Histogram and Curves Part 6

by Ronald V.

Let's start with a review of the way the computer shows us color. All of the colors we see on the monitor are created from various levels of Red, Blue, and Green.

For example: Black is a zero amount of Red, Blue and Green. White is level 255 of Red, Blue and Green. (use a magnifing glass on a white area of your monitor display!)

What is the significance of 255 being the highest value? This gets into basic computer math. Our computers can only count in two's! (and unlike humans, computers start counting at zero.) With just one bit for color, we could have a count of 0 or 1. For two bits, we could have:

00
01
10
11
Which works out to 0, 1, 2, 3 or four counts.

If you're into math, the exact formula for the number of counts is 2^n (two raised to the n power, or the number of bits we use.) Now we normally use 8 bits for each color, so 2^8 is equal to 256.

256? Yes, recall that a computer starts counting at zero, so a count span of 0 to 255 is 256 levels!

I know, less than as exciting as watching grass grow if you're non-math type. However, even non-math types can understand that we'll normally use a range of from zero to 255.

Back to Colors: An Red, Green, Blue (sometimes just listed as RGB) of 0, 255, 0 is no red or blue and full green, so the pixel would be a bright green.

Next time you're in the Materials pallet, looking at the color wheel, look down at the RGB values listed. Try mixing various numbers of RGB to see what colors you can create. (PS- did you know that chocolate hues are dark levels of Orange?)

What does this have to do with the Histogram Adjust? Up to this point, we've been working with the Luminosity of the image. In addition to the Luminosity, the Histogram can also do it's magic on the three primary colors, creating a different response for all four! Luminosity, Red, Green, and Blue!

Just above the spectra display window are two radio buttons, Luminance and Colors. Selecting Colors also activates the Color window and enables the drop-down list to select one of the three.

Here's an item to burn into memory!: Just because a color or luminance is not selected does NOT mean that the effect is not in operation! For example, you do an operation to set the Blue with the Histogram, then go off and do some editing. You decide to go back into the Histogram Adjust to change the overall luminance, and guess what? That blue adjust is still in effect and controlling the current operation, added on top of the previous setting!

This is not a 'Bad Thing' (©). In fact, here's how you can turn it to your advantage.

Let's say that you have an image where you want to boost the Green Gamma (this is the exercise later on...). You boost the green, then notice, that since all colors are made of the same three colors, that a deer in your image has taken on a slight green tinge that you didn't want, but all of your adjustments are exactly where you want them.

Do an exit from the Histogram Adjust, without applying the effect. Make a selection around the item you don't want changed, invert the selection, then open the Histogram Adjust. Your previous settings are still in effect! All you need to do is apply them.

Now this could also be a problem if we're just coming in and don't want any previous effects! Big Hint: That is why there's a Reset to Default button up between the 'Presets' and the disk icon used to save presets.

So... the first thing you need to decide upon entering the Histogram Adjust is - Do I need to Reset it? (There are other tools that work the same way. In this series, you'll see this again for Curves)

Open deerColorGamma1 and examine it in PSP.

This is a delightful picture that has had soft focus applied to it. However, I want to make the greens a little greener, to give is more of a Spring/Summer appearance, rather than Fall.

Open deerColorGamma1Adj image and look at what I did to the histogram. I selected color- Green. I expanded the dark region of the green to make the greens have a little darker shade on the low end, then I kicked up the Gamma to 1.2 (remember that the Histogram is a tool of subtle effect!)

I could have left it alone, but I used the exclusion method listed under '..not a 'Bad Thing'... above, to exclude the deer. In fact, I made a pretty sloppy selection around the deer on the left in order to let you get a better idea of what the adjustment was doing.

Look at image deerColorGamma2 to see the final product.

Result: The greens have been boosted and the image appears less Autumny. In particular, the purple shade that was in the upper left has almost disappeared, yet white and other hues are almost un-effected.

Now I'd like to make it absolutely clear that there isn't a Right or Wrong way to do things here! There's only the way I want to do things to my images, and the way you want to do things to your image. The Histogram Adjustment is a powerful tool for you to use, and it should be one of the very first you apply to images, but make it do what you want it to do, never allow it to run wild!

Try applying the various curve adjustments to various images. See what you like, and what you don't. Experiment, you never know what you'll find!

And that concludes my little tutorial on the Histogram Adjust. Hope you learned a few things along the way.

Next- the Curves tool.

RonV

Tangling with the Python