CSCE 441 Lecture 14

« previous | Friday, February 14, 2014 | next »

Light and Color

Parts of the Eye

• Cornea
• Iris
• Lens
• Retina
  • Rods (brightness only; most sensitive)
  • Cones (147K / mm² centered around fovea; but less in peripheral)

Image is focused on Fovea of retina

Current display systems maintain one focus plane

• Even for stereo (3D) displays
• Even if the image tries to simulate different focus planes

Intensity and Luminance

Intensity/Luminance How much light energy is carried by photos

Brightness is the interpretation of intensity

• Interpretation is logarithmic to actual intensity
• doubling luminance is a linear increase in perceived brightness

Display levels have discrete intensity levels.

Maximum intensity level of display device is 1.0

$\sum _{i=0}^{n}r^{i}\,I_{i}=1.0$

Human eye can discern between $r=1.01$ difference in brightness levels.

$n=-\log {(1.01\,I_{0})}$

Gamma Correction

$I_{out}=k\,I_{in}\,\gamma$

$\gamma$ and $k$ are device-specific terms

$\gamma \in \left[2.0,2.5\right]$ (usually 2.2)

Color space intensity is non-linear, so we have to convert to linear space and then convert back

Gamma meant to model CRTs only

LCDs have very different response curves that do not match the gamma curve

Dynamic Range

${\frac {1}{I_{0}}}$ is called dynamic range

ratio of maximum to minimum intensity (remember 1.0 is max)

varies by device

Contrast

Maxumum vs. minimum that can be displayed (in adjacent pixels) at the same time.

Limited Display Levels

Printing: either ink is there or not there (we can adjust the amount of ink, but not intensity)