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By Matt Spinelli, 4/16/2006 (updated on 4/19/2006, see box below)
In part 2b of this user guide we discussed
how to apply tone curves, contrast, brightness, saturation, and hue adjustments
using the Adjust2 tab. In this section we will examine the Detail tab which
contains the sharpness, moire cancel, and noise reduction controls. Please wait
for this page to load as there are many images for this tutorial.
Making Adjustments - Sharpness, Moire Cancel, and Noise Reduction
Sharpness
In the most basic terms, sharpening enhances the edges of an image. Almost
all digital cameras that provide RAW capture need their images sharpened. This
is primarily due to the demosaicing process, which converts the data saved
from the sensor into data that when viewed thru an editor becomes a picture
to our eyes. This demosaicing process produces images that are not very sharp
and thus most cameras apply some amount of sharpening to the image. As mentioned
before, with RAW capture in-camera processing is by passed. As such we need
to sharpen the image.
With s7raw we have four options when it comes to sharpening. The first three
are the presets of Hard, Normal, and Soft. The forth option is Custom. Below
you will see how the presets compare to the non-sharpened test image. For sharpening
comparisons, moire cancel has been set to 0 and noise reduction has been disabled.
These images are 100% view crops.
The forth option, Custom, allows us to adjust the amount of sharpening to apply
precisely as needed. This is more useful than the presets as the amount of
sharpening to apply normally varies form one image to the next. Below you will
see six images, ranging from a sharpness amount of 0 to 100.
As we can see the higher the sharpening amount the more the edges are enhanced.
Once we get to around 60 it begins to become too much. I am not
sure what type of sharpening algorithm is being applied in s7raw, but just for comparison
purposes below is the s7raw sharpening (set to 60) verses Photoshop's unsharp mask set to
a radius of 1.0 and amount of 350% (this is an high amount, but was the closest
match with regards to sharpness and detail to the 60 setting in s7raw).
It is obvious that s7raw's performance with regards to sharpening is close to
the standard Photoshop unsharp mask filter. But I did notice some obvious dark tracing around
the larger edges on the s7raw images. This is best seen in the top left area of the leaf
crop. I think unsharp mask is a little better overall, but
as long as s7raw's sharpening tool is set somewhere below 40 the results seem great and
the dark tracing is not noticeable.
Lastly, we can also both sharpen and soften the image at the same time. Adding a bit of
softening to the image while sharpening will reduce some of the coarseness of a sharpened
image, but at the cost of some detail.
Moire Cancel
Moire is a complicated subject. It is beyond the scope of this article to
talk about it in technical detail. Nonetheless, I will cover the basics as they apply
to how we would come across this effect in digital photography. Simply, moire
is distortion and undesirable in images. The word
moire comes from a French word that means "to water" referring specifically
to the rippled, water-like look caused by the distortion. There are two
primary types of moire that can show up in digital images. The first type is a
wave pattern. The second type is color artifacting that occurs along with the
wave pattern. To visualize a simulated wave pattern of the effect click
here.
The large wave-like patterns are caused by fine detail patterns in a scene
interfering with the resolution of a camera's sensor.
Unfortunately, I was unable to find any detail patterns in the test image we
have been using throughout this tutorial that showed moire. As such I took
a picture of a sports coat that had a pattern on it. I shot the photo from
such a distance that the detail in the pattern conflicted with the resolution
of the camera. In other words, each pixel in the image would be close to the
size of the fine detail (or since it is a pattern, a grouping of detail)
in the scene. This took some trial and error, but I finally produced the effect.
With s7raw, the moire cancel function seems unable to remove the ripple pattern.
However, what the control does remove is the color artifacting (which helps
reduce the visibility of the ripple pattern). Below is a before and
after example of moire cancel being applied to the sports coat example. The
images are 100% view crops.
Moire cancel has 5 levels of adjustment, with the higher numbers resulting in stronger
cancellation. Higher levels will work better with larger ripple patterns and
lower levels will work better with smaller ripple patterns. Once the amount of
moire cancel needed is determined, next focus on adjusting the threshold.
The threshold control should be adjusted to the highest value that
still eliminates the color artifacting. Start at 0 and slowly increase the threshold
amount. Once it is noticeable that the moire cancellation is loosing effect, decrease
the threshold amount some. The reason for doing this is because we only want to
cancel moire in trouble areas and limit its effect on the rest of the image
if possible. The higher the threshold the more likely non-moire areas of the
image won't be affected by the moire cancel.
