Histograms: luminosity or saturation?

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I've been learning Photoshop for quite a while, I've done lots of tutorials and read a few books, but I can't find the answer to this simple thing: In the histogram palette menu there are red, green and blue histograms to give the saturation of those colors, and a luminosity histogram to give the overall ( greyscale) brightness. There is also an RGB option, presumably to give a composite version of the red, green and blue saturation.
In Levels, however we have the same thing, except there is no luminosity option, only RGB. But RGB in this case cannot be a composite of the three color channels, as we use it to adjust brightness and contrast; in other words it is presumably the same as Luminosity in the histogram palette. Despite this, when I do adjustments to an image the RGB histogram in Levels looks identical to the RGB histogram in the histogram palette. I'm obviously missing something, but I don't know what. Can anyone help?...I'm using PS 3 Extended.

On Sat, 11 Oct 2008 12:38:05 -0700, [email protected] wrote:
Good questions. It's no wonder that the histogram tool is a bit confusing,
since very little is said about it in the PS documentation. There are five
histogram "modes", RGB, red, green, blue, luminosity, and colors.
> 1) In the histogram Palette ( and indeed in Levels) do the color channel histograms
> represent luminosity ( which I take to be brightness) or color saturation? (These
> are presumably separate entities).
Speaking only of RGB images, the red, green, and blue channel histograms
are the first ones to sort out. They represent the number of values
(0-255) of each color channel. The Color histogram is a superimposed
display of red, green, and blue histograms. The absolute height of the
peaks in the histogram is determined by the height of the maximum peak, and
this is done on a per channel basks.
> 2)Does the black RGB histogram represent a composite of the 3 color
> channels( as I think you've said ) or the grey tonal values? If the
> former, why do we use it to adjust tones ie. make the image darker
> or lighter,increase contrast etc?
The black RGB histogram is a combined view of the histograms for each of
the three channels, however all the channels are vertically scaled by the
same amount, instead of being scaled individually per channel. This means
that the location of the peaks for each individual channel stays the same,
but that the height of each peak will be smaller for two of the channels.
> 3) What is the luminosity option for in the Histogram Palette and why is there not one in Levels?
Luminosity is calculated by taking the weighted sum of the red, green, and
blue values for each pixel. The weight is smaller for blue, since it is
the darkest color, and largest for green. This corresponds closely to the
histogram seen in many camera displays, and is the reason that clipping can
occur, but not show in the histogram.
> 4)In the channels palette, do the greyscale images for each channel represent colors,
> or are there really greyscale images behind each color channel?
Each channel is a grayscale image. For RGB, each image can be thought of
as providing a certain amount of red, green, or blue to the image.
> 5) Do Curves adjustments increase saturation or brightness ie. Luminosity.
Again, speaking only in RGB, Curves can increase saturation and luminosity
in a variety of ways, since saturation, hue, and luminosity information is
mixed together in each of the red, green, and blue channels. Many people
find that working in the Lab color mode is simpler, since the L channel
contains all the luminosity information, and a and b contain independent
color information. Try Lab - you might like it very much indeed.
> 6) Why is there a black channel in CMYK if each channel has its own greyscale image?
The black channel borrows data, more or less equally, from the C, M, and Y
images. This produces a fourth channel, called K, that provides excellent
control over shadows and textures. Since the CMY channels contain more
color information, they provide a very sensitive adjustment for color,
which is good for things like skin tone variation.
> As you can see, I am having problems understanding what precisely color channels
> are, and also what exactly histograms are telling us.
It takes a while for all of this to sink in, but it does not require a PhD.
Things like additive and subtractive color, and the idea that organizing
color data into different color models, such as RGB, Lab, HSB, and CMYK can
provide leverage.
> I realise this might be a tall order for a forum such as this but I would appreciate
> any help you can give.
I think it's important to work these things out from basic principles, as
you are doing. Grab a book by Dan Margulis, and some light bulbs will
start to turn on regarding the different color spaces.
BTW - histograms are a poor tool for color correction. If there are
problems with clipping or other issues, it is much better to look at the
individual channels of the image, rather than the histogram.

