Question #1

Graded by: Pat Hanrahan

An important factor when viewing images on a monitor is the monitor's gamma. For the purposes of this question, assume the monitor as a gamma of 2.

A. (5 points) Suppose no gamma correction is being performed, and the following two images are shown side by side:

These images have been zoomed, the actual images are displaed so that each square represents a single pixel. A white square indicates a pixel value of 255, a black square a value of 0, and a grey square a value of 128. Which image - the left or the right - will appear brighter overall? Explain your reasoning.

Answer: The right image.

White pixels are 2x the value of gray, but since gamma=2 they are 4x as bright. The checkerboard is 1/2 black + 1/2 white = 1/2*0 + 1/2*4 = 2. They grey image has a relative brightness of 1.

B. (5 points) Suppose a monitor has been gamma-corrected. What does that mean?

Answer: Gamma correction is used to compensate for non-linearities in the display process on a monitor. The result is that the output light intensity is linearly related to the input framebuffer values.

C. (5 points) What are the advantages of using a gamma correction table? What are the disadvantages?


Advantages: Gamma correction allows the programmer to operate in a linear intensity space. This linear space is more intuitive than a non-linear space. Linearity is also required for anti-aliasing and compositing. Hardware tables allow gamma correction to be efficiently performed in real-time.

Disadvantages: Most gamma correction tables have limited precision, and hence multiple input values may map to the same output, and vice-versa. Linear color space is also not perceptually uniform.

Copyright © 1996 Pat Hanrahan