Line Drawing Features on a Coin

Features on ancient coins are not always easy to identify if some parts of the design are not clearly visible. The toning or residual corrosion on the coin can make it difficult to get a clear scan or photograph of it. To avoid any damage to coin one should not over clean it. Many of the details are however visible under magnification and in such cases, it is useful to make a line drawing. If there is text that is only partly visible (corroded or outside the flan), it may be possible to guess and draw the rest of the letter particularly if characters are known from other specimens.

Many numismatic publications include line drawings that are usually clearer than the photographs. Coins from Southern India by Sir Walter Elliot, published in 1886, illustrated the differences between prints from copper plates engraved by Indian artists. These plates make visible the minute detail of the coins. Even in the reprint of this book, the engraved first plate is very clear in comparison with the auto-type plates. Some of the low-cost publications from south-asia have used pencil rubbing but they are rarely useful and lot worse than good well-printed photographs.

Computer technology has now made line drawing easy for even non-artists like me. This web page contains my notes on how to create line drawings of coins. I hope you will find it useful to avoid some of the mistakes I made in my learning experience. I hope it encourages you to draw some of your ancient coins.

Image processing software like Photoshop has various tools that can be operated using a mouse. However, a mouse gives poor control compared to the drawing with the human hand. For example try signing your name with the mouse, a task we now often do in shops on a computer screen using a pen-like for credit card authorization. In reply to a question on Internet discussion group Moneta-L, Grzegorz Kryszczuk told me about WACOM products. They seem to be the trademark of choice for computer aided drawing instrument. There are many models over a wide price range. They include the older Graphire products, the more expensive Intuos products. For a few $K, the Cintiq tablet PC allows artists to work directly on a LCD touch screen. The USB Graphire2 and the newer Graphire3 models are reasonable entry-level devices and are probably as good for those without the artistic expertise to make optimum use of the more expensive and sophisticated devices.

I was able to purchase a Graphire3 4x5 (MSRP US$100) for $63.50 after rebates (included a 2-year warranty and Sales Tax) from Circuit City. It sells for around $90 on eBay. The 8.3 x 8.2 inch tablet is plugged into a USB port and has a 3.6 x 5.0 active area. The light cord-less, battery-less pen operates like the mouse on this tablet. Although it works with Photoshop 7.0, it has clearly been optimized for Adobe Photoshop Elements.
Also included is a cord and ball-less mouse that works on tablet if you want to get rid of a standard roller ball mouse. The tablet is attached by cord to USB, rather than the mouse to computer

Unlike the relative motion of a mouse, the pen on tablet uses absolute motion. Each location on the tablet maps to a specific pixel on the screen. The larger and more expensive tablets may not be better for coin tracing since then drawing require larger movement of hand. It is better to display parts of the coin image on the screen and look at a magnified view, minimizing the hand movements for the drawing. Choice clearly depends on application and preference.

After a bit of practice, drawing on tablet while looking at the screen is no different to drawing on paper. In fact since your hand and pen don't cover or shadow what you are drawing, and not needing to look down are distinct advantages of this setup over the very much more expensive tablet PC. You can use the pen to do all of the mouse operations as well; so you don't need to switch to mouse to select options etc. The Graphire pen with tablet is just a replacement of the mouse as the input device. You are operating within "Photoshop Elements" software so you have ALL of it's full range of image processing tools.

The top end of pen has been defined as an eraser, so you can use it just like in a pencil. The Graphire pen activates when 5 mm from tablet so switching between the pen and mouse is automatic. For this reason, do not leave pen on tablet but replace it when not in use in the molded inkpot or penholder on back of tablet.
(An advertised improvement in Graphire3 which I understand is not significantly different from older slightly less expensive Graphire2. See FAQ).

As an example my first line drawn coin: the Abisheka Lakshmi token. I purchased the Graphire3 motivated by the need to draw this unique token found in Ruhuna, south east of Lanka. See JavaScript page which allows you to flip between images with and without line drawing and coin. On a high resolution(1024x768 or larger) screen view page which displays at full imaged resolution and allows you to flip between images including the images of coin before it was cleaned. The standard HTML page does not need JavaScript and displays coin at full imaged resolution.