Similar Messages

Luminosity and saturation blending modes in FW CS4 differ from their equivalents in PSE 7

Example:
Upper bitmap is filled with RGB(232,168,89) color and the lower with RGB(61,114,158) .
Blending results are as follow:
Luminosity in FW CS4: RGB(67,121,175)
Luminosity in PSE 7: RGB(137,190,234)
Saturation in FW CS4: RGB(60,109,158)
Saturation in PSE 7: RGB(41,119,184)
I think PSE uses chroma as saturation and luma Y' 601 with gamma compression (2.2 or 1.8) as luminosity.
So it's compatible with PS. And what about FW. Does somebody know how FW works. I was experimenting with
HLS and HSV and wasn't able to find it out. Maybe FW's modes are better in some web cases, I don't know.

What happens to your blended colors in PSE if you turn off the color profiles?
Fireworks is intended to produce graphics for display in Web applications, which have only recently begun to support color profiles. Even then, you don't want to use them on your page graphics (like nav buttons), because the profiles won't apply to HTML/CSS colors and you'll get mismatches.
PS is primarily intended for photographs and for printing, and so the standard workflow in PS (and I suspect also PSE) is to apply at least a default color profile. One use of color profiles is to correct the colors for printers, but profiles will also change the colors in images.
Have a read:
http://www.viget.com/inspire/save-for-web-simply
http://www.rumblingskies.com/blog/?p=46

Nikon D700 Opens .6 to 1 stop over exposed in ACR 5.2

My D700 histogram shows the pix to be properly exposed but when I open them in ACR all images are .6 to 1 stop over exposed. While I can reduce exposure in ACR I have not had this problem in CS3 with my D100 & D200 cameras. What's the fix?

>The histogram you see on your camera is NOT based on your raw image directly, but on the JPEG preview created by the camera's built in software and to which adjustments are automatically applied.
What Ramón says is quite true, but you can compare the raw histogram to the camera histogram, With the Nikon D3 set to Adobe RGB and the standard Picture Control, I have found that the camera histogram and blinking highlights are slightly conservative but are generally reliable indicators of the raw histogram. Since the D3 and D700 have essentially the same image processing, I would think that this would also be true for the D700. (since inline images are not allowed in these forums, the message is hard to read since you have to click on the link to see the image. Ramón has figured out a way to have thumbnails with Pixentral, but I have not found a way to do this with my site, Smugmug)
Bill Janes, "ETTR and ACR" #3, 12 Nov 2008 6:37 am
One must be aware that the black and white camera histograms are luminance histograms which are strongly weighted towards the green luminance, whereas the ACR histogram is an RGB histogram and is similar to the color histograms of the camera superimposed on one another.
Both of these histograms reflect white balance. The red and blue channels may appear blown on the camera histogram or ACR histogram when in fact these channels are far from clipping in the raw file. Some users load a UniWB into the camera to get a better approximation of the raw channels.
Since the widest space in the camera is aRGB, the camera histogram may show saturation clipping with intact raw channels. It would be nice if Nikon would also provide ProPhotoRGB as a color space.

Photoshop - how do I make a surface soft and plain ?

Hey guys, been using PS "unofficially" for years now. I'm making this pic where I dropped a face from another pic and everything's fine except I just want to make the skin on the face look like the rest of the body. I've done everything I could luminosity/vibrance/saturation/contrast wise, but it still looks too sharp and "shiny", and I want it to look soft and plain, sort of less shiny/contrast-y. Know what I mean ? I'd provide the example, but it's kind of personal, unless I really really have to. Let me know, would really make this thing perfect if I could just get that info.
Have tried loads of stuff from the "Filter" menu, but nothing worked. I use a Mac, and PS CS5.
THANK YOU THANK YOU !

There are many threads on making composites and working on skin often how one address this depends on the actual image content. I think you may find what you looking for faster if you do some web searching.

Can an action set the dimension of the crop and put in the center of the image?

