How can I reduce/smooth the colors of an image?
I have an drawing that was created with colored markers. I've scanned it and would like to use it as clip-art/icon style image.
I'm wondering how I can smooth out the colors of the image such that the marker strokes are no longer visible. I've tried posterizing, but this also ruins the edge of the image and ends up appearing too extreme.
Are there some other techniques I could use to make an acceptable-looking icon or clip art from a scan of a marker/crayon/colored-pencil drawing.
Here are two techniques you can use in Photoshop, commonly used for exactly this kind of situation. They give different results, so pick the one that achieves what you're looking for.
A basic thing to keep in mind is that marker strokes (including the texture of the paper than is usually visible) are really just a form of noise. In this type of situation, though, the "noise" pixel areas are usually beyond the scope of
Filter > Noise > Reduce Noise or
Filter > Noise > Dust and Scratches.
Here is a detail, zoomed to 250%, from a scan I received from a client. (The artist did a terrible job of the scan using some kind of homeowner-grade Mac imaging software, but the marker strokes, which is what we're interested in, are clearly visible.)
Filter > Noise > Medianmerges the colors in adjacent pixels. Isolate the drawing from the background first and keep it selected so that the filter doesn't blur the edges. Adjust the radius to get the effect you're looking for. It will be small -- 2-5 pixels, depending on the size of the image, is usually plenty. You can apply this more than once to build the effect gradually.
Filter > Blur > Surface Bluris often more useful. It blurs regions where the brightness differences among adjacent pixels are below a threshold setting. (No need to make a selection, because it doesn't affect hard edges.) It's a matter of playing with the values until you see the effect you want. The higher the threshold value, the more pixels will be affected. Again, you can apply this one more than once. Sometimes applying the Median filter first, then Surface Blur, gives the best result.
Both of these can be applied to the composite RGB image or to individual channels. In some cases, changing the mode to Lab (Image > Mode > Lab), applying Surface Blur only to the Lightness (L) channel, then switching back to RGB and applying again to the RGB image will give you "better" (depending on the effect you want) results.
Sometimes one of these will get you all the way. Usually you'll still have to do a little bit of touch-up with the Spot Healing Brush, Patch Tool or Clone Stamp. Experiment a bit, and you'll soon get the hang of which one, or which combination, works best in different situations.
I would recommend using Adobe Illustrator for this, if you have access to it. Simply Live Trace the image (Object>Live Trace>Tracing Options) and adjust the settings there until it looks the best. I would reduce the number of output colors to match the number of colors in your original image. Once you are done live tracing the image, you may need to convert it to Live Paint, which will let you edit the points easier.
The benefits of using this method are that it renders a cleaner, editable vector graphic that can then be scaled infinitely without losing quality. This is very important for clip art and icons.