HistogramTutPic1.
It's a picture of the Cassini launch a few years back. The satellite is currently exploring the larger planets of our solar system. However, disregarding the subject, what can we tell about the luminance of the overall picture? (brightness/darkness)
Since the launch was at night, the background goes from about a medium gray to nearly black, and the exhaust is mostly near white.
Now, if we were to convert this image to gray scale, we'd have 256 possible colors, going from 0 (pure black) to 255 (pure white) with all of the grays in between. Now if we added all of the same value colors and plotted them with a vertical line representing the number of an individual luminance, and its horizontal positions with black on the left, and white on the right....
Look at TutPic2. If you want to see this in your copy of PSP, just open the TutPic1 and do an Adjust, Brightness and Contrast, Histogram Adjustment.
TutPic2
Be sure to select the Luminance radio button, and set all adjustments as shown. This is actually the starting point for the Histogram until you know it well enough to jump right in.
There are enough adjustments around the display box to confuse an airline pilot! Right? Well, we're not going to do anything with them until we understand that center display!
Look at the red squiggly line across the bottom of the display window. That squiggly line is made from 256 vertical bars. The first bar on the left is the relative number of black pixels in our image!
Why Relative? Well, we really don't care at this point how many there are, we just want to know where the major and minor number of light/dark places there are. The operation of the Histogram display is to automatically scale the vertical part of the graph so that we get a general idea of what is going on.
Now, what does the graph tell us. Try it on your own, study the graph and come back and then read my statement below, I'll even add a hidey space!
Ok, remember that the original image had a lot of near black in it! There is a burst of lines near the left side, indicating those dark areas. There's also a lot of near white, which is shown by the spikes on the right! There are fewer grays, although they're still present as shown by the middle of the graph.
Homework time! (didn't think you were going to get off easy, did you?)
Open a new image and fill it with pure black. What do you think the Histogram will show? Open the Histogram Adjust and look. Did it show what you expected? What about a solid white image?
Yes, I know this sounds far too simple, but I want to make certain you're up to speed on the basics, so the more advanced items don't throw you.
When you are ready, delve into Part 2.
RonV
Tangling with the Python