Hi
running cs6 64bit under w7 64bit
can an action set the dimensions of the crop height & length and put the crop in the center of the image?
or should i need a script?
i mean for example i taken many photos about 20mp mega pixel
i would like to run a script or an action that set the height & length and set the crop in the center of the image
thanks

mantralightroom wrote:
i have cs6 but i haven't in the action option conditional actions
ps i tried to record a script to set the histogram channel RGB or Luminocity , but the script plugin doesn't record it and even the action can do it
you know cs6 starts with histogram in colors mode
I have a hard time with English even though it may native language. I'm having some problem with what you wrote.
Yes cs6 does not have what Adobe calls conditional actions they put into CC. That feature more or less an IF statement condition on some document attributes. Like if document width is greater then document height play action landscape else play action portrait.   With CS6 you need to write one line script to use in actions. Most of the utility scripts I wrote for use in actions are more complex then a simple if statement.
The way you record scripts is PS is be using Adobe Scriptlistener Plug-in when installed the plug-in records everything you do that can be recorded into two log files one in JavaScript and the other in VBS. The code generated works by passing parameters to Adobe Photoshop's Action Manager. To make scripts toy need to extract the steps you need from one of the log files.    However some thing you can do in Photoshop can not be recorded. And while your right that the scriptlistener plug-in does not record anything for setting the histograms to RGB channel or Luminosity channel your wrong when you write the Action Recorder can do it.
However scripts can get the histograms channel data. I have never process a histogram in a script however there is a sample Photoshop Histogram in the Photoshop javascripting guide. That creates a text graph log.
// Function to activate all the channels according to the documents mode
// Takes a document reference for input
function TurnOnDocumentHistogramChannels(inDocument) {
    // see how many channels we need to activate
    var visibleChannelCount = 0
    // based on the mode of the document
    switch (inDocument.mode) {
        case DocumentMode.BITMAP:
        case DocumentMode.GRAYSCALE:
        case DocumentMode.INDEXEDCOLOR:
            visibleChannelCount = 1
            break;
        case DocumentMode.DUOTONE:
            visibleChannelCount = 2
            break;
        case DocumentMode.RGB:
        case DocumentMode.LAB:
            visibleChannelCount = 3
            break;
        case DocumentMode.CMYK:
            visibleChannelCount = 4
            break;
        case DocumentMode.MULTICHANNEL:
        default:
            visibleChannelCount = inDocument.channels.length + 1
            break;
    // now get the channels to activate into a local array
    var aChannelArray = new Array()
    // index for the active channels array
    var aChannelIndex = 0
    for(var channelIndex = 0; channelIndex < inDocument.channels.length;channelIndex++) {
        if (channelIndex < visibleChannelCount) { aChannelArray[aChannelIndex++] = inDocument.channels[channelIndex] }
    // now activate them
    inDocument.activeChannels = aChannelArray
// Save the current preferences
var startRulerUnits = app.preferences.rulerUnits
var startTypeUnits = app.preferences.typeUnits
var startDisplayDialogs = app.displayDialogs
// Set Adobe Photoshop CC to use pixels and display no dialogs
app.preferences.rulerUnits = Units.PIXELS
app.preferences.typeUnits = TypeUnits.PIXELS
app.displayDialogs = DialogModes.NO
// if there are no documents open then try to open a sample file
if (app.documents.length == 0) { open(File(app.path + "/Samples/Fish.psd")) }
// get a reference to the working document
var docRef = app.activeDocument
// create the output file
// first figure out which kind of line feeds we need
if ($.os.search(/windows/i) != -1) {fileLineFeed = "Windows" }
else { fileLineFeed = "Macintosh"}
// create the output file accordingly
fileOut = new File("~/Desktop/Histogram.log")
fileOut.lineFeed = fileLineFeed
fileOut.open("w", "TEXT", "????")
// write out a header
fileOut.write("Histogram report for " + docRef.name)
// find out how many pixels I have
var totalCount = docRef.width.value * docRef.height.value
// more info to the out file
fileOut.write(" with a total pixel count of " + totalCount + "\n")
// channel indexer
var channelIndex = 0
// remember which channels are currently active
var myActiveChannels = app.activeDocument.activeChannels
// document histogram only works in these modes
if (docRef.mode == DocumentMode.RGB ||docRef.mode == DocumentMode.INDEXEDCOLOR ||docRef.mode == DocumentMode.CMYK) {
    // activate the main channels so we can get the documents histogram
    TurnOnDocumentHistogramChannels(docRef)
    // Output the documents histogram
    OutputHistogram(docRef.histogram, "Luminosity", fileOut)
// local reference to work from
var myChannels = docRef.channels
// loop through each channel and output the histogram
for (var channelIndex = 0; channelIndex < myChannels.length; channelIndex++) {
    // the channel has to be visible to get a histogram
    myChannels[channelIndex].visible= true
    // turn off all the other channels
    for (var secondaryIndex = 0; secondaryIndex < myChannels.length;secondaryIndex++) {
        if (channelIndex != secondaryIndex) { myChannels[secondaryIndex].visible= false }
    // Use the function to dump the histogram
    OutputHistogram(myChannels[channelIndex].histogram,myChannels[channelIndex].name, fileOut)
// close down the output file
fileOut.close()
alert("Histogram file saved to: " + fileOut.fsName)
// reset the active channels
docRef.activeChannels = myActiveChannels
// Reset the application preferences
app.preferences.rulerUnits = startRulerUnits
app.preferences.typeUnits = startTypeUnits
app.displayDialogs = startDisplayDialogs
// Utility function that takes a histogram and name
// and dumps to the output file
function OutputHistogram(inHistogram, inHistogramName, inOutFile) {
    // find ouch which count has the largest number
    // I scale everything to this number for the output
    var largestCount = 0
    // a simple indexer I can reuse
    var histogramIndex = 0
    // see how many samples we have total
    var histogramCount = 0
    // search through all and find the largest single item
    for (histogramIndex = 0; histogramIndex < inHistogram.length;histogramIndex++) {
        histogramCount += inHistogram[histogramIndex]
        if (inHistogram[histogramIndex] > largestCount)
        largestCount = inHistogram[histogramIndex]
    // These should match
    if (histogramCount != totalCount) {
        alert("Something bad is happening!")
    // see how much each "X" is going to count as
    var pixelsPerX = largestCount / 100
    // output this data to the file
    inOutFile.write("One X = " + pixelsPerX + " pixels.\n")
    // output the name of this histogram
    inOutFile.write(inHistogramName + "\n")
    // loop through all the items and output in the following format
    // 001
    // 002
    for (histogramIndex = 0; histogramIndex < inHistogram.length;histogramIndex++) {
    // I need an extra "0" for this line item to keep everything in line
    if (histogramIndex < 10)
        inOutFile.write("0")
    // I need an extra "0" for this line item to keep everything in line
    if (histogramIndex < 100)
        inOutFile.write("0")
    // output the index to file
    inOutFile.write(histogramIndex)
    // some spacing to make it look nice
    inOutFile.write(" ")
    // figure out how many X’s I need
    var outputX = inHistogram[histogramIndex] / largestCount * 100
    // output the X’s
    for (var a = 0; a < outputX; a++)
        inOutFile.write("X")
    inOutFile.write("\n")
inOutFile.write("\n")

