IMAGENER'S RESHARP FUNCTION
SHARPNESS CONTROL
Techniques to Get The Most Out of
Your Enlargements
All digital images can benefit
from being sharpened at some stage in their lives. This
is especially true for enlarged images, but enlarged or not,
all captured images - digital or analog - suffer from blurring
or softening of detail in some way. This is precisely the
motivation behind Kneson Software’s addition of the
Resharp function in Imagener Professional and Imagener
Unlimited.
However, our customers have been asking for more information about this function
as use of it does require a certain level of
skill
and knowledge to master.
Blurring occurs when the representation
of an object is at a lower contrast or retains
less detail than is present
in the original. Sharpening compensates for this loss by
improving the visibility of the information in the image.
It works by applying a matrix of numbers over an array of
the pixels in the image with the matrix centered over one
pixel at
a time. Sharpening can add image data that
improve the appearance or impression of sharpness.
This function is formally known as "Unsharp Mask." We named
it "Resharp" to avoid any confusion, but the technique is actually
a darkroom technique for improving the sharpness of paper prints
that was used before the age of digital pictures.
There are three variables to the function, Amount, Radius, and
Threshold. Each variable interacts with the other so it is possible
to obtain nearly identical effects with very different settings.
Keep the following characteristics in mind:
Amount |
Radius
|
Threshold
|
- A measure of the strength of sharpening, roughly
as a percentage of the increase in edge contrast.
- Works best within the 50-200% range (enter 50 up
to 200 in the amount box in Imagener).
|
- A measures of the number of pixels over which
the function operates.
- Needs to be set with care as radius has the
greatest effect.
- A radius of three
may in fact cover seven or more pixels.
- In general, low figures give crisp edges; larger
figures produce broader edges, and increase overall
contrast.
|
- Measures the minimum difference between
two boundaries that the function will operate on.
- Is based on 255 levels and therefore will only
accept a numerical value of 0 to 255.
- Zero
threshold tells the function to operate over
the entire image.
- A value of 127 (about
half of the 255 levels) will cause sharpening to
occur only where pixels are next to other pixels
that
are 50% lighter or darker
|
| For example, the images
below were enlarged using Imagener Unlimited. The image
used was of somewhat low resolution, hence the slight
blur that appears in the image to the left. Using the
Resharp settings shown at the right, the image below
right is made to look much more visually appealing.
Edges contrast better, and the overall appearance is
improved. |
|
|
Image Resized
|
Image Resized and Resharpened
|
|
|
|
|
These photo examples are unusually small ... most image enlargement
uses start with images much larger with higher resolution. These
images are for this discussion as examples only.
General Guidelines
Start your resharp operations with the following settings as a guide.
Image consists mostly of fine details |
|
Image has large areas of even tone |
| Set a strong amount of sharpening, with very small radius
and a low threshold, such as Amount: 200, Radius: 1-2, Threshold
1-2. This improves all boundaries in the image over a small
distance. While these settings will also increase grain or
noise, they will be hidden by the detail. |
|
Set a modest amount of sharpening, and a large
radius with a relatively high threshold, such as Amount: 50,
Radius: 20, Threshold: 10. The high threshold ignores noise
while the high radius compensates for the moderate amount. |
You may discover that the wrong settings can
create bright, bare patches. Make sure you remember your enlargement
settings as you will need to click the Reset button if this occurs.
Keep experimenting. With practice, you will increase the visual
clarity and appeal of your enlarged images.
|
