Synthetic Lighting for Photography
Light from different light sources add together to illuminate objects in a scene. We can use this super-position principle to modify the lighting of a scene after it has been photographed. We can also simulate negative light sources.
In this scene, two desk lamps illuminate a few objects on a table top. There's a statue, some rolls of ribbon, two small match boxes, and a folded piece of paper.
Photographing the SceneIf we save the following three pictures of this still life, we can alter the lighting of the scene freely after the pictures have been taken.
1. We save an image of the ambient light.
Both of the lamps were switched off for this picture.
It's kind of dark because there is only a little ambient
light floating around in this office.
2. We save a picture with just the left lamp switched on. This picture contains the ambient shown above light plus the light coming from the lamp on the left.
3. We save a picture with just the lamp on the right switched on. This picture contains the ambient light plus the light coming from the lamp on the right.
Finding the Contribution of each Light SourceBy subtracting the ambient image from the picture taken with the lamp on the left, we can create an image that has just the light contributed by the left lamp. Similarly, we can subtract the ambient light image from the picture illuminated by the lamp on the right. This gives us an image that shows exactly what light is contributed by each light source.
Changing the Color of Light SourcesNow we can simulate what the scene would look like if the lamp on the left was blue instead of white. To do this, we multiply the image of the light from the left lamp by a blue color. The red channel is multiplied by 0.0 contributing no red, while the green channel is multiplied by 0.45 and the blue channel is multiplied by 1.0. Next we add in the ambient light to make this image.
By applying a similar process to the lamp on the right, we can now synthetically illuminate the scene with multicolored lamps. The brightness and color of any number of lamps can be controlled in this way. The only requirement is that the scene is not in motion, and the images are digitized accurately.
Negative Lights!We can also create negative lights by subtacting light that would normally be added by a particular lamp. Below, the lamp on the right has been made into a negative 30 watt bulb. It removes photons from the scene instead of creating them. Notice the bright negative shadows projected to the left of the little match boxes.
Another ExampleHere are the original pictures. Each photograph was made with just one light on.
These images were scaled to simulate red, green and blue lights.
Then these colored images were added together to make this final image.
This technique can also be used to control the lighting of rendered geometric objects.