How to create a non-destructive luminosity/b&w mask for (de)saturation?

Someone asked me this question: CG renders (created in Modo, Max, Blender, Maya, Vray, Octane, etc) can be made to look more realistic by desaturating the colours in the highlights. It was also requested that this move would be done in 32bpc as well (something that is often required in a CG compositing workflow).
This is easy enough to do: create a selection based on luminosity (or a black and white version of the image), and use that as a layer mask in the Hue/Saturation adjustment layer, and turn down the saturation.
However, this is a destructive action: suppose we want to import an external file as a smart object, and then we need the option to have the file automatically update with any change we make to the original artwork.
In that case we would have to recreate that luminosity mask again and again as well.
Ideally, an update of the external file will result in an automatic update of the entire layer stack.
Now, in Photoline this is easy to do:
Since layer masks behave like regular layers in PL, and layers can be virtually cloned (not unlike smart objects), it is a mere case of cloning the original external file layer twice, and adding adjustment layers. Finally, that one instanced layer is used as a layer mask for the adjustment layer that controls the saturation. Done.
Replacing or editing the original source will then automatically update and cascade the changes through the entire layer stack.
This is a very handy technique to have! Since I teach Photoshop classes, I thought this would also be handy to know how to do in Photoshop, and I tried several methods (clipping masks included), but I cannot seem to achieve the same non-destructive result. Clipping masks do not work with adjustment layers. Groups cannot be used as clipping masks. Layer masks cannot reference a smart object.
I have a feeling there ought to be a reasonably straightforward method to achieve this, but how?
Would anyone have a suggestion how to solve this in Photoshop in a non-destructive way?

