Monday, August 27, 2012
Posterization Prevention And Remedies—08/27/12
Posterization can ruin digital images. Here’s how to deal with it.
|This Article Features Photo Zoom|
- Work with a 16-bit image file. The color bit depth of an image is a function of how many million or billion different colors are available in an image file. The difference between a million and a billion seems like an arbitrary comparison of huge numbers, but in fact the visualization of this difference is huge—and something you'll definitely see in pictures. With fewer subtle color changes, gradations turn into jagged, stepped color blotches. But if you have more colors at every pixel's disposal, posterization will be a thing of the past. The best way to eliminate posterization is to start with at least a 16-bit image file and not downsize the bit depth when converting a RAW file or opening it in Photoshop.
- Get a good exposure. Keep an eye on your histogram to ensure that you're not maxing out any one color and losing detail—which will also contribute to a posterized image. The worst-case scenario is to underexpose an image. Since there are fewer available tonal values at the shadow end of the spectrum, you're more likely to lose detail there—and loss of detail equates to posterization. This is the same principle as “exposing to the right” to maximize image detail that lives in highlights, and it's why a lighter exposure of the same exact scene will produce a larger image file. Simply put, dark images just don't have the detail necessary to prevent posterization in subtly graduated tonal areas.
- Fix it with blur. Sometimes no matter how careful you are with your image files—from exposure to bit depth and deliberate editing—you're going to get some posterization. In these cases you may have no choice but to try repairing it. To do this, a subtle blur of the Gaussian variety (found in Photoshop's Filter menu) can eliminate some of the stronger edges in a posterized file. This works especially well when the posterization is most evident in the background—say a blue sky, for example—because you can duplicate the image onto a new layer, run the blur to mitigate the posterized background, and then mask away the top layer to combine a sharp original subject with a blurred—and less posterized—background. You can also try masking the posterization with a bit of intentional noise added to the image as well. Beware, though, that these techniques work best when applied to scenes with minimal amounts of posterization.
- Save carefully. Even if your image is perfectly posterization-free throughout the editing process, if you save your file incorrectly you'll find it posterized the next time you open it. For instance, when saving as a plain old JPEG you're going to go from a 16-bit to an 8-bit file. This will automatically increase the chances of posterization, which will be exaggerated by saving a smaller file size with higher compression. TIFF files are able to be saved as 16-bit files, as are certain JPEGs, and that automatically helps minimize posterization from the smaller bit depth. The lack of compression in a TIFF helps prevent it as well, so when in doubt save as a 16-bit TIFF file and you'll minimize the risk of posterization.