Part 3 of the Histogram Tutorial.

At this point the histogram display window should be at least a little familiar. However, let's start with an exercise that will give you a little more 'Hands On'. Look at HistogramTuttestpics.

HistogramTuttestgrams.jpg

This are three different images, with their histograms shown in a stack in the image HistogramTuttestgrams.

Your task, and it shouldn't be very difficult, is to match the picture to it's histogram!

Also, see if there's anything that attracts your attention to picture C. You may not have the words to describe it yet, but I'm sure you'll spot what's wrong with it.

Picture A is dark, overall. The water appears white, but this is only because it's in contrast to the dark woods. It's histogram, therefore, is the bottom one. A big lump of dark luminance, then some medium grays in the middle of the chart.

Picture B doesn't have any dark or white, all of it's luminance falls in the middle of the graph, which turns out to be the middle graph, too.

Picture C is mostly light luminance. In addition, it really doesn't have much of a spread of luminance hues, so they show up as spikes in the graph, and are all on the right side of the graph. Picture C tells us one more little piece of information. It has white that has run into saturation. That spike at the pure white point indicates that some previous adjustment has driven the light colors into saturation. Once you adjust any luminance into black or white, it can go no further.

Imagine a picture of clouds for example. Nice fluffy and white. The only thing that defines the texture of the cloud are some very subtle shades of gray. If we adjust the luminance so all the grays become white, we lose the texture of the cloud and are just left with a big white blob in the sky.

However, don't get the idea that all saturation is bad! There will be times when you'll drive one side or other of the luminance span into saturation, in order to bring out the luminance span in another section of the picture.

Now let's do some adjusting. Open HistogramTutPic4 in PSP and open the Histogram Adjustment. Look a little familiar? You've already seen this before, in the previous exercise.

HistogramTutPic4.jpg

Look at the bottom of the graph window. Just under it are three triangles, representing on the left- Black, middle- gray, and right-white. These are slider adjustment 'Thumbs'. You can click and drag them along the bottom of the display with just one little caution! Don't try to overlay them! If you do, and cannot manage to separate them, just stop the Histogram operation and re-open it.

Now click on the black thumb and drag it to where you start seeing some luminance value in the graph. As you move the thumb the 'Low'. will change, showing where the thumb is located, in both luminosity step and percentage. (I moved it to step 25, but I caution you not to pay too much attention to the actual numbers, but the image and the chart!) Remember that for this exercise, we're not shooting for a work of art, but to see what happens. (else we likely wouldn't be working on an image of a concrete plant!!!!!)

What happened? Look at the image and the histogram! The response curve moved along the bottom of the chart! Dark values of luminance became darker! And surprise, even the lighter luminance values became darker, but look at the range. The amount of change is much more at the dark end than the light end.

The Response Curve changed! This is important, as it'll be a factor of these type of adjustments from now on! You'll see it pop up in other tools, later on.

Recall that the span across the bottom of the chart represents the range of input luminance. With the curve set this way, any luminance value to the left of the black thumb will be output as black! A way to think of it, is that the input value comes in from the bottom of the chart, up the chart until it hits the response curve, then the output is set to the value of the output span, to the left.

Now, Let's grab that White Thumb control and drag it to the left until it's under the Input display where we start seeing light luminance values. (Luminosity step 212. Remember that the black is the input, while red is the output. If the action of the output chart moving bothers you, then un-check the 'Overlay Result Histogram' box in the lower left and you'll only see the Input histogram.)

Well, congratulations! You've modified an image with a low luminosity range to give it both lighter and darker, and appear less washed out!

Still a lot more coming! When you are ready, proceed to Part IV.