csuebele wrote:
I'm still not seeing it. Your example does not have a pixel based layer clipped to the group which is causing my problem. I hope I can explain this. See below. the layers in the group create the "mask". In this case a b&w adjustment layer was added to control how the "mask" is converted to b&w. You can see what these layers look like in the mid section top section of the image. any color showing though is from the very bottom layer, as the group has a blend if on it allowing the shadows to come through. The top image has a curve layer with no adjustment, while the bottom image has a curve layer to clip more of the shadows. You can see this change in the upper right corner of both images. The curves are allowing more of the image to be clipped in the area is masked out on layer "Layer 0 copy 1", as more of the color of the base layer is showing though. However, that b&w areas are suppose to be just the mask and you should see the b&w, just color as seen in the bottom image. However, the curves are not changing the transparency in the areas that have the layer to return the color.
I don’t get it – what good is the pixel Layer ("Layer 0 copy 2") clipped to the Group supposed to do? Don’t you want to use the Mask on an Adjustment?
Edit: Maybe this can help clarify how I suppose the issue would be approached.

Histogram and saturation in Photoshop

1. Photoshop histogram panel is the average of the color intensity of the selected areas and layers of color channel statistics mean?
2. In the Photoshop histogram panel do you mean the average of the brightness of the image luminance channel?
I represent the average value of the maximum to the average of brightness from 0 to 255?
3. To obtain the saturation current number of images should I use any method or menu or panel?
I would appreciate a response.
Mail: [email protected]

Not sure I follow all the details you mention.
Hope this helps:
Photoshop Help | Viewing histograms and pixel values

Can I set histogram to always display in luminosity?

I am using PSE 8 for Mac. I am teaching beginners. I would like the histogram to always display in luminosity mode as default.
Is this possible?
Mary Lou

I would encourage Adobe to consider making this something which can be done in Preferences. Since Elements is often used by beginners to digital photography, it would be helpful to limit the histogram to luminosity.
Mary Lou

Why are saturation results different than Matlab's imhist() function?

We have a machine vision application where we had set some threshold levels in Matlab, and had planned on applying them to NI's IMAQ libraries. However, when trying to reproduce a set of Matlab results to verify our VB code was working, we found that we could reproduce Matlab's results on the R, G, B, and H planes, but not the S plane.
For example, if you take the 50% red image at http://www.cae.tntech.edu/~mwr/red-50.png and load it into both Vision Builder and Matlab, you get the following results:
Matlab:
rgb=imread('red-50.png');
hsv=rgb2hsv(rgb);
[hist_s,bins]=imhist(hsv(:,:,2),101);
bins(find(hist_s==max(hist_s)))
ans =
0.5000
Vision Builder 6.0.5:
Load red-50.png
Extract Col
or Planes: HSL - Saturation Plane
Histogram -> results in a peak in bin 64 on a 0-255 scale.
How many different ways are there to calculate saturation? So far, Matlab and Corel Photo-Paint agree on their calculations, and Vision Builder is the odd one.

Mike,
Thank you for posting on the forum. We spoke regarding this question, but I wanted to reply to this post with some information for other readers. You can see how we are calculating the HSL plane by examining the equations in the Vision Concepts Manual. Using the formulas, I was able to confirm that we are calculating the value for the HSL plane from the equation listed there:
S = 255(1-3min(R,G,B)/(R+G+B))
where R, G, and B in this equation are normalized. So given an RGB image (255,127,127) then we get the following values:
S = 255(1-(3*.5)/(2))
S = 63.75
I also created an image in just MS Paint with the RGB values of (255,127,127) and that gave me HSL values of (0,240,180) -- or in MS Word HSL values of (0,255,191). These latter values match
ed the HSI values returned from LabVIEW (using the IMAQ ColorHistogram VI).
Regards,
Michael
Applications Engineer
National Instruments

