Using Calculations to Extract Hidden Details

Hidden in plain sight on the Adobe Photoshop® Image menu, is an often misunderstood, yet potentially powerful tool for the processing of color images. Unlike many of the other tools found in Photoshop, Calculations gives the more experienced user access to some of the basic mathematical functions that formerly were only available to students of Electrical Engineering - or those who were willing and able to write their own computer code. Calculations gives you the ability to perform mathematical operations upon specific color channels within the same image, or between two different images, without having to write any code.

So why should you care if you can subtract one color channel from another? Suppose you're trying to identify a fingerprint that you processed with ninhydrin, and the piece of paper on which the print is located has some red in it. It becomes nearly impossible to distinguish the print from the background.

When ninhydrin reacts with amino acids in the latent print, the result is a purple- or violet-colored print that derives much of its color information from red. The rest is made up of both green and blue. By subtracting a portion of the red spectrum (adding green), you can lighten the background and darken the fingerprint ridges.

Mixing various amounts of three different color channels - Red, Green and Blue - creates a color image in the computer world (RGB). In the case of a 24-bit color image (also known as a true color image), there are 256 possible numeric values (ranging from 0 to 255) for each of the Red, Green and Blue image channels.

By mixing different amounts of each channel, it is possible to produce more than 16.7 million different combinations (256 x 256 x 256). Because the human eye can only visualize approximately 350,000 different colors, the eye sees many subtle differences in hue as the same color. If these subtle differences happen to represent a fingerprint on a piece of paper, it is going to be nearly impossible for the average latent examiner to even see, let alone identify, that fingerprint without some kind image filtering.

In the old days, we might have put a filter on the lens of a camera, photographed the fingerprint, and then hoped that there was enough difference between the fingerprint ridges and the background so that the filter would pass more reflected light from the ridges than from the surface of the paper. The result would be a higher contrast image on film, which the eye can more easily distinguish.

Calculations gives you the ability to separate information by color, much the same as the optical filter does in traditional photography, but with much more control over the result.

It is also possible to build effects by performing operations upon the result of previous operations. This may sound complicated, but once you understand the basic principles and what each function actually does in terms of pixel values, you'll find Calculations to be a very effective tool.

As unlikely as it might sound, even a fingerprint developed with fingerprint powder reflects color information. Since the powder selectively absorbs, and then reflects a percentage of the original light that illuminated it, a color shift can occur. This color shift can then be used to further separate the fingerprint from background elements that might reflect the light differently. If we were to record the image with a grayscale camera rather than a color camera, much of the information that would have been available to separate the fingerprint from its background would be lost. The real power of Calculations comes from the ability to utilize color information to extract useful image data. A list of Calculations functions is presented at the end of this section.

The best way to start using Calculations is to open an image in Photoshop, and using the list of functions as a guide, try to predict what the results of the function might be. The best part about using Calculations in this kind of situation is that when you choose a particular function, Photoshop provides an instant preview of the result, giving you the opportunity to try different operations before committing any one particular function to your image.

Using Calculations with Ninhydrin Prints

Use the following steps to process a ninhydrin image using Calculations:

With your image on the screen, click on Photoshop's Image menu and choose Calculations.
Select Red for channel in source 1 and Green for channel in source 2.
Select Overlay for blending and set opacity to 45%. You can vary the opacity to obtain a darker or lighter effect. A higher percentage of opacity results in a lighter image overall. A lower percentage results in a darker image.

Caution: Unless you change the target (result) or the target channel, the result of this operation is saved to a new channel in this image. Since the only image format in Photoshop that supports more than three image channels is the proprietary Photoshop® format, you will be unable to save this image as a .TIF or any other file format supported by Photoshop®, until you have changed the image to Grayscale. If you are using MoreHits® to track your images, you must convert this image to Grayscale before you exit Photoshop and return to MoreHits®.

Click OK to accept the results of the Overlay function.

Using Calculations with a Forensic Light Source

The whole idea behind the forensic light source is to use fluorescent dyes and powders to cause a color shift to occur along the ridges of a latent print - so that the print can be visually separated from extraneous background interference. The Calculations function can be a very useful tool when a light source is used to isolate visual information.

As newer dye stains and more discriminating light sources are developed, color and the ability to work with color is going to become very important.

Don't be afraid to experiment with these functions. Because all fingerprints are different, it is very difficult to have any kind of hard and fast rules for using Calculations (or any of the other functions Photoshop provides). The following table lists the functions available in the Calculation Dialog, and provides brief descriptions of each.

Function	Description
Normal	Combines the results of the pixel values in each channel
Multiply	Multiplies the pixel values in the two channels selected and divides the result by 255. The resulting color is always a darker color. This function is analogous to superimposing two positive transparencies on a light table.
Screen	Multiplies the inverse brightness values of the pixels in the two channels selected. The resulting color is always a lighter color. This function is analogous to superimposing photographic negatives of the two source channels, and then printing the result on photographic paper.
Overlay	Performs a combination of multiplying and screening. Colors are overlaid on the existing pixels, but the highlights and shadows are maintained. The two channel pixels are mixed to reflect the lightness or darkness of the original color.
Soft Light	Multiplies or screens the pixels in the two selected channels. It produces the effect of shining a diffused spotlight on the image.
Hard Light	Multiplies or screens the pixels in the two selected channels. It produces the effect of shining a harsh spotlight on the image.
Darker	Compares the brightness values of the corresponding pixels in the two selected channels and displays the darker of the two. When two grayscale images are selected, using darker simply retains the darkest values from each image or channel. When color images are selected, the result can be very different from the colors in either of the two selected images or channels.
Lighter	Compares the brightness values of the corresponding pixels in the two selected channels and displays the lighter of the two. When two grayscale images are selected, using lighter simply retains the lightest values from each image or channel. When color images are selected, the result can be very different from the colors in either of the two selected images or channels.
Add	Adds the pixel values in the two selected channels. Because higher pixel values represent lighter colors, adding channels lightens the image. Black areas in both channels remain black (0 + 0 = 0). White in either channel results in white (255 + any value = 255 or greater). The Add function divides the sum of the pixel values by the scale amount, and then adds the offset value to the sum. For example, if you wanted to find the average pixel value in two channels, you would add them, divide by 2 and enter no offset value. The scale factor may be any number between 1.0 and 2.0. Entering a higher scale value darkens the image. The offset value lets you lighten or darken the pixels in the destination channel by any brightness value between +255 and -255. Entering a negative offset value darkens the image even more. Entering a positive offset value lightens the image.
Subtract	Subtracts the pixel values in the source channel from the corresponding pixels in the target channel. As with the add function, the result is then divided by the scale factor and added to the offset value. Subtracts the brightness value of the pixels and displays the absolute value of the result in a new channel.
Difference	Difference is useful for comparing information in two different images or to see if the information in two copies of the same original are different. For example, you can use this function to compare two frames, taken from a video frame grabber, that have the same background but contain different foreground elements. The light areas in the resulting new channel show areas in the frames where changes occurred.