The Story Behind the 2006 Holiday Card

xmas2006 (104K)
The image was created by a computer program that uses a computer graphics technique called ray tracing.
A bit of geometry is involved but the simple explanation is that each (of 960,000) pixels in the image is painted a color that is determined this way:

Imagine a camera pointed at the scene and a clear sheet of blank pixels right in front of that camera.
Imagine a line from the camera, through a single pixel of the sheet.
Then, extend the line into the scene to see what it hits.
If the line hits, say, a blue object, then paint the pixel blue.
Do this for all the pixels in the image.
When you're done, you'll have an image of the scene painted onto the sheet.
The line is called a ray because it represents a ray of light.
The image can look more real if, when the ray hits something, make a new ray and point it towards a light source.
If the new ray hits the light before it hits anything else, make the color brighter.
Otherwise, it's in shadow so make the color darker.
Rays can also bounce off shiny objects and pass through transparent objects.
These rays go on to perhaps hit other objects.
The colors gathered by the reflected and transmitted rays are added together and passed back to the original camera ray.
The pixel is painted with the composite color from all those rays.

The entire process is called ray tracing. In real life, light sources emit rays of light but there are an infinite number of them
and most never reach the camera. So, we start with the rays that do reach the camera and we trace them backwards to see
where each one came from. For this reason, sometimes ray tracing is called backwards ray tracing.

The camera, the pixels, the rays and the scene that we imagined above are all created in the computer
using geometry to set positions, directions and object shapes. The sheet of pixels is the computer screen.

The scene:

The scene is made up of objects that are modeled by geometric shapes.

Santa	Santa's image comes from a catalog of Santa costumes. The manufacturer chose a rather slender model so I had to fatten him up a little. I also gave him a new hat. He doesn't look quite right but since only his shadow appears in the scene, I'm not concerned with his details. I use blue screen effects to cut out his silhouette from a thin (ironic, isn't it?) slab. He looks like a 2-dimensional paper doll but his projected shadow is all I need in the scene.

Graffiti	The graffiti is painted on the wall using a white mask (same as blue screen but white). The wall also has another layer with a dirty stucco-like texture.

Floor	The floor texture is a procedural texture where the pattern is calculated mathmatically instead of overlaying an image onto the surface of the object as I did for Santa's image or the graffiti. Here, the basic beige color of the floor is made darker by intersecting its flat surface with concentric cylinders. The cylinders are tilted very slightly to vary the pattern but the intent is to simulate how the tree rings make patterns when the floor boards are cut from the trunk of a tree. Randomly, the base color of certain boards is made a little darker so some of the boards are darker to begin with.


Tree	The Christmas tree is a purchased model from 3dplants.com. I converted it from 3D Studio format to be suitable for my software.


Flyby	This flyby shows how the elements come together to make the scene. It resembles a theatrical stage with flat scenery. The view only appears properly when viewed from the camera position represented by the red cone. Not seen is a point light source which shines through the doorway and projects the paper doll Santa's shadow on the opposite wall.

Return to Cards of Christmas Past