Bug: Histogram Does Not Update when Using Painting Tools

I was investigating something else, and I saw something not working... I'm under the distinct impression that the Histogram panel should automatically update after every operation. Having the Histogram panel displayed full-time on the screen, it should always be up to date.
However, when I Dodge or Burn or even paint color on an image, the Histogram is not updating.
If I select one of the History states I've just created, it updates.
Consider this simple sequence of operations:
I'm able to reproduce this failure in Photoshop CS5 as well, BUT NOT IN CS4 (it updates in Photoshop CS4 after every painting operation), so clearly it's not a new bug, though somehow I've managed not to notice it in my years of using Photoshop CS5.
Is it broken like this for everyone in Photoshop CS5 and 6?
Could this be some twisted attempt to try to optimize painting performance? If so, why isn't there an option to allow a user to request it to update on every operation on a fast computer with cores out the wazoo?
-Noel

More info:
Just poking around in Photoshop CS6 to see what, if anything would make the Histogram display update...
Even though the Refresh icon is disabled (and the "Uncached Refresh" entry in the fly-out menu is grayed-out), double-clicking in the topmost histogram display will force it to update (as described above by Conroy in post #1 above).
If I change the selection Channel field in the Histogram panel, mostly it does not update, but when I change between some of the settings - for example between RGB and Luminosity - the graphs update. Amazingly they revert back to the un-updated state if I switch the channel BACK to what it was!
Making a selection DOES cause it to update.
Choosing Select - Color Range (just starting the function) DOES cause it to update.
Hitting the quick mask mode button DOES cause it to update. Notably Edit in Quick Mask Mode is in the Select menu. Could an association with Select be a clue?
This bug is easy to reproduce, is shown to be a problem across multiple versions and multiple OSs, and in at least one case (Photoshop CS4 x64 on Windows 7 x64 here) it can be shown to work properly, so the code is probably not far off being right. Seems to me this should be the kind of thing a junior engineer could find and fix so someone, say, at Chris Cox's level could have it done before 5 today...
-Noel

Why are my RAW images imported in to Lightroom 5 being de-saturated?

Lately, in LR 5.3 and 5.5, every time I import RAW images they show up de-saturated. In the thumbnail they look right, but in both the library and develop mode main screens they show up very flat and de-saturated.
This is the first time I have had this problem.....never had it in V2-4.
I am shooting with a Canon 5D MIII.
I thought that there may be a preset during import that is doing this, but I do not think that is the case.
I need some help, any ideas what is causing this? I cant process all my weddings with this issue.....it would take forever to fix.
JP

Images can be different between the camera-rendered preview you see before Import and the default Adobe rendering, but after import the thumbnail in the LR grid view shouldn’t be a different tone than the image in the main area. Is that what you’re saying you’re seeing?
Can you show us a screenshot with both the thumbnail filmstrip and the central area visible as well as the histogram?

Can I create a saturation mask based to a gradient map?

Hi
can i create a saturation mask based on a gradient map?
for the example a gradient blue from 2441f6 to 6075f6
and have a luminocity mask based of this gradient
can i do it?
how can i do it?
thanks
cheers

The gradient maps are not use to sample color, but to recolor, so you cannot use it to create the selection, use the selection to limit the gradient map to the blue areas of the sky.
You could MASK a gradient map using the saturation of the sky, to recolor based on the actual saturation of the sky.
Select the sky with your favorite method, jump it to a new layer (CTRL+J)
Use the technique in the linked tutorial to extract the luminosity, it should land on its own layer.
Hide that layer, and delete the selective color one.
Now add your gradient map adj layer, enter the two colors as desired.
Show the layer with the saturation converted to a layer. Select all, copy.
ALT+click the layer mask of your gradient map adj layer, and paste. You might need to paint in black the areas you do not want affected.
I tried it here, and had semi satisfactory results given the very heavy compression of your file, and also because the colors are pretty unrealistic.
Here is the image showing the saturation mask, and the results on the side:

Histogram Shadow Clipping Indicator - What do the colours indicate?

