Novice Corner - Histograms RGB, Luminance & Color

[Diff]

Histograms – RGB, Luminance & Color

What’s exciting as a novice learning PS CS3 these last few months is that I know about numerous concepts. As I progress things start popping & become clearer. Here’s an example and feel free to add to it and/or point in another resource to take this further.

Just finished reading “Camera Histogram: Tones & Contrast” (source: cambrigeincolour.com site.) I know enough about (in camera & RGB) histograms & knew a little about luminance, but I didn’t have any understanding how the color luminance histograms worked.

Hmm, luminance histograms are more accurate since they show how color intensity is distributed through an image. Mainly, that they keep track of the location of each color pixel & RGB discard that information.

Help me out here. What I’m taking away from this (short) article is that when using PS I should 1st use the RGB histogram to access overall tone/contrast. Second, always check/use the luminance (color) histograms to determine if any one color is being clipped. Right?

How does this information become useful when it comes to Levels or Curves? Curves, is my Mt Everest, along with a few other things. Let me take a broad stab. I can use the color histogram to make fine adjustments when it comes to any possible color clipping and/or seeking to add detail?

As always, much thanks for your feedback!

[LiquidAir]

Help me out here. What I’m taking away from this (short) article is that when using PS I should 1st use the RGB histogram to access overall tone/contrast. Second, always check/use the luminance (color) histograms to determine if any one color is being clipped. Right?


Actually its the other way around. If you want an overview of how luminance is distributed in your image, the Luminance histogram is the way to go. If you need to find out where a channel is clipped, that's where the RGB histogram comes. For the way I work, I almost never look at the RGB histogram after I have finished my RAW conversion. Once I am out of Lightroom or ACR, I rely on the gamut warning rather than the RGB histogram to tell me where I have issues with saturated colors.


How does this information become useful when it comes to Levels or Curves? Curves, is my Mt Everest, along with a few other things. Let me take a broad stab. I can use the color histogram to make fine adjustments when it comes to any possible color clipping and/or seeking to add detail?


When setting my black point and white point in Photoshop (usually I take care of this in the RAW conversion instead), I prefer to use the clipping indicators to the histogram to judge my end points. While there are lots of ways to use the histogram when setting curves here's a simple one:

A simple way to think of the luminance curve (RGB curves are another, far more complicated matter) is that if the slope of the curve is steeper than 45 degrees it increases contrast. If the slope is less steep than 45 degrees it decreases contrast. So, all things being equal, where to you want to add contrast to your image? More likely than not you want the increased contrast to affect the largest area of your image possible which means you steepen the curve where there are spikes in your histogram. CS3 makes this much easier because it shows the histogram in the curves dialog. A simple approach to popping a photo is to identify the largest spike in the histogram and steepen the curve over the spike.

[jjbong]

More likely than not you want the increased contrast to affect the largest area of your image possible which means you steepen the curve where there are spikes in your histogram. CS3 makes this much easier because it shows the histogram in the curves dialog. A simple approach to popping a photo is to identify the largest spike in the histogram and steepen the curve over the spike.
Nice answer overall, but I disagree with this particular point. Where you want most of the contrast has nothing to do with the histogram. You want it in the most interesting part of the image, whatever you think that to be. If this point is contentious, I could work up some examples.

[LiquidAir]

Nice answer overall, but I disagree with this particular point. Where you want most of the contrast has nothing to do with the histogram. You want it in the most interesting part of the image, whatever you think that to be. If this point is contentious, I could work up some examples.

Certainly there are lots of counter examples. I did not at all intend to imply that as the right answer for every image. Here is, maybe, a better way of putting it. If you want to steepen one part of the curve, you will have to flatten another. For most images (but, of course, not all), I want to sacrifice contrast in a way which impacts the least amount of my image possible. So, while I may not steepen every spike, I'll probably (but again, not always) be flattening a low spot of the histogram.