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By Matt Spinelli, 8/30/06
In part 2c of this user guide we went over
the Detail tab which contains the sharpness, moire cancel, and noise reduction
controls. In this section we will examine the Curve tab which contains the free
tone curve control. Curves can be a little intimidating to those that do not
know how to use it. This article will help to explain the basics on how to use
curves in s7raw.
Making Adjustments - Free Tone Curve
Understanding Curves
To comprehend how the curve control works we need to understand what is going
on when adjustments to the diagonal line are being made. The diagonal line is
perfectly straight in its default state. This means that nothing is altered
to the image. When we click on the line and create a control point and then
move it, the entire line curves.
Curving the line above its original state (the original state is represented
in the background as a blue line) causes the image to become brighter. Curving
it below its original state will cause a shift to darker tones.
Curving Above
Curving Below
At this point we have 3 control points (small red dots on the curve), 2 of which were there by default. The first
one is at the bottom left hand corner, the second at the top right, and the third is
the one we created when clicking on the line.
Notice the box to the right of the curve control? The first set of numbers correspond
to where on the curve the mouse cursor is when it is pointing to the curve. Go ahead,
move the mouse around the curve control and you will see these numbers change. The
first number is the input level (the brightness level of the tone before the curve
is applied) and the second number after the slash is the output number (the brightness
level of the tone after the curve is applied).
These numbers represent brightness values ranging from darkest (0) to brightest
(255). The box contains the input and output values for each control point. If the
output value is greater than the input value then that tone becomes brighter. If the
output value is less, then the opposite happens. If you click on the numbers in the
box it will select the control point it corresponds to.
Normally though the curve control is not used with just 3 control points. More can
be added. Click on the line again to create another control point. Moving this forth
point around causes the curve to curve some more. This time though the curve can be
moved into different shapes not just up or down.
Below I made an illustration to summarize how to understand the curve control. First,
notice that there is a gradual transition from bright towards the top and dark towards
the bottom. Thus as has been mentioned, when the curve is pushed up the tones become
brighter. When pushed done, they get darker. Notice too that I superimposed the histogram
on the curve graph. I did this to illustrate how the curve line corresponds to the histogram.
The center of the line represents the center of the histogram (brightness level of 128). Moving
down the line is the same as moving to the left on the histogram, which represent darker tones. Likewise,
moving up the line is the same as moving right on the histogram, which signify brighter tones.
Contrast adjustments
If you have played around with the control for a while you probably have noticed
that the image can be severely affected by changing the shape of the curve. In most
cases only subtle changes are necessary with this control. One of the most useful
things it can do is apply custom contrast adjustments.
To start, let us reset the curve by clicking "Reset All." To adjust the contrast to
become stronger we need to add 3 control points. To start, use the grid as a guideline
and add control points where the grid lines intersect the diagonal line.
Next, curve the control to look as follows:
The result is that more contrast is applied overall to the image and the histogram
is expanded to the left and right. This is called an S-Curve since it resembles the
shape of an S. Why does this happen? Remember if the curve is above the original line
tones become brighter and if the curve is below tones become darker. In this case the
curve is both above and below the original line. The control point we added to the
lower left has an input value of 80 and output value of 51. This shows that the darker
tones become darker. The third control point has an input value of 180 and an output
of 200. This causes the brighter tones to become brighter. The middle point stays the
same and thus middle tones around this point are either not affected or slightly
changed by the curve.
S-Curve Applied
The contrast can be fine tuned by adding additional control points. Let's say the
darkest shadows are too dark and we want to brighten them up a bit. To do this we
can move the lower left control point up a little bit but still have it be below the blue line. Then
add another control point to the left of it and move the new point up to meet the blue line. Also,
maybe we want to brighten up the middle tones a bit and moving the control point in the center
up slightly. After these adjustments the curve and resulting image should look something
like this:
Fine Tuned S-Curve Applied
Fine tuning like this can be applied anywhere on the curve. If we want to reduce
contrast overall in an image we cause the curve to be an inverse S shape. In other
words move the curve such that the bottom left section is above the original line and
the top right is below it.
Inverse S-Curve Applied
One thing to watch for when using curves is to be careful not to clip the image. If
the output values become 0 or 255 for tones that were not already completely dark or
bright, you will loose shadow or highlight detail. An easy way to prevent this is to
make sure the curve does not create straight lines at the top or the bottom of graph
as pictured below. Notice that all tones above the brightness level of 140 will be shifted
to 255, and all tones below the level of 40 will be 0. This should be avoided unless the
effect is desired.
Instead make sure the curve gradually bends to the bottom left (brightness level of 0)
and top right (brightness level of 255). By doing this the contrast of the image can be
adjusted without fear of clipping the highlights or shadows.
Color Adjustments
Curves can be used to adjust color as well. Although I find the selective color
replacement tool and the fine tuning of the white balance in s7raw to be more useful,
I will still discuss this as it is a feature of the curve control.
Notice the drop down box that currently reads RGB. This means that the curves are
applied to the entire image and to all colors equally. However, each of the color
channels can be adjusted, whether red, green, or blue.
For the red channel, moving the curve above the original state will cause a color shift
towards more red, whereas moving the curve below the original state will add cyan.
For the green channel, moving above will shift towards green, and below will add magenta.
Likewise for the blue channel, moving above shifts towards blue, below adds yellow. Below
is an example of the effect on the red channel.
Red Channel Curve Up
Red Channel Curve Down
S7raw's curve tool also has a linear option. Enabling this prevents the curve from
curving and instead forces the line to be angular. When using a linear curve on the
color channels it is possible to precisely change the values of a specific tone
without affecting other tones. The example below shows this on the red channel. A very small
section of red tones in the flower and elsewhere are more pronounced than the rest of the tones.
On the test image this is not a modification that we would want to keep, but it does show how
the linear option can be used to select a very narrow portion of tones and change their values.
Also note that the linear option can be used much less aggressively than in this example. It can
be used anytime that a curving line would not work well.
Red Channel Linear Curve
Curve Tab Tutorial
To sum up the adjustments in this article lets continue with editing the test image.
Although the contrast, brightness, and gamma curve controls have already been used to
correct the image, we can use curves to further fine tune the contrast of the image.
Below is the curve that I feel works best. The curve is applied only to the RGB setting.
No adjustments need to be done to the individual colors. Notice too that the curve
adjustment had a positive side effect in that it helped reduce the vignetting to some
degree.
Curve Parameters
Before Curves
After Curves
I hope you have found this tutorial useful. In our next article we will cover the Lens
tab which contains controls to adjust lens distortion, chromatic aberration, vignetting,
and correction.
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