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Here's the original image loaded into the GIMP. It is
noticeably soft, probably due to bad autofocus. It really shows
up in prints.
Note: be sure to do any other editing that you want to
on the image first, especially downsampling or upsampling
(resizing). Sharpening should always be the last step that
you do.
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Duplicate the image by pressing Ctrl+D or right-click and select
Image/Duplicate. It might be wise to minimize the original
image window now. You can open it later to compare to the
result.
Right-click on the duplicate image and select
Image/Mode/Decompose. In the decompose dialog box, select LAB
(or HSV; see Note 1). Check the "As layers" checkbox (see Note
2) and click OK.
This will create a new window with the image decomposed into the
Luminosity ("brightness"), A and B channels as 3 layers. This
will allow us to isolate and sharpen the luninosity channel which
is a) the most important, and b) should be the cleanest in terms
of noise.
At this point I usually turn off visibility of the A and B
layers in the Layers dialog by clicking on the "eyes".
Note 1: if you happen to have an older version of the
compose/decompose plug-in it may not have the LAB option. In
that case use HSV (hue/saturation/value) instead. For any
reference to the "LAB" image or "luminosity" channel below,
substitute the "HSV" image/"value" window. The LAB option was
not in my initial version of the GIMP (ver 1.2.1), but I got the
newer version of the plug-in at the
GIMP Plug-in Registry
and compiled it myself. Don't worry if you don't have it; HSV
works great too.
Note 2: in the older version of the decompose
plug-in there is no "As layers" checkbox; instead it will create
the three channels as separate windows rather than three layers
in one window. It really doesn't matter if you do it this way;
the layers way just reduces the clutter on your screen. If you
don't have the "As layers" option, I suggest you minimize (but
not close) the hue and saturation (or A and B) windows.
We'll need them later, but you don't want to accidentally pick
one of them in the next few steps.
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Go to the duplicate image. Right-click and select Filters/Edge
Detect/Edge. In the Edge Detect dialog box, select a parameter
value of the appropriate size for the edges in your image. You
may need to experiment with this. For the example image I chose
6.
Now convert the edges image to grayscale by right clicking on it
and selecting Image/Mode/Grayscale. This image is going to
become our sharpening mask.
Note: in some tutorials they recommend converting to
greyscale and then running the edge detection filter.
The premise there (my guess) is that perhaps you don't want to
find noise as edges. My thinking is that there may be
an edge in color, but not tonality (e.g. transition between two
light colors). Once you have converted to greyscale you may not
be able to detect that edge any more. Besides, with a high
enough edge detection parameter (here 6) you will avoid
detecting random CCD noise as edges.
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The next step is to adjust the tonality a little so that areas
that need sharpening are really white and anything that
doesn't need sharpening at all is really black.
Open the Levels dialog (Image/Colors/Levels). Bring the black
point up and the white point down to filter out any
insignificant edges. You may need to play around with the exact
positions of the sliders. Click OK when the image outlines the
significant edges in white and most everything else is black.
How much you'll need to tweak this really depends on the image.
There wasn't a lot of spurious edges shown here, so a small
adjustment was all that was necessary.
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Now apply a gaussian blur (Filters/Blur/Gaussian Blur) to smooth
out the edges a little. A radius of between 3 and 10 pixels
(horizontal and vertical) should be enough. In the example I
used 7.
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If you want to you can use Levels again to adjust the white and
black points of the mask. I find this is usually a good idea
after the blur.
In this example I pulled the white point slider down a bit.
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Open the Layers dialog. Select the LAB image (if HSV, value)
in the drop down box at the top and then click on the
Channels tab.
Click on the new channel button ( ) at the bottom of the dialog to
create a new channel. Name it "Sharpening Mask".
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Right-click in the blurred/edges image window and Select/All.
Right-click again and select Edit/Copy. (Alternatively you can
use keyboard shortcuts Ctrl+A and Ctrl+C in succession).
In the Layers dialog, make sure the Sharpening Mask channel is
selected. Go back to the LAB (value) window and paste
(Edit/Paste or Ctrl+V).
Select the Layers tab in the Layers dialog and click on the
anchor button ( ) to anchor the
floating image.
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Click on the Channels tab in the Layers dialog. You should see
a tiny version of the sharpening mask in the channel icon,
indicating that you properly pasted the sharpening mask into the
new channel. At this point you may wish to click on the eye
icon to make the sharpening mask invisible.
Select the Sharpening Mask channel and click the "Channel
to Selection" button ( ).
You should see "marching ants".
You can experiment with feathering the selection here if you
want; I usually don't.
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Important: in the Layers dialog, click on the Layers
tab and make sure the luminosity layer (if HSV, Background layer
of Value window) is selected, and is the only one
selected. We only want to sharpen the luminosity channel.
Apply an unsharp mask (Filters/Enhance/Unsharp Mask). You may
want to experiment with the settings here. You will find that
you can use much higher amounts of sharpening (than if
the entire image were selected) without significantly degrading
the image.
In the example here I used Radius=1, Amount=2.0 (200% ?),
Threshold=0.
Now get rid of the selection (right-click and Select/None) and
zoom in (= key) to examine the results. If you're not happy
with the results, undo and try the sharpen step again with
different parameters.
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It's time to reconstruct our original image from the LAB (HSV)
components.
Right-click in the luminosity (value) window and select
(Image/Mode/Compose). In the Compose Options dialog, make sure
that you have selected LAB (HSV) on the left and on the right
that the Luminosity, A and B (Hue, Saturation and Value)
components are matched up with their respective images.
Click OK; this should create a new composite image.
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Examine the resulting image, zooming in to examine edge detail.
It may help to unminimize the original image for comparison.
If you feel your image needs more/less sharpening, then go back
to the LAB window, undo twice and redo the unsharp mask step
above.
When you are satisfied with the result you can close all the
extraneous windows.
The images on the right are zoomed to 100% and cropped:
- Top right: original image
- Center right: smart-sharpened image
- Bottom right: sharpened with a typical unsharp mask
filter (Radius=1, Amount=1.0 (100% ?), Threshold=0)
The smart sharpened image hardly shows any additional noise
over the original image. Notice how the "regular" unsharp
masked version at half the sharpening amount
has much more visible noise, especially in the shadows on the
side of the face and in the door frame of the car.
Check out the shadow noise in the larger images on
the smart
sharpening shortcuts and variations page.
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