To see what colours in the shadows are clipping to zero, and exactly where they occur in your image, you can hold down the Alt (Windows) or Option (Mac) key and move the Blacks slider to the left (in ACR 7.x and 8.x).
To quote from the top of page 206 of Jeff Schewe's and Bruce Fraser's 'Real World Camera Raw with Adobe Photoshop CS5' (which deals with ACR 6.x - but AFAIAA nothing has changed in ACR 7.x or 8.x), you interpret the displayed result as follows:
"Unclipped pixels display as white. The other colors show you which channels are being clipped to level 0. Cyan pixels indicate red-channel clipping; magenta pixels indicate green-channel clipping; yellow pixels indicate blue-channel clipping. Red pixels indicate clipping in both green and blue channels; green pixels indicate clipping in red and blue channels; and blue pixels indicate clipping in red and green channels. Black pixels indicate that all three channels are clipped."
Does the same result hold true for the colour of the Histogram Shadow Clipping Indicator?

Thanks, Eric. I'll check out your suggestion about the point curve.
I've already started to do some testing of my own, based on the Histogram's reaction faced with simple images composed only of various combinations of pure (fully saturated) Black, White, Grey (dark, mid and light), Red, Green, Blue, Cyan, Magenta and Yellow. The results so far are a little surprising, but I'll report in full when I've had the chance to complete the test.

RGB, Luminosity, and Black Point

I have an image shot in a forest, with a footbridge over a stream and a path in it. It has a lot of green, naturally, and a few blown out places in addition to dark shadows. While I like it, I've always found it a little flat.
Looking at the histogram under levels adjustment, the left side of the "mountain" is right up against the zero point of the graph, so I never bothered to try tweaking it.
But yesterday I noticed that, if I use the histogram palette and change the drop-down box to "Luminosity", then the left side of the mountain is a bit to the right of the zero point on the graph. Curious, I switched to the levels adjustment, selected the black eyedropper, and clicked in the darkest part of the image that I could find. The colors became much deeper -- especially the greens -- and even the blown out parts look better. Some of the shadows are maybe a bit too dark, but overall it's a nicer photo. And I noticed that on the histogram palette, the luminosity mountain is up against the zero point of the graph now.
So, finally my questions. What exactly is luminosity? And is it always preferable to use the black eyedropper rather than the little slider on the levels adjuster, as I had always done before? Is there a way to see a luminosity "mountain" in the levels adjuster, rather than just RGB or the individual colors? And for that matter, what is RGB?

"What exactly is luminosity?"
Luminosity is a representation of the brightness of an image separate from the color/hue. A histogram of the luminosity will show a pretty reliable map of the tonal range of an image. Adjusting the range of the red, green and blue histograms so they cover a full range of tone may be a better correction in Levels than using Luminosity mode as the r,g,b sliders will correct for some color casts and shifting that luminosity will not.
"And is it always preferable to use the black eyedropper rather than the little slider on the levels adjuster, as I had always done before?"
I would stick with the slider adjustments. The eyedropper is convenient, but not easy to control, and not as accurate.
"Is there a way to see a luminosity "mountain" in the levels adjuster, rather than just RGB or the individual colors?"
If I think about this, there might be an easier way, but this will give you the right view of the histogram:
1. Create two new layers at the top of the layer stack for the image you are working on. Name the bottom of these two layers Commit and the upper Source.
2. Activate the Source layer and stamp visible to the layer (Ctrl+Alt+Shift+E/Command+Option+Shift+E [PC/Mac]), then set the layer mode to Luminosity. At this point the image will look the same.
3. Activate the Commit layer and fill it with 50% gray (Use Edit>Fill Layer). After the fill the image will appear to be grayscale.
4. Activate the Source layer again and merge down. Name the resulting layer Luminosity.
5. Create a new Levels adjustment layer and view the histogram. You will see that this result is a somewhat different representation than you get from the RGB graph.
"And for that matter, what is RGB?"
RGB is red, green and blue...it is a light-based color model where the set of these three colors mixed in different amounts make up the total set of available colors. 100% of red, green and blue is white, 0% of each is black and even amounts of each is gray (at varying brightness depending on the percentage). RGB theory is what much of digital imaging is based on and much of what my book relies on to help people with color and other corrections.
Richard Lynch

