Tools for Philatelists – Image Comparison. 
Using our computer and imaging software, we have the capability to compare images. In the example shown, the logo on official mail is known in 52 different varieties. They can be easily separated into four major groups by language used. However, beyond that point it become difficult.
The difference between the first group of logos with Irish and English wording to the second group of Irish language only is easy. The “Official Paid” disappeared. What is interesting is the transition ! One would assume that the draftsman would draw a new master and omit the English lettering. That was what was done in most cases. However, in the rush to get things done, a few of the older Irish-English logos were modified to remove the offending English language. Who did it, and with what sort of tool is unknown, but the result was very unprofessional. (Figure 1). While most of the letters were removed, the stubs of their bases remained. A portion of the base of the harp was also axed, and the “D” in Diolta was also affected.
It is possible to show that the resultant logo was actually a previous logo butchered by overlaying the two logos. The procedure for doing this requires a photo processing software, such as Corel Photopaint. The two logos to be compared is the Irish-only shown in Figure 1 and what we believe to be the original logo as Figure 2. (Figure 2) The method is as follows:
1. Scan both logos,. and make them parallel. The easiest way is to find a straight line, in this case the back of the harp, and using the Straighten tool, set both logos to the same aspect. The cancels have been erased from the Irish-English logo.
2. Change the color on one or both logos to contrasting colors. Here Figure 1 has been changed to magenta. (Figures 3). Figure 2 remains black. This is done since It is impossible to distinguish black from black.
3. Remove the background from one of the logos using the Background Remover tool. In this case the modified log, Figure 3, since we are looking for where the missing letters belong. Save the resultant image as a transparency.
4. Overlay the transparency on top of the of the original image, taking care to align the outside circle as best as possible. Save the result (Figure 4) which, in this case, shows the letters and letter stubs align, indicating that they came from the same original Irish-English logo..
This procedure works with other type images as long as a portion can be designated as background and be removed. For multi-colored images, it is sometimes helpful to first convert the image to black and white then recolor to a single color..
This method is a useful tool to have in your box of tricks.
“Visual Comparison of Complex Objects”, Murphy, R., American Philatelist, June 2012,