Sun, 26 Oct 2003 22:25:29 +0000, Shangara Singh
wrote:

in article , Don wrote on 26/10/03 10:32
am:

Why do auto levels (and, indeed, auto curves) set output levels of individual R, G and B channels to start around 30?

White and Black clips are set to 0%.

Is there some other setting which influences Auto operation which I may have set inadvertently?

Don

There are two settings, one in the Auto Color Corrections Options dialog box under Target Colors & Clipping (accessed by clicking on the Options button in Levels/Curves in Photoshop 7.0) and one in the Color Picker.

I have Photoshop 6.1 but in the Levels dialog box, after clicking on
[Alternate] Options another dialog "Auto Range Options" pops up
containing "Black clip" and "White clip", which are both set to 0.50%.

If you click on the black, gray or white color swatches in the ACCO dialog box, you can set the levels where the shadows, midtones and highlights are clipped. Alternatively, you can set them by double-clicking on the eyedroppers in Curves/Levels. I suspect you have R=G=B set to 30 (or thereabouts).

BINGO!

That’s it! Double-clicking on the eyedroppers does indeed bring up another dialog box and, as you guessed, black clip was set to 30, gray to 70 and white to 245.

So, I was right! It was a "hidden" setting!

Question: Are those default settings? If not, who or what could have changed it? OK, it was probably me in the early days… 😉

A good ball park setting for shadows is H0 S0 B5 (R=G=B 13), and for highlights H0 S0 B95 (R=G=B 242).

OK, changed as we speak… Out of curiousity, what are rule-of-thumb setting for midtones?

Thanks very much indeed! Yet another Photoshop mystery solved.

Don.

in article , Don wrote on 27/10/03 2:56
pm:

If you click on the black, gray or white color swatches in the ACCO dialog box, you can set the levels where the shadows, midtones and highlights are clipped. Alternatively, you can set them by double-clicking on the eyedroppers in Curves/Levels. I suspect you have R=G=B set to 30 (or thereabouts).

BINGO!

That’s it! Double-clicking on the eyedroppers does indeed bring up another dialog box and, as you guessed, black clip was set to 30, gray to 70 and white to 245.

So, I was right! It was a "hidden" setting!

Well, they are sort of hidden. It depends on your starting position. If you start from the Auto Curves/Levels dialog box, they are hidden. If you start form the Color Picker, they are hard to miss – if you know what you are looking for! <g>

Question: Are those default settings? If not, who or what could have changed it? OK, it was probably me in the early days… 😉

I think you may be right. <g> I can’t remember what the default settings were but I suspect they are more likely to be R=G=B 0, H=S=B 0. Maybe Marc or Chris can help here if they’re reading.

A good ball park setting for shadows is H0 S0 B5 (R=G=B 13), and for highlights H0 S0 B95 (R=G=B 242).

OK, changed as we speak… Out of curiousity, what are rule-of-thumb setting for midtones?

R=G=B 128. If you set the shadow and highlight, the midtone should take care of itself (all things being equal!). Alternatively, enter 50 in the Brightness text field in the HSB section of the Color Picker (remember to enter the Color Picker by double-clicking on the eyedropper in Curves/Levels). These setting affect only the eyedroppers and the Auto corrections, AFAIK.

Thanks very much indeed! Yet another Photoshop mystery solved.

And life was never the same again on the mothership….<g>

–/ Shangara Singh.
:: Photoshop 7.0 Essential Tips
:: Photoshop Glossary of Terms, Phrases & Acronyms
:: http://www.photoshopace.com
:: Exam Aids for Photoshop, Illustrator & Dreamweaver :: http://www.examaids.com

"Don" wrote in message
[snip]

That’s it! Double-clicking on the eyedroppers does indeed bring up another dialog box and, as you guessed, black clip was set to 30, gray to 70 and white to 245.

So, I was right! It was a "hidden" setting!

Question: Are those default settings? If not, who or what could have changed it? OK, it was probably me in the early days… 😉

As I recall, the defaults are: WP=255, BP=0, GP=128.

A good ball park setting for shadows is H0 S0 B5 (R=G=B 13), and for highlights H0 S0 B95 (R=G=B 242).

OK, changed as we speak… Out of curiousity, what are rule-of-thumb setting for midtones?

If your intent is to use these for color balancing, the suggested settings of 242 and 13 are, imo, a little conservative; iow, they will clip colors and compress the tonality more than is necessary for today’s high end photo ink-jet printers. For example, I’ve found that my 2200 will use and print values between 6 and 248.

