THE IDEA
A string for every point
Dürer's 1525 manual shows a draughtsman using a strong thread from a point on the wall to a lute, marking where it crosses a frame.
Meet the rays
HOW COMPUTERS DRAW LIGHT
Follow the light,
one ray at a time.
Every pixel asks one question: what would I see?
Ray tracing follows the path of light through a scene as it bounces off objects. The idea is old: a 1525 woodcut shows an artist plotting a lute point by point with a taut string. Computers now do the same, over and over, for every pixel.
ONE RAY per question
Meet the rays- Rays start at the eye
- Shadows are one more ray
- Mirrors bounce the ray on
- Three ideas behind a ray tracer
THE SHORT ANSWER
Work backwards
from the eye.
Light in the real world travels from lamps to eyes.
A ray tracer usually runs the journey in reverse.
THE METHOD
Rays from the camera
Most renderers send viewing rays from the camera into the scene to find what is visible at each pixel, then check the light at that point.
Step by step
THE COST
Hours for two spheres
In 1986, path tracing a picture of two spheres took 7 hours on a computer that cost roughly $280,000 when new.
See the numbers
HOW A RAY TRACER WORKS
Four steps
for every pixel.
Every ray tracer simulates at least cameras, ray and object intersections, light sources, visibility and how light scatters at surfaces. In outline, each pixel goes through these steps.
Shoot a camera ray
Send a ray from the eye through the pixel, as if the screen were the film of a pinhole camera.
Find the nearest hit
Work out where the ray meets each object and keep the closest, with its surface direction and material.
Check each light
Send a shadow ray towards every light. A light counts only if nothing blocks the path.
Bounce if shiny
For mirrors and glass, trace a reflected or transmitted ray and add what it finds.
TWO NUMBERS WORTH KNOWING
Slow by hand,
slow by machine.
Tracing rays has always taken patience,
whether with string or with silicon.
7 hours
To render two spheres in 1986
When Kajiya introduced path tracing in 1986, an image of two spheres took 7 hours on an IBM 4341, a computer that cost roughly $280,000 when first released.
PATH TRACING · PBR BOOK
2 people
People needed to plot each point
When the University of Melbourne's print collection rebuilt Dürer's lute device, each point needed at least two people to plot, and the slightest movement spoiled the drawing.
DÜRER'S DEVICE · UNIV. OF MELBOURNE
The Physically Based Rendering authors quote Blinn's law: as technology advances, rendering time remains constant, because artists use extra computing power for more complex scenes.
MEET THE RAYS
Which ray does
which job?
Pick a kind of ray to see what it is for. The bar shows how far along the process it is traced, from the first ray out of the camera to rays that bounce on.
Based on Physically Based Rendering (pbr-book.org) and Scratchapixel's introduction to ray tracing.
WHY NOT START AT THE LAMP?
Most light never
reaches the eye.
In nature, light travels from a source and scatters in all directions, and only a tiny fraction of it reaches an eye. Simulating light from the lamp forwards would mean tracing an astronomical number of rays to fill every pixel.
So a ray tracer usually starts at the eye and works back towards the light. Path tracing, introduced in 1986, follows these paths further, bounce by bounce, to capture soft, indirect light.
Questions
Questions people ask
What is ray tracing?
A way of making images by following the path of rays of light through a scene as they hit and bounce off objects. The Physically Based Rendering book calls it conceptually a simple algorithm.
Why do rays start from the camera?
Scratchapixel explains that of all the light leaving a lamp, only a tiny fraction reaches the eye. Starting from the eye means only the rays that matter for the picture are traced.
What is a shadow ray?
A ray sent from a point on a surface towards a light. If another object blocks it, the point is in shadow from that light.
Who first used ray tracing for lighting effects?
The PBR book says Turner Whitted's 1980 paper introduced ray tracing for global lighting effects, producing images unlike any seen before, often of mirrors and glass balls.
What did Dürer have to do with it?
His 1525 manual on measurement includes a woodcut of a draughtsman plotting a lute with a taut thread from a point on the wall, recording where it crosses a frame, one point at a time.
Sources
- Pharr, Jakob and Humphreys, Physically Based Rendering (4th ed.): A brief history of physically based rendering
- Physically Based Rendering (4th ed.): Photorealistic rendering and the ray-tracing algorithm
- Scratchapixel: The raytracing algorithm in a nutshell
- The Fitzwilliam Museum: Dürer, Draughtsman drawing a lute (woodcut, 1525)
- University of Melbourne Archives and Special Collections: Finding Dürer's Perspective
Updated