Histogram and Curves Part 2

by Ronald V.

We're not done with that display yet! There's still something that's normally hidden except to those that know the trick! Look at Tutpic3!

Tutpic3

I've expanded the display by 150% and added some text. Across the bottom of the display is an implied INPUT display. Black is on the left and White is on the right. There's an output too! The left vertical side is the output.

A way to visualize this is to imagine incoming Luminance of Black to White coming in across the bottom of the display, going up to intersect the diagonal line. Where it hits the diagonal line, it goes to the left to the output.

Just so you don't get too surprised later on, this diagonal line is a 'Response Curve' often just called a Curve. (sound familiar?)

A straight response curve such as the one we're looking at here is called linear. What comes in, goes out. Yes, I know, we haven't changed anything.... yet.

Look at the box on the left side of the display window with a black circle in it. This is a moveable handle that you can click and drag up & down. Do so now and pay attention to 2 areas. One, the JNE box to the left of the handle named Min., and Two, the Curve.

The Min box displays the minimum value of Black that we'll allow the image to have! And, as we drag the box up, the starting point of our curve goes up on the output values.

Drag it up to about halfway (using the Histogram Adjust on Tutpic1.)

Now look at the spectrum display and the output image in the right hand display window. What have we accomplished?

Many things have gone on.

1. The spectrum display has split into two different displays. A red part and a black part!
The black part is the original spectrum (this is a fancy name for the display graph data)
The red spectrum is the new output overlaying the output section!
2. The output doesn't change, but the start point of the curve has been move up. Now, black coming into the process is output as the value of the Min. (I ran mine up to 127, so incoming black goes out as 127). Since the curve is still linear, All other luminance are moved PROPORTIONALY, until you get to luminance 255 (white)
3. The image has lost all of it's black values, and a medium gray is now the darkest luminance in the image. (Yes, this is just demonstration, not an actual image adjustment, although there are ways to use it productively. If I forget, remind me at the end of the series.)

Well, if the black circle did that, what does the box with a white circle in it do as you drag it? Set the Min back to zero and try it.

Yes, it changes the end point of the curve and sets the Maximum (white level) of the image.

Ok, when you are ready. Go On To Part 3

RonV

Tangling with the Python