|
When banding occurs in images, it can be mistaken for posterization, but it’s more complex than that.
Banding is caused by the failure of an algorithm during an image conversion, such as switching to a new
color mode. If too many near-identical image tones spread out farther than its computational area, the
algorithm can fail, and the conversion may produce tonal edges called bands. To eliminate banding, use
the Add Noise filter to add pixel variation to adjacent tones.
|
|
To understand the cause of banding
and how you can fix it, you are going to
intentionally force an algorithm failure
on an image and create banding. Make
a new 1024 x 768-pixel RGB document
with a white or light-colored background.
With a 300-pixel hard black
brush, click once in the center. You’ve
only created one obvious tonal edge, so
converting an image like this would be
an easy task for any algorithm. Since you
want to purposely create an algorithm
failure here, you are going to make the
image much more difficult for a conversion
to compute.
|
|
Many operations can generate conditions
ideal for banding, but blurring is
the most common culprit. This is because
blurring is literally the spreading of one
pixel’s tonal value into adjacent pixels.
If taken to extremes, the blur will spread
out all tones too smoothly, which causes
conversions to fail. Since you want to be
extreme in this case, choose Filter > Blur
> Gaussian Blur and set a 75-pixel Radius.
Now you have an image that will test the
limits of any conversion algorithm!
|
|
If you have an image ripe for banding,
the most common occurrence is when
you convert to CMYK for printing either
in the printer or the software. Since
every printer setup is different, just
use Photoshop’s CMYK conversion here.
Zoom into the top-right quarter of the
blurred image and choose Image > Mode
> CMYK Color. See how the conversion created banding? You wouldn’t want an
image like this to print that way, so press
Command/Ctrl-Z (Undo) to go back to
the unbanded version, and you’ll learn
now how to fix the problem.
|
|
While pressing the Option/Alt key,
click the Create a new layer icon at the
bottom of the Layers palette to open
the New Layer dialog box. Set Mode to
Overlay, and you’ll notice the Fill with
Overlay-neutral color (50% gray) option
appear. In Overlay mode, 50% gray is
transparent, so check this option, and
click OK. Even though you won’t see a
visible difference in the image, the new
layer has been filled with 50% gray.
|
|
Now it’s time to add some tonal variety
that will allow for a conversion without
banding. Select the Overlay layer in
the Layers palette, choose Filter > Noise
> Add Noise, and set a 2% Amount with
Uniform distribution and Monochromatic
unchecked. You may think you’ve
just ruined a perfectly good gradient, but
remember you’re zoomed in and it won’t
be noticeable when you print. Due to the
Overlay blending mode, the noise only
appears in the image’s midtones. Had
you applied noise directly to the image,
it would’ve added noise to the highlights
and shadows and created a muddy look.
Now go ahead and convert to CMYK;
choose to flatten the image. This time,
the noise provides some tooth for the
conversion algorithm to use, so there’s
no banding
Tips
This is a destructive procedure, so
make sure you’re working on a copy
of your original.
Each image will require its own
custom noise settings, so just add
the minimum amount of noise to
eliminate your banding.
In certain circumstances, any
conversion algorithm can fail, even
one that simply interprets how your
image appears on your screen. For
this reason, always zoom in close (at
least 200%) to inspect and ensure
the banding is genuine and not just
a video artifact.
|
 Don't miss the next Photoshop tip on Graphics.com. Get the free Graphics.com newsletter in your mailbox each week. Click here to subscribe.
|
Doug Nelson is a freelance writer,
technical editor, and founder of
RetouchPRO.com, an online community
for photo retouchers.
|
|
