IMCO: Defining colors

Who Am I ?

Ricardo SAPAICO

• PhD in Computer Science, Tokyo Institute of Technology
• 10+ years of expereicne

About DXO Labs

• ~18 years
• 50 people working in France

Products

• DxO PhotoLab
  • LightRoom like
• DxO PureRaw
  • Smaller version of PhotoLab
  • Editing images quickly
  • EISA award
• Nik Collection by DxO
  • Plugin for Adobe
• DxO FilmPack

Back to the course

Why color matters ?

• Useful for quality inspection
  • Rely on color for a small object
  • Gives shape (shine color light on objects and create shadows)
• Cultural heritage
  • Paintings
  • If you take a picture of a painting, you want the photo to represents the painting well
  • Same for printing a copy of the painting
• Printing
• Computer graphics
  • Even tho not physical, you have to make sure you have the good colrs
• Photography
  • Sometimes, it's just a matter of doing the right thing to have a good picture
  • From color to B&W is not easy
  • Style Transfer IA on photography
  • Hyperspectral images

Evaluation

• Attendance
  • Each class attended, you get
    $+0.5$
• Writtent report
  • Exercises using the data we will obtain during the TPs
  • A litlle bit of investigation for solving a typical color-related probleme in cameras
  • Group of 2 people
• Participation bonus: You can get up to
  $+2.0$ in your grade if:
  • You ask me technical question by mail
  • You ask technical questions in the IMCO group (Teams)
  • You reply a technical question in the IMCO group (Teams)

Defining colors

White becomes pink with reflection

What is (not) color

• Color is NOT a property of an object
  • Once in the dark, no more color
  • Someone colorblind will see the color differently
• Color is NOT a particle
• Color is a sensation like touch
• Color is a sensation produced by a physical reality

Grass is NOT green and sky is NOT blue,
They are perceived as green and blue
They have physical properties that makes them look that way

Color is in your HEAD

How is color produced ?

• Sun: light
• Apple: object
• Human: sensor

Electromagnetic spectrum

How can you see the black if it doesn't reflect any light ?

Because there is a shape (ex: a TV)
Black isn't a color but we can perceive it

If we have a white surface and a red beam of light coming in, it would reflect the red.

Object

Surface and subsurface interaction with light

• specular
• absorption
• diffuse
• transmittance
• refraction
• scattering

To know of colors will work on this textile, you can only know it by printing hundreds of colors and extrapolate for the other ones

Color is no 'part' of an object

• Dimension, shape, material (what it's made of), volume, density, porosity

Sensor

Human vision system

Reminder
Refractive index:

$$n=\frac{c}{\nu }$$

• Cornea
  • The most significant image-forming element of the eye
  • Eye problemes, such as nearsightedness, farsightedness, astigmastism, can be attributed to it
• Lens
  • Serves the function of accomodation
  • It is layered, flexible structure that varies in index of refraction. This feature serves to reduce some of the aberrations
  • Distant object: it becomes "flatter" resulting in the decreased optical power
  • Nearby object: it becomes "fatter" thus has increased power
  • As we age, we lose flexibility. When we are about 50 years, the lens has completely lost its flexibility
• Iris + pupil
  • controls the pupil size
  • Pupil is the hole in the middle of the iris through which light passes. The pupil size is largely determined by the overall level of illumination
  • Pigmentation in the iris is what gives us color
• Retina
  • A thin layer of cells, approximately the thickness of tissue paper
  • Contains the visual system's photosensitive cells + initial signal processing and transmission "circuitry"
  • Photoreceptios: Rods and Cones
• Fovea
  • Area on the retina where we have the best spatial and color vision
• Optical nerve
  • Transfer info from the retina to the brain
  • 1 million fibers vs 130 million photoreceptors
    • There is a compression of the visual system
  • There are no photoreceptors where the nerve is
    • blind spot

Rods and cones

Cones: useful to perceive color
Rods: useful in low-light

3 types of cones:

• Long
• Medium
• Short

Fun fact
Most mammals have only blue and green cones, only primates have red

Color matching experiment

The observer adjusts the intensity of the primary colors until the results matches the test
This test was like 100 years ago

Chromaticity diagram

Spectrum Locus
Chromaticity of monochromatic light at specified wavelength

Spatial properties of Color Vision

In JPEG we have an image divided in luminance and chromatic information. Our eyes are less sensitive to chromatic infos, so JPEG subsamples then reconstructs the image

How to (re)produce color ?

• Additive process (RGB)
  • Light mixing
• Substractive process (CMY)
  • Used in printing
  • Paint mixing

Color framework

Bonus

Facts about animals

• Most fish, frogs and turtle and 3 to 5 cones
• Most mammals have retinas where rods predominate
  • Is it because the Earth was dark when first mammals appeared ?
• Nocturnal mammals like rats and mice have retinas dominated by rods (only 3 to 5% of cones)
• Snakes can see ultraviolet light

Why the eyes reflect the light ?

In the eyes, there are epithelial cells

Epithelial cells
Prevents light from going back to the retina

$\to$ keeps the sharpness and contrast

Nocturnal animals exchange image quality for image power. Their cells reflects the light back

$\to$ the photoreceptors have a second chance to absorb energy

Different than having red eyes !

Structural color: Iridiscence

Used for camouflage

Facts about humans

• Red-green color blindness
  • $8\mathrm{\%}$ males (XY chromosome)
  • $0.5\mathrm{\%}$ females (XX chromosome)

of northern European descent

Around

$12\mathrm{\%}$ of the female population could be tetrachromatic

To think over

• What is fluorescence ?
• Why a banana stays yellow under 2 different light sources ?