On Sat, 11 Oct 2008 12:38:05 -0700,
karl_vitti@adobeforums.com 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.
- Curvemeister
