The following is a brief tutorial on how you can take and edit multiple distinct digital photographs taken from a single vantage point for creative photo-manipulated effects. This includes separate scanned film images, or separate images from a digital camera.
If you've worked through the example in the previous page, you may already be guessing the solution to creating translucent objects.
The trick here is to take two photographs, one with a certain object in view, and one of the same scene without that object in view. The combination of these two images using GIMP's layering feature now becomes child's play.
The image above was built in a matter of minutes. I used a tripod and two desk lamps to take a photograph of my computer's mouse. I took a second photograph with my hand in place... being very careful not to move the mouse that was already in the scene. (You may try it the other way around, taking the hand-shot first.)
With both images in GIMP as separate layers, I reduced the opacity of the top layer to approximately 50%. My photographed hand became a ghostly translucent object, while the mouse which appeared in both photographs stayed perfectly solid.
Note that for a two-photograph shot, it really doesn't matter which layer is on top. The trick will work either way. If the hand layer were on the top, reducing its opacity lets the mouse show through. If the no-hand layer were on top, reducing its opacity lets the hand show through. Try it for yourself both ways to understand the effect.
Other projects may be easier with a certain ordering of layers. To change the ordering of layers in GIMP, just open the Layers dialog and drag the layer thumbnails above or below each other in the list of layers. If it seems like GIMP won't allow the layer ordering to be rearranged, right-click the bottom layer's thumbnail and choose the Add Alpha Channel option. This tells GIMP that the bottom layer can have other layers added below it.
I saved the image in GIMP's XCF format to keep the layers separate, just in case I wanted to go back and adjust the transparency at some future date. I also exported the image in JPG format so that it could be used in this web-based tutorial page.
Another fun example using this technique is to take a group photograph twice, and then turn one of your friends into a ghost. A more challenging project is to make an opaque person sit in the driver's seat of a translucent car. Remember, everything in the scene which needs to appear opaque will need to be in both (or all) photographs.