Luminosity (Luminance) and R+G+B histograms

[tomczak]

I've been trying to figure out what exactly the in-camera histograms are showing: it looks like most newer cameras show the Luminosity histogram and then, possibly, 3 individual RGB channels for developed JPG image. I'm trying to understand the UniWB concept better, hence the interest. Would it be useful it the PWP histogram tool had two additional kinds of histograms (or at least the first one): Luminosity and R+G+B histogram (the 'photoshop style'?).

While the Luminosity can be calculated and extracted using Extract Channels, and the histogram tool used on it if needed, it would be much more convenient to have it under Histogram tool options, I figured.

[tomczak]

Here is a post in similar vein and Sean's answer/addendum to his article on histograms. I experimented and I confirm that at least Canons I have access to show Luminosity histogram, if only one histogram is shown.

Clipped channels won't show as clipped, only pure black/pure white will. One way of looking for clipped channels is looking for spikes (from left to right: G, then R, than B - according to luminosity formula). All these histograms are based on the in-camera JPG (with RAW only, there is still a JPG embedded in the CR2 file).

http://www.cambridgeincolour.com/forums/thread89.htm

[Charles2]

One of the most practical aids that many digital cameras offer is a preview of the image with blinking red on highlights and blinking blue on shadows. Many times a portion of sky is the only red blinking. When that happens, the photographer need not reduce exposure and shoot again since she can tame the sky later in PWP. This is another reason in support of the proposition that UniWB is a technical curiosity but not a valuable tool for photographic work. I do agree that the process of learning about UniWB is educational.

[tomczak]

The issue, I think, is what's blinking? In the cameras I checked, only the pure-white pixels are marked as overexposed - those that have 1 or 2 channels blown would not be marked. This is consistent with the Luminosity histogram. Also, unless the WB is set to UniWB, both the histogram and the overexposed pixels preview map correspond to in-camera JPG, not RAW values (the latter may be within the dynamic range still, and reducing the exposure may not be necessary).

I think more experiments are needed to make some better recipes for using the histogram(s) while shooting in order to optimize the exposure. I think it's a worthwhile exercise as there seem to be a lot of conflicting opinions on the topic and being able to set a more radical exposure without damaging the image stands a chance to significantly improve the quality of pictures, all other things being equal.

[jsachs]

What the cameras ought to be showing for the histogram is not luminance but HSV Value which is simply defined as the maximum of the red, green and blue channels. The problem with a luminance histogram is that the blue and to a lesser extent the red channel are weighted much less than the green channel so you can clip red or blue without even coming close to clipping luminance. The HSV Value channel, however clips if any of the RGB channels clips so it is a much better measure of loss of highlight detail.

You can see any of these histograms using the new Histogram tool or using the Color Curves transformation and selecting the channel you want to histogram.

[tomczak]

What's the semantic difference between 'luminance' and 'luminosity'?

[Dieter Mayr]

Maciej,

You may want to take a look at the Wikipedia Article of Luminosity as a starting point:
http://en.wikipedia.org/wiki/Luminosity
Luminnosity is often incorrectly used as term in photography for the terms Luminance or Lightness (in HSL), but is used in astronomy to described the radiated energy per time of a astronomical object (so, it describes it's power).

[jsachs]

Luminance is a weighted sum of red, green and blue channels that reflects the eye's differing sensitivity to each color. The green channel has the highest weight followed by the red and finally blue -- roughly the weights are 60% G, 40% R and 10% B. Because the weighting reflects the eye's sensitivity, the luminance channel is a good starting point for realistic conversion of color to B&W images. Some perceptually-based color spaces such as Lab use luminance the represent brightness. Luminosity is not a term normally used in the context of image processing.

[Pierre]

Amazing. Always something new I can learn here.

I've always thought I learned more about the actual nuts and bolts of digital image editing by using PWP over other editors, and this forum is unmatched for adding to that. Thanks.