Synthetic Lighting for Photography
Paul Haeberli
Jan 1992
Introduction
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 Scene
If 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 Source
By 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 Sources
Now 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 Example
Here 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.
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