Read Beneath the Lines! Learn How Amped FIVE’s Color Deconvolution Filter Helps You Take Out Relevant Elements from Color-Mixed Images

Dear Tip Tuesday lovers, thanks for being here in the heart of summer!
Today’s hot tip is about the Color Deconvolution filter, which is one of the (many!) “wow”-generating tools in Amped FIVE.

We had a case where a calligraphy expert was asked to authenticate a signature on a document. We can’t publish that image on the blog, so we’re using a home-made sample for this tip:

Unfortunately, as in the case above, most of the signature was covered by a stamp, which made the expert’s analysis a bit complicated. Luckily, though, the signature and stamp were in different colors. “It’s trivial, then!”, I can hear you say. Well, it’s indeed feasible, but it’s not that trivial. Just subtracting a color channel could deteriorate both the stamp and the signature, and playing with channel balancing (which you can do with Amped FIVE, of course) could easily be time-consuming and rather subjective. That’s because, as long as we work in the classical RGB color space, both the stamp and the signature will usually share some green, some red, and some blue (although in different amounts, hence their different colors). Probably, thus, working with RGB is not the best option!

Amped FIVE features a powerful tool, called Color Deconvolution, which creates a color space tailored to our needs: a color space where the difference between the undesired color and the color of interest is better expressed.

Figure from “Berger, Charles E. H. et al. ‘Color separation in forensic image processing.’ Journal of forensic sciences 51 1 (2006): 100-2 .”

Let’s add the Color Deconvolution filter (under the Extract category) to our chain. We just need to tell the filter which is the color we’re interested in, which is the disturbing color, and which is the background color (in our case, the color of the paper). To do so, simply click on the “Desired Color” tab and then click on a pixel of the signature.

Then, do the same for the “Undesired Color 1” tab (this time, pick a pixel on the stamp) and for the “Background Color” tab (pick a pixel on the empty paper).

And here’s the result!

There could be cases where you need to take away two undesired colors. For example, imagine we’re asked to enhance the readability of the red writings in the image below (you can find this image in your Amped FIVE’s Samples folder, accessible through the Help menu; it’s in the folder “color-deconvolution-graffiti“).

Both black and blue are disturbing. If we were limited to removing only one color, we should probably choose black, and obtain this image (we’re also adding a Level adjustment):

We see that blue writings are still there to disturb. Once more, Amped FIVE has got us covered! We just need to set the desired and background colors as we did before, and then select a pixel on the black writing in the “Undesired Color 1” and a pixel on the blue writing in the ” Undesired Color 2″. Then, we just move to the filter’s Settings tab and choose “Remove First and Second Color”.

As you can see below, the possibility of removing two colors produces an image where red writings are way more readable!

One more thing before we say goodbye: in the filter settings caption shown above you may have noticed there’s an option called Size. Since we’re dealing with reality, your element of interest will usually not be entirely made of an exact, single color: instead, there will be minor variations. For this reason, the filter gives you the possibility to sample the desired/undesired/background colors by averaging pixels in a neighborhood around your selection, whose size is customizable. Use larger values of Size when your element of interest has noticeable color variations. Of course, make sure to click on a pixel that’s well inside your object of interest, otherwise, the neighborhood could include other elements of the image and pollute the sampling.

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