Eliminating Image Banding in Photoshop
By Doug Nelson
Dateline: May 19, 2006
|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
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.
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Doug Nelson is a freelance writer, technical editor, and founder of RetouchPRO.com, an online community for photo retouchers.