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Photoshop CS5 - how to "normalize" colors in an image to reduce the noise?


Question

New to photoshop so bear with me...

I have a scanned image that, ideally, should be just three colors: red, black, and white. The original was a pen an ink drawing and the act of scanning added all sorts of slight color variations.

To make things worse, the image was saved as a .jpg before it was sent to me, so the compression has created some artifacts as well.

I've already tried "reduce noise" filter in a hundred different ways and, while somewhat helpful, does not produce the effect I'm looking for. Is there some way to convert, normalize, or blend all of these different shades of red to a single "red" color? Likewise for the blacks and whites. I don't want 35 different shades of light gray around the edges of the my image.

Thanks in advance for any help you can provide.

2012/01/11
1
7
1/11/2012 6:10:00 PM

Accepted Answer

There are three "brute force" approaches you can take:

  • Add a "Posterize" adjustment layer and set the color count to 3. One you have it to your satisfaction, flatten the image. This is probably the quickest and simplest approach, and will likely work fine for your purposes. This is the only approach that doesn't mandate making a copy of the base layer first.

  • Use Filter > Blur > Surface Blur to smooth out the variations without affecting edges.

  • Eliminate the noise on a channel-by-channel basis using Surface Blur or Filter > Noise > Median. You can even go into Lab mode and run the noise filter on the a and b channels, then Surface Blur on the L channel, but in a simple case like this I doubt that's needed.

If the jpeg artifacting is really obnoxious around your hard black/white boundaries, as it sometimes is, you can prep the image for the Posterize treatment by using a hard-edged white brush in Overlay mode (or the Dodge tool set to "Highlights" and about 50% exposure) to turn the gray blocks white without affecting the black.

2011/12/23
11
12/23/2011 6:00:00 AM