(H) Ed Halley's

GIMP Tutorials

Using Multiple Exposures in Photography

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.

Overlaying Transient Objects

Sometimes we want to develop a scene with more people or more cars than there actually available at any given moment. We can hope to wait for a few hours for just the right crowd to come along, or we can develop a crowd of our own using the GIMP. Again we will build upon the previous example to make a more complicated effect.

I went to a local college campus and stood on a suitable open-air overlook at their central plaza. A couple of people were milling around with some poll-taking paperwork, also hoping for a good crowd of passersby. Many students went this way and that, but not quite enough to establish a good crowd. So I set up my tripod and took about twenty shots of the foot-traffic.

As before, I combined the layers into one XCF-format GIMP file. You can see the assembled layers in the first screenshot below, where I've set each layer to about 10% to 20% opacity. This let me get a good guage of which shots bunched up too much, and which shots I wanted to keep. I threw out a few shots and set to work.

Although this step is optional, it helped me focus on the crowd. Just as before, I made an empty scene which had no people in it. I copied the XCF file and erased everybody to achieve this, just as I had done with my highway scene on the previous page. When I finished making an empty plaza, I flattened all of the layers into one, from the Layers menu, and copied that image to the bottom of the original XCF file's stack of layers. This left me one layer which had an empty plaza, and many layers with a few people each.


combining many layers

selecting objects in each layer

crowd with nowhere to go

The next step, as shown in the second screenshot above, was to isolate the people in each layer and clear everything else away. I wanted the people layers to have nothing but people on them.

While it's quite possible to grab the Eraser tool and go crazy with it, and you don't have to worry about rubber eraser crumbs all over your floor, there's a faster way.

Instead, I used the Quick Mask mode to select the parts of each layer I wanted to keep. In the lower-left corner of any GIMP image window, you'll find two small buttons as pictured here. One refers to the normal "marching ants" selection mode, and one has a red box to indicate the Quick Mask mode.

First, I selected the Quick Mask. While in that mode, a translucent red film covers the image. Painting light colors, such as with a Brush or Pencil or Airbrush tool, makes the red film more transparent; conversely, painting dark colors restores the red film. I chose a soft-edged brush and a perfect white color to remove the mask entirely, and brushed away the red film on top of each person. I used the airbrush to soften the edges even more near the people's feet, so that some of the shadowing on the ground was captured too.

When I turned off Quick Mask mode, any clear areas were converted to an active selection, complete with the little marching ants. (Although the marching ants ringed each person with a crisp line, the selection still remembered any soft or feathered edges I painted in the mask. The ants merely marched on the midpoint edges.)

With the people carefully selected thus, I chose SelectInvert from the image's menu. This reversed the situation: the people were not selected, but the rest of the layer was selected. To tell the difference, watch the marching ants as they circle the whole picture as well as the border around each person. I then chose EditClear to erase all the buildings and sidewalk, but left the people alone.

If you're new to this concept, I recommend you play with the Quick Mask a lot to learn how it works. Once you've mastered the Quick Mask, you'll find it a lot easier than this description makes it sound. Unlike the Lasso selection tool, Quick Mask can make very complicated, softly and variably feathered and multi-aread selections as easily as you can paint with a paintbrush. You can even edit a selection made by the lasso or other selection tools.

The results of editing each layer will look something like the third screenshot above. I've turned on the eyeball icon for each people layer, and turned off the bottom-most background layer which contained the empty sidewalks and walls.


finding tangled layers

untangling the layer order

The rest of the job is in the details. Depending on your project, there may be a lot of careful detail work to do. Some details matter, while others won't be noticed in the final work.

For instance, some of the people in this scene are behind red metal railings, which I have not tried to erase from them. This detail doesn't really matter, since they'd have to appear behind a rail anyway, in the final image.

However, as the above two screenshots show, there can be some problems when two layers interfere with each other. The student in yellow carrying a backpack is being obliterated by another pupil's foot on another layer. They need to be untangled somehow.

In many such cases, this can be accomplished just by dragging the layer thumbnails in the Layers dialog, adjusting their stacking order. The girl with a backpack obviously should appear in front of the boy with the foot.

In other cases, they can't be swapped so easily. Swapping two layers may fix one such case while creating another problem in another part of the image. One of the layers probably has too many objects on it, causing problems when you want to adjust their ordering in relation to the other layers.

The remedy here is to make a Duplicate Layer for the most offending layer, one with multiple distinct objects on it. Erase one object from one copy, and erase the other object from the original layer. Now there are more layers which can be adjusted for ordering, and fewer objects on each layer to conflict with each other.

The final touch-up task is to carefully erase the soft edges around those objects which are overlapping objects on other layers. Anywhere you forget to erase carefully, you will see a soft line of the background image showing through. The girl in yellow above a good example which shows the outline that has to be removed, both at her backpack and her foot. Erase those objects carefully, shaving the edge away until the two objects look like they were really in the same photograph at the same moment. Edit the shadowing below people's feet, soften them up a little with the Blur/Sharpen tool to "weld" them to the ground where they're standing.

The GIMP can edit images of pretty much any complexity, but how quickly it performs on complex projects like this depends mostly on your machine's memory. If you want to do this sort of editing with large images and lots of layers, you will want plenty of free memory. Also, the more layers are visible, the harder (and slower) the GIMP must work to display them all on the screen. Keep only one or two layers visible at a time while you work, until you want to preview the whole scene.


Next Section: Controlling Lighting


Contact Ed Halley by email at ed@halley.cc.
Text and artwork are Copyright © 1996-2005 Ed Halley
Copying in whole or in part, with author attribution, is expressly allowed.