Personally, I use multiple samples and individual channel adjustments. But if I were to use a picker, it would be the mid-point picker in curves, not levels. Just find something that is NOT supposed to have any color cast, sample it, average the RGB values, and insert that value into the tool. Click and voila. You’ll end up with slightly tweaked curves, usually with little or no clipping, unlike what happens with the WP and BP picker.

Also, you can use these tools for quick and dirty "color matching" between images. Sample a mid-point value on anything in one image, note the values (they do NOT have to be grey balanced), go to the next image, insert the values, and click on what you want to have the same color.

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On Mon, 27 Oct 2003 16:34:23 +0000, Shangara Singh
wrote:

Question: Are those default settings? If not, who or what could have changed it? OK, it was probably me in the early days… 😉

I think you may be right. <g> I can’t remember what the default settings were but I suspect they are more likely to be R=G=B 0, H=S=B 0. Maybe Marc or Chris can help here if they’re reading.

Oh, it’s no big deal. I was just curious. If it really bugs me I can always force Photoshop to reset.

Thanks very much indeed! Yet another Photoshop mystery solved.

And life was never the same again on the mothership….<g>

:o) Live long and prosper! _\\//

(That’s the best I can do to emulate Vulcan greeting with ASCII art.)

Don.

in article 3f9d696a$, Flycaster wrote on 27/10/03
6:49 pm:

A good ball park setting for shadows is H0 S0 B5 (R=G=B 13), and for highlights H0 S0 B95 (R=G=B 242).

OK, changed as we speak… Out of curiousity, what are rule-of-thumb setting for midtones?

If your intent is to use these for color balancing, the suggested settings of 242 and 13 are, imo, a little conservative; iow, they will clip colors and compress the tonality more than is necessary for today’s high end photo ink-jet printers. For example, I’ve found that my 2200 will use and print values between 6 and 248.

I should’ve added those settings are for a "universal" offset press (you have to find the optimum from your prepress house or run off proofs).

And also, it’s worth bearing in mind the white point on all inkjets vary wildly. The only way to get the absolute best out of an inkjet and to pin the white point is to have it profiled for the paper and ink you intend to use.

So, while one inkjet may print values between, say, 6 and 248, given a paper and ink combo, it won’t on another paper or with a different ink set or another inkjet. And we are still talking about the same model! If you switch models, well…it’s a lottery…you may hit the jackpot but it’s more likely you won’t. <g>

–/ Shangara Singh.
:: Photoshop 7.0 Essential Tips
:: Photoshop Glossary of Terms, Phrases & Acronyms
:: http://www.photoshopace.com
:: Exam Aids for Photoshop, Illustrator & Dreamweaver :: http://www.examaids.com

….snip

Also, you can use these tools for quick and dirty "color matching" between images. Sample a mid-point value on anything in one image, note the values (they do NOT have to be grey balanced), go to the next image, insert the values, and click on what you want to have the same color.

….snip

Thanks very much for all the great hints and suggestions. Much appreciated!!

Don.

On Tue, 28 Oct 2003 08:12:11 +0000, Shangara Singh
wrote:

in article 3f9d696a$, Flycaster wrote on 27/10/03
6:49 pm:

A good ball park setting for shadows is H0 S0 B5 (R=G=B 13), and for highlights H0 S0 B95 (R=G=B 242).

OK, changed as we speak… Out of curiousity, what are rule-of-thumb setting for midtones?

If your intent is to use these for color balancing, the suggested settings of 242 and 13 are, imo, a little conservative; iow, they will clip colors and compress the tonality more than is necessary for today’s high end photo ink-jet printers. For example, I’ve found that my 2200 will use and print values between 6 and 248.

I should’ve added those settings are for a "universal" offset press (you have to find the optimum from your prepress house or run off proofs).
And also, it’s worth bearing in mind the white point on all inkjets vary wildly. The only way to get the absolute best out of an inkjet and to pin the white point is to have it profiled for the paper and ink you intend to use.

So, while one inkjet may print values between, say, 6 and 248, given a paper and ink combo, it won’t on another paper or with a different ink set or another inkjet. And we are still talking about the same model! If you switch models, well…it’s a lottery…you may hit the jackpot but it’s more likely you won’t. <g>

Well, at least inkjet odds are better… :o)

Seriously though, thank you very much indeed for taking the trouble to clear it all up.

Don.