Aperture and it's Histogram

Hi. Sorry if must of you already know this information but I've done extensive testing with my images and the Aperture Histogram. I've been posting on another thread where it really does not belong so I thought I would just repeat here. I hope you don't mind but I've found that a few people really have a rough time getting their heads around the Histogram and to me, a) Aperture's is the very best and most acurate when it comes to capturing the Midtones and b) It's ALL ABOUT THE MIDTONES in your images.
So......
There is a very small difference between RAW and jPEG IF you know how to expose correctly. I mean very small difference. If your shooting in RAW with the hopes of adding Photoshop sludge to "compensate" for the lace of proper exposure you will never be happy with your photography. Never.
Here is my suggestion:
Learn, in great detail, about your histogram. EVERYTHING in the digital world is about MIDTONES, meaning between 1 and 254. I can promise you once you really learn about the histogram and Midtones you may never go back to RAW again. Really invest the time you I'll bet anything your photography will go up at least a huge level.
It drives me nutz when someone says, "oh, your histogram should be like a camel hump" or even better, for RAW photographers, "Just move the right or left slider over to where the beginning of the "Hump" begins" AGH!!
Learn about that Histogram, shoot a gray card and play with your exposure until you see that shot of the graycard have a spike right at the 128 mark (middle) and then use that same exposure for your shot.
I promise, the time it takes you to do this, will be much less time it take you to process a single RAW image, and the output of your work will be tens of folds better.
There are lot's of resources out there, understand digital exposure and how it relates to the histogram and you'll fall in love with your images.
How I check out a gram before I make a purchase, and I did this with both trail versions of Aperture and LR, is take a picture of my son wearing his black shirt.
Now, I'm sure all of you, or most, know that the Histogram represents black to white, or, 0 to 255.
1 thru 254 is your midtone. So taking a picture of my son, close up in his black shirt should have a huge spike at 0, correct? NO. 0 is complete black or the deep fold under his arm in a shadow etc.... His shirt has wrinkles, creases, shades from lighting etc... this is what makes my image 3D. These are also called the midtones, even if they appear black to you or me on our eyes, they are midtones. That's what I'm looking for when I look at the histogram and that's why I chose Aperture and made the purchase. It's that darn great to me.
White, or 255, of course is not white. 255 will be things like a chrome bumper in the sun, etc.... the polarbear in the snow is not white, it's all shades of grey but will of course be on the right side, or at least more information will be on the right side, of your histogram.
Working with Aperture for awhile now, I've found the best way to capture my midtones is going into my camera's menu and taking down both the contrast and Saturation one level. The difference to me when I first learned this was mind blowing. My midtones where gaining by a huge margin in Aperture's Histogram but not as much in LR. I was capturing more midtones or making my image more 3D and as we live in a 3D world, much more lifelike.
My outside shots jumped a great deal in the midtone area when I really learned how to use fill flash. Here's a simple test. Take a picture of your subject without fill flash in manual, then add fill flash with a difusser but take the same exposure down just one stop. Look at both in your histogram.
Bottom line is Aperture's reading or representation of my captured image regarding the Mid-Tone range is really wonderful.
When you take images as suggested before, open up your histogram in Aperture, then go into your Mac ~ Apps/Util folder and open up the Digital Color Meter. Run this meter over your image and watch how the sync or compare to the Aperture Histogram. Fun to do but a great learning tool to learn about your Aperture Histogram.
I'm sorry if most or any of you know this but I simply find the Aperture histogram one of the best I've ever seen and would like to help anyone that could use a better understanding of it.
(in my old light I was a fiber optic engineer, but I'm much better now.)

Hi David, and I'm sorry, I should have taken it to the next step.
So do this.....
Open you color meter and the image you are working on in Photoshop.
Run your color meter over the image until you get a value around 128 for each, not perfect but get close.
Make a new Curves layer, now when the curves window opens up, take the middle eyedropper and go back to the area whereas your color meter read 128. Click on that spot with your color meter.
You've just told your image where the MidTone is for that specific image. This should also remove any color cast in your image as well.
Now, back to aperture, i meant it's simply fun and a great learning tool to go over your image with the digital color meter and relate it to the Aperture Histogram. You can actually learn about midtones and values this way. For example, if I'm running my + over a underarm shadow and can relate the numbers to the histogram and then move it and watch the values in relation between the image and the Aperture histogram.
This is frankly how I found Aperture to have the most acurate histogram between products.
It really is a good way to learn.
A bit tedious now, but when you learn this it truly is a giant leap into making better images.
Best,
Tom

Histograms: luminosity or saturation?

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