Processing Coin Photographs in GIMP

Whether taking photographs of coins to catalogue a collection, show them off online to other collectors, or to help sell them, it is important to get the best image possible.

This of course starts with having the right equipment and lighting, both of which are complex subjects in and of themselves (I would highly recommend checking out this great discussion on photo lighting and camera rigs). To keep things simple, the photographs in this example were taken with an iPhone 6s held parallel to the coin and stabilized on a stand, and taken under a white light over a uniform white background. This achieves passable results, although better can be achieved with more specialized equipment.

But taking the photo is only the start of the journey. Some post photo processing is usually required to achieve an accurate, attractive and consistent result. This article sets out a relatively simple approach.

For this I use Gimp, which is a powerful cross platform open source image editor. It can be downloaded here.

Phase 1 - Stitching the Obverse and Reverse of the Coin into one Image

Given the two sided nature of the subject our image processing journey will start with two photos, one of the obverse of the coin (“heads”), the other of the reverse (“tails”). Because of this it is important to try to take the photo of the coin from the same height, angle and against the same colour backdrop (either mat white or mat black) with the same lighting, to ensure consistency across the images.

For this example I am using a late Roman bronze coin of the Emperor Valens. Reduced sized pre-processing photographs are below.

Once the pre-processed photographs have been downloaded to your computer:

  1. Open one of the photographs in Gimp (it doesn’t matter which);
  2. From the File Menu, select Open as Layer then open the other photograph;
  3. Create a new layer (there are a few ways to do this but for the sake of this tutorial from the Layer menu select New Layer). In the Width and Height box enter a square amount. This can be any size so long as it is larger than the coin (in these examples I use 1000 x 1000 pixels). Make sure that the new layer has a colour (it doesn’t matter what, it just makes it easier to see where the layers is relative to the photographs and to select it);
  4. Select the new layer, center it over the obverse of the coin (reducing the Opacity of the new layer can assist here), then, ensuring that the new layer is selected (use the Fuzzy Select tool, and ensuring the feathering option is deselected), select the layer with the obverse and copy then paste the obverse as a new layer (name it O);
  5. Select the 1000 x 1000 pixels layer again, then, after making sure it is centered on the coin, make a new layer from the reverse (name it R);
  6. Hide all the layers except O and R, then, using the Move Tool, align the two layers so that the O layer is to the left of the R layer and both are perfectly flush (it helps to zoom in and use the arrows on the keyboard to move the last few pixels);
  7. Once the O and the R layers are aligned*, go to the Layer menu and select Create New Layer from Visible. Call the layer that has been created OR;
  8. Hide the O and the R layers so only the OR layer is visible. Use the Fuzzy Select tool again to select the OR layer (it can be easiest to click outside of the borders of the layer (ensuring Select transparent areas box is checked first) then use ctrl + i to select the inverse (i.e. the OR layer)). Once selected Copy the OR layer to the clipboard (ctrl + c);
  9. Finally, from the File menu, select Create, and then select From Clipboard. There will now be a new image of the OR layer no larger than the layer itself.

*If the colouration of the O and R layers are significantly different you may find it convenient to correct the colour of each layer individually prior to competing this step. If so, skip ahead to Phase 2 (below), and come back to this step when you’re done.

Phase 2 - Colour Correction

More often than not, the lighting conditions and limitations of the camera being used will mean that the colour of the coin in the photograph is not true to life. But now that there is an image of both the obverse and the reverse on the same layer and the excess background has been reduced, we are ready to start correcting the colour to ensure that the image matches the coin.

Go to the Colours menu, and from that select the Levels tool. You will see three eye drops icons (coloured black, grey and white). Pick the icon that aligns to the colour of the background used in the photographs, then click on part of the image that is meant to be that colour. This will adjust the colour levels to make the part of the image you have clicked the same colour as the eyedrop tool being used, and will adjust all other colours in the image proportionally.

Most of the time this will adjust the colour of the coin in the image to pretty close to the colour of the coin in hand, but you may find some times that you may need to repeat this several times to get the colour close.

Other tools that can assist here are also found under the Colours menu. The ones I most commonly use are:

  • Colour Balance; and
  • Brightness/Contrast

Subtle tweaks to the values here (usually less than 10 points) are often enough to bring out the coin’s true colour if the result of the Levels tool isn’t quite spot on.

Phase 3 - Uniform Background

Here, the Valen's coin I have been working on is pretty good, and like my experience with this coin you may find that after completing Phase 1 and Phase 2 your image might be perfectly satisfactory. However sometimes this won't have been achieved or you may wish to change the colour of the background. To achieve this requires a little patience, but with the right tools is relatively quick and can achieve a pleasing result:

  1. From the Layer menu, select New Layer, and under the Layer Fill Type select White. Rename this as Background;
  2. If you want your background to be any other colour than white, at this point use the Bucket Fill tool to change the Background to the desired colour;
  3. Move the Background layer behind the OR layer;
  4. Click on the Fuzzy Select tool, and ensure that the following settings are set:
    1. Antialiasing is ticked;
    2. Feather edges is ticked, with Radius set to 10;
    3. Threshold should be set to 15;
    4. The Select by menu should be set to Red for bronze coins, while Hue works better for silver coins
  5. Use the Fuzzy Select tool to select the area around the coin by clicking the areas of the background of the OR layer;
  6. If the selection isn’t perfect (zoom in to check), holding shift click on areas of the background of the OR layer that have not been selected to add them to the selection, or to deselect areas of the coin that have been selected, holding ctrl click the selected areas that you wish to unselect. If this does not work to perfectly, you may find that you need to select or deselect parts of the image manually using the Free Select tool;
  7. Once happy with the selection, hit delete to reveal the background.

Finally, from the File menu, click Export As to save the image in the desired format.

Below is my Valen's coin with the background set to black (you will notice that changing the background from white to black can have a significant impact on how the colour of the coin comes across, showing how different coins may suit different approaches ). For a similar finished example see the (rather worn) Caligula As at the heading to this page.


Copyright Aion Art