Curves

by Ronald V

Now that we've gone through the Histogram, Curves should be quite easy, since the tools work very similar. Drag HistogramTutPic4.jpg into PSP and do an Adjust || Brightness and Contrast || Curves

The very first thing to notice here is that the tool is located in the Brightness and Contrast family, along with such tools as the Gamma and Histogram tools.

Next, since this is the initial opening, we want to start at the default settings, so click the Reset button located between the Presets and Save Presets in the Curves window.

Let's do a quick run though of the controls, top left to the bottom.

1.Presets list box. click the down arrow to see saved presets.
2. Reset push button.
3. Save Current as a Preset. A new box opens for the Name.
4. Input preview window.
5. Output preview window.
6. Zoom Preview Out. Only the preview changes.
7. Zoom Preview In.
8. Navigate. Click & drag to move the preview around.
9. Preview windows on/off.
10. Input range of luminosity, or RGB. Color coded.
11. Output range of Luminosity, or RGB, Color Coded.
12. Channel. A display/drop list selector to select Luminosity (RGB) or Red, Green, Blue.
13 A JNE box (JASC Numerical Entry) that displays the selected breakpoint along the input.
14. A JNE box that displays or places the selected breakpoint on the output.
15. OK, the apply button.
16. Cancel, exit, but retain the Last Used settings.
17. Help. Access the help file for the Curve tool.

Right off, you should see some things that are familiar and some things that are different than the Histogram.

This tool doesn't show a spectrum display! It does show the input and outputs as graphic elements and in the same direction as the Histogram- black is on the left for input, bottom for output. White is on the right for input, top for output.

Doing a reset placed the white input/output breakpoint at the upper right and the black at the lower left.

There is No cutoff on the output, such as we had in the histogram. Instead, the two default breakpoints serve both the input and output control.

For example, click and drag the black breakpoint until the input display reads about 30 and the Output reads 60. Remember that you can wipe over and enter number from the keyboard, or click one of the control arrows, or even click the box and use your roller on the mouse.

Now, what's happened? Any input luminosity between 0 and 30 will show up in the image at a luminosity or as it's known in this tool- RGB value of 60. The remaining span of input RGB is spread linearly from an output of 61 to 255.

Recall the first thing we did in the Histogram? Setting a cutoff level? Here we've done that plus we've already altered the response curve.

Now, I want to demonstrate something interesting with the White breakpoint. Click and try to drag it to an input less than the black breakpoint! Hold the button and observe the input display. It turns red to indicate an illegal operation! If you release the mouse button, it will move to the right until the operation is legal.

Not much of an operation! We know the operation of a response curve is such that the input span at the input goes to the intersection of the response curve and at that intersection, goes to the output. (Just as it did in the Histogram) If you followed me exactly, your output will only be showing the luminosity of 30 - 31, and the rest is gray.

As with the Histogram, clipping the output to a fixed gray scale is a limited function, and one that we'll rarely use. Hit the Reset to get back to the default values.

Let's set the RGB to the same values we used in the Histogram. Click the black breakpoint to select it, then drag it to input 25, output 0.

Then click the white breakpoint and drag it to input 212, output 255.

Hey, guess what? We've expanded the luminance of the image to the same display as we did with the histogram.

Now, here comes the power of curves! Watch carefully!

Click on the curve response line, about the middle of the graph and drag upwards about the length of the Arrowhead!

Wow! A New Breakpoint was created, and it pulled the entire response curve up! Instant Gamma!

Not exactly, though. Actual Gamma has a fixed amount of curve, all along the length of the curve between breakpoints. We can click on this new breakpoint and move it around... do it gently at first, then get wild with it and see what happens!

The White and black breakpoints form an rectangular area (not shown by the graph) and whenever you move a created breakpoint outside that boundary, you get a red flash in the input box and the breakpoint disappears.

This is the way you'll normally get rid of excess breakpoints!

Breakpoints? Plural? Yes, certainly! Create three or four new breakpoints along the response curve. Move them around and see where the new extinction boundaries appear. Also, pay attention to the shape of the curve.

I'll bet you had fun doing that! Now, what did we observe about the action of the response curve and the breakpoints?

Open CurveTutPic1.jpg to see my results.

1. The breakpoints create 'Graceful' curves along the length of the Response Curve. Breakpoints other than the white and black attempt to have entry and exit curves along the line so that effects are not abrupt.

2. Progressive breakpoints can be anywhere on the Output, but they cannot be placed higher or lower on the Input than any other breakpoint position!

If that's not clear, just play around with creating breakpoints and positioning them to see what happens. It should become clear fairly fast.

While doing that, you may want to see what you're doing to the actual output image.

And if that's not enough to make you flip... Remember that you can do this to the primary colors in addition to the luminosity.

Incidentally, negative going curves can do strange things to an image. Look at the output side of the picture, and see what it did to the edge of the pipe and the building.

Now this is not an effect that you'll be constantly doing unless you really go for psychedelic images, so let's do something real!

Look at CurvetutPic2 This is a picture I took from my backyard last month. It's just as the camera saw it.. other than a few power lines and pole cloned out.

Let's go bold with it! We'll take the high reds and low blues and boost them, then control the overall effect with RGB!

CurveTutPic4 shows all of the adjustments I made. From top to bottom: I wanted to keep the darker reds pretty much as they are. so I created a breakpoint at the center, using the grid to help hit the center. The next breakpoint was about 3/4 of the way into the lighter reds and boosted those, but the very lightest reds still needed a little boost so I added one more and pushed it very high.

Blues, I wanted almost the opposite. I wanted deeper blues, so I pegged the upper part of the curve linear and boosted the lower.

Overall Luminance RGB, I felt the darker values were still just a little too light, so I used a single breakpoint to lower all of the luminance, but especially at the low end.

Result.. and quite bold is image CurveTutPic3.

And that, my friends, is it. The rest is up to you. As I said for the Histograms, Make those pictures do what strikes your fancy!

Enjoy your new powers with the Histogram and Curves adjustments.

©RonV 2005

Tangling with the Python