Visualization for Machine Learning

Spring 2024

Color

Vasas et al, PLOS Biology, 2024

Visible Spectrum

Light

• Visible range: 390-700nm

• Luminance has a large dynamic range:

  – 0.00003 – Moonless overcast night sky

  – 30 – Sky on overcast day

  – 3000 – Sky on clear day

  – 16,000 – Snowy ground in full sunlight

• Colors result from spectral curves

  – dominant wavelength, hue

  – brightness, lightness

  – purity, saturation

Physiology of the Eye

The Retina

Photoreceptors

• Discrete sensors that measure energy – Adaptation
• Rods
  • active at low light levels (scotopic vision)
  • only one wavelength-sensitivity function
• Cones
  • active at normal light levels (photoptic)
  • three types: sensitivity functions with different peaks

Cone Sensitivity

Density of Cones

Cones and Rods

Color Stimulus

Color Matching Experiments

Color in Visualization

• Trichromacy: Humans perceive colors according to three channels

• Most usable and useful way to describe colors (especially for visualization):

  • Hue, Saturation, Luminance

How do we use color in visualization?

• Quantify

• Label

Quantify

Label

Quantitative Color Scales

• Desired Properties of Quantitative Color Scale:
  • Uniformity (value difference = perceived difference)
  • Discriminability (as many distinct values as possible)

Quantitative Color Scales

• Desired Properties of Quantitative Color Scale:
  • Uniformity (value difference = perceived difference)
  • Discriminability (as many distinct values as possible)

Single Hue Sequential Scales

• Choose one hue
• Map value to luminance

Categorical Color Scales

• Properties:
  • Uniformity (uniform saliency / nothing stands out)
  • Discriminability (as many distinct values as possible)

Categorical Color Scales

• How many distinct values can one perceive?

• how many can you use in a visualization?

• Estimates are between 5-10 distinct codes.

• Healey, Christopher G. “Choosing effective colours for data visualization.” Proceedings of IEEE Visualization’96, 1996.

Diverging Color Scales

• Sometime useful/necessary to distinguish values above and below a threshold.

Created using these data: link

Color Blindness

• Missing or defective photoreceptors:
  • 10% male and 1% female have some color deficiencies

Oliveira, Manuel. “Towards More Accessible Visualizations for Color-Vision-Deficient Individuals.” Comput. Sci. Eng., 2013.