Just how does moire cancel affect non-moire parts of an image? Well, it reduces
some of the sharpness and detail of an image to a small degree. It also reduces color accuracy near
color boundaries. The below images illustrate this when moire cancel is applied to the
test image. The negative effects are very minor, but they do exist. If we
look closely along the border of the flower edge and white background, it should
be somewhat visible that some of the red and orange color of the flower has turned
grayish in appearance. Notice too that some areas appear to be slightly blurred as a
result of moire cancel. However, I don't see the negative effects of this control
preventing it being used if moire is present in an image.
Noise Reduction
Noise comes in two main categories: grain (luminance
noise) and color blotches (chroma noise). Most consumer digital cameras
produce moderate to high noise depending on the ISO setting (higher is worse), lighting
conditions (darker is worse), and the pixel density of the sensor (denser is worse).
Often the camera has built
in noise reduction to minimize it's presence in the final image. However,
with raw images there is no in camera processing. Too much noise will
severely lower picture quality. Keep in mind, however, that any
noise reduction will result in some loss of detail. The best noise reduction
algorithms try to reduce noise while maintaining as much detail as possible.
In an effort to reduce noise, s7raw has a noise reduction control.
I have created a series of 100% view crops of the flower and background
area of the test image. I have set the radius and amount values of the
control to a variety of combinations ranging from a radius of 1-5 and
an amount of 25-100. Sharpness has been set to normal and moire cancel to 0.
Keep in mind that the test image was shot under
conditions that do not produce overly noisy pictures to begin with.
However, I wanted to be consistent with the rest of the s7raw tutorials in
using the test image so that you too can play with s7raw to produce the
same results illustrated below.
Out of all the above options, a radius of 2 and an amount of 50 seems to be
best to my eye if our goal is to reduce noise and maintain a reasonable amount of
detail. Overall, I'm somewhat disappointed in s7raw's noise reduction system as it
seems to be a bit weak in its current version (v0.4.3). The RAW editor apparently
uses a median filter. This is virtually the same as using the median filter
in Photoshop (filter -> noise -> median), except s7raw has an extra
slider to adjust the amount between 0-100% to apply.
A median filter calculates each pixel value by taking a median
value of the surrounding pixels. As a result, fine noise is removed, but so
too is fine detail. With s7raw, luminance noise is removed fairly well,
but chroma noise seems unaffected and possibly a bit worse than in the
non-noise reduced image. Hopefully s7raw will improve this feature in a
future version.
I would suggest using a more sophisticated noise reduction solution such as
PictureCooler 2, Noise Ninja, or Neat Image. Those three options are not
free however. For those of you that have Photoshop 6 or newer, look for
De-noise
Deluxe. These sets of actions are the best free option that I am aware
of. Also those that have Photoshop Elements 1 or 2 can also install these
actions using the method described here. Lastly, for more
information about how well these other noise reduction programs work,
Michael Almound wrote up a very nice and extensive comparison
of many noise reduction solutions.
For our comparison purposes, I will show the non-noise reduced image, the
best s7raw noise reduced image, and the results from PictureCooler 2.
Overall, PictureCooler 2 maintained more detail, sharpness, and had less noise,
especially chroma noise in the gray area of the flower crop, than s7raw's result.
Notice too, that the veins in the leaf loose detail the most with s7raw's noise reduced image. Remember,
though, that the test image used in this tutorial and user guide was shot at
ISO 64 on a 4 megapixel camera. Noise is not terribly high to begin
with so you will need to look closely at these images to see the subtle differences.
UPDATE 4/19/06: It was brought to my attention that some checkboxes have three states
(checked with a white background, checked with a gray background, and not checked). Well after
hearing back from the s7raw's development team, apparently a gray checked noise reduction checkbox
indicates that noise reduction is on (just like if it had a white background), but it also
means that moire cancel is boosted by +5. Thus a white background means no moire cancel boost and
no check of course means noise reduction is off.
Also, for those interested, s7raw uses an ordinal Laplacian filter for sharpening.
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Detail Tab Tutorial
To sum up the adjustments in this article lets continue with editing the
test image. Below are the settings I felt worked best with the image. Even
though I am not a fan of s7raw's current implementation of noise reduction,
I decided to apply it considering that some may want to use s7raw as their
primary or only editor. The before and after shots don't show much difference,
but the full resolution images do differ to a noticeable degree.
Detail Tab Parameters
Before Detail Adjustments
After Detail Adjustments
Up to now, we are able to adjust the orientation, white balance, exposure,
tone curves, contrast, brightness, saturation, hue, sharpness, moire cancel,
and noise reduction. This is a lot of fine tuning that can be applied! However,
we aren't done yet. In our next article we will cover the Curve tab which contains
a more advanced version of Adjust2 tab's tone curve control.
Continue to part 2d - making adjustments with
the Curve tab
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