Sometime after writing up these notes I spoke with a good friend Nare Ratnapala who lectures on Art in the University of Maryland. He told me the Pen-tablet was not very popular among professional artists. The reason is interesting. If you have already trained your brain to master drawing with the pen looking at what you are drawing, it is more difficult and maybe even bad to retrain your brain to the Pen-tablet independent screen format of the Graphire. He said that even the TabletPC the screen does not the same texture of a canvas or paper; the media on which one learns to draw. It is therefore simpler for them to master using the mouse which is a different kind of device than use a pen-like instrument for which the hand has already been trained to draw on a paper. He also commented on the larger hand movements needed on the tablet as mentioned above, which is a clear case for getting the smaller tablet not just because of lower price.

For beginners like me with no prior art training I give in text below the sequence of operations needed to draw an outline of a coin. Menu commands are shown in Bold font with - to indicate sub-menu option. All commands take place in the environment of Photoshop Elements (version 1.0 provided with product).
Elements version 2.0 is NOT compatible with Win98SE

The line drawing can be shifted (Move Tool) on to different image of the same type of coin and scaled (Image - Resize - Scale) and rotated (Image - Rotate - Free Rotate) to match it. Resize and rotate operation may need to be done alternatively a few time to interactively get a proper registration of the features.

When moving rotating images it is useful to first View - Fit Screen to see all of coin image as well as to increase the size of the canvas (Image - Resize - Canvas Size) to be about 25% larger. This pads an additional blank border around the coin image. Remember the original pixel size so that it can be cropped back to the original size using the same option.

The operation of moving the line drawing to a new image as some useful applications.

Text along periphery

PhotoShop has a builtin image transformation Filter - Distort - Polar Coordinates to change a circular map with Pole at center to a square Mercator Projection. This can be used to obtain a straight line image of the text along the periphery of the coin without needing to copy each letter out one at a time and rotate and position in order on a new canvas. The PhotoShop projection of circle is to a square image of the same size needs an additional horizontal stretch by π (3.14) to get very readable Brahmi text along the edge with hardly any distortion. To stretch Image - Resize - Image Size with Constrain Proportions unchecked multiply Width pixels by 1.772 and divide Height pixels by 1.772 to keep the image area the same.

Brahmi text is written anti-clockwise with feet of letters to edge. Roman Text written clock-wise with feet of letters to the inside would also require an additional 180° rotation of top to bottom and left to right.
This transformation is clearly useful for presenting text written along the periphery on ancient coins in a more readable image format. Probably more useful for Brahmi text we are less accustomed to reading at different angles like we can Roman alphabet.

One needs to rotate the start point to the top of the coin image. Rotation is best done in one step to avoid image degradation. Image - Rotate - Free Rotation does not give a quantitative value to remember last rotation. Image - Rotate - Canvas Custom allows rotation by an angle input numerically. This greatly simplifies interactively selecting the angle of rotation needed to get the start point of text on top. The Canvas is increased in size and would need to be Crop Tool selected and Image - Crop back to original size keeping coin centered. Else one may cut and move a rectangular section after transformation to get the proper beginning of text.


Unresolved Problems

These notes are being updated and corrected in my learning process.

Below are issues to which I am seeking the proper solution. If you know an answer please E-mail me.

Random thoughts

A high resolution quantitative 3D image or scan of the coin could the best starting point for automated image drawing but that equipment is extremely expensive. The features of a coin need not however have the same depth and a contour plots may not highlight all of the feature. The 2D image of a scanned coin casts shadows of the features depending on the direction of illumination. These shadows can be avoided by digital photography in defuse light. However shadows do help highlight the features. It may be possible to analyze a number of images of same coin taken from different orientations of illumination and both take away these shadows and obtain an automated line drawing.

The task is more similar to 3D face recognition than 2D fingerprints which cast no shadows. However unlike face recognition all coins do not have standard design features. Large scale designs on coins are a lot more different than the set of fingerprint or set of faces. Each coin type would need to be independently setup and pre-identified. Within a single coin type it may be possible in theory to cross identify different images to find die types and varieties as currently done by eye. However any automated coin analysis is unlikely to be as good as the human eye which can do very much better qualitative feature recognition on a well illuminated high resolution coin image.

Some interesting related links
The Stylistic Analysis of Celtic Coins