My apologies for not posting this sooner. Here is my work through. For tips on composition and choice of images – see the previous article on double exposure.
1. Open Gimp. In my version of Gimp there are 3 separate windows, the image window in the centre, a toolbox/tool options dialog on the left and the layers/brushes dialog on the right. The layers box will also have tabs for channels, paths and history dialogs. All of these are required. Make sure all of these windows are visible.
If not, you have probably closed them inadvertently and Gimp has remembered this as your preference. You will have to go to the windows menu and try “recently closed docks”. If you can’t find them here go to “dockable dialogs” and tick each of the respective boxes to make them reappear. (This was a common problem at our workshop)
2. Open the first image – This is easy. You just drag and drop the image into the central window from your image browser (eg windows explorer)
3. Paste the second image as a separate layer over the first image. The easiest way to do this is to simply drag and drop the second image on top of the first image.
If you have opened 2 separate windows in Gimp (one for each image) just copy the second image (Ctrl-C). Then select the first image and paste (Ctrl-V). The new image may appear as a floating selection in which case you will need to convert this to a layer. (Right click on floating selection in the layers dialog and this option is offered to you in the menu that appears.)
4. Merge the two layers. The merge functions are in the “mode” drop down box at the top of the layers dialog. In this instance you only want to change the “mode” for the top layer. It affects how a layer interacts with the layers below it (not above). Select the function that suits the image best. You have a lot to choose from. See the previous articlefor a quick guide to what the functions do. Remember you can also reduce the opacity of this layer if the merge is too strong. The “opacity” slider is directly below the “mode” drop down menu.
5. Move the images in relation to each other. In order to improve your composition you may want to move or scale the layers.
There is a blue four pointed arrow in the tool box that will allow you to move a layer. Unfortunately this tool will only move the upper most visible layer. You can move lower layers by turning off the visibility for all layers above it (eye symbol in the layers dialog). There is also a scale tool that works in the same way.
(NB It is worth moving and scaling layers before making duplicate layers to avoid creating double images.)
6. Improve contrast. I do this with the curves tool. Select the layer that you wish to adjust in the layers dialog and then go to the “colour” menu and select “curves”. For a full explanation of how to use the curves tool, here is a previous article you can refer to.
If you want to adjust both layers at the same time. You may have to merge the layers first.
7. Improve detail. You can improve detail of either of your 2 layers by goings to “filters” and then “enhance” and then “sharpen”. Remember the filters will work only in the selected layer.
If I want to strengthen the lower image I found that sometimes it was helpful to duplicate this layer and paste it over the top of the second image. Then I merge it with “soft light” and reduce the opacity until you achieve the effect you want.
8. Flatten image. You can combine all layers into a single image using the “flatten image”command which is available if you right click any of the layers in the layers dialog. Sometimes it is useful to flatten an image so that you can work on the contrast or details of the combined image..
9. Export as .jpg. Gimp does not allow you to save in any format other than it’s native file type, .xcf. The way around this is to export the image as a .jpg (or .tif or .gif or whatever format you choose)