kees van overveld from light to enlightenment the physical layer origin and nature of light light as...

Kees van Overveld

From light to EnlightenmentFrom light to EnlightenmentThe physical layer•origin and nature of light

•light as particles

•light as waves

•light as energy

•the illumination equation

•absorption and scattering; color perspective

•the spatial behavior of light

•refraction and lenses

•the perspective projection

•summary

-1-

Kees van Overveld

From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light

• Radiation of 'black(=non-reflecting)' body: a continuous distribution of energy over , only depending on the temperature of the body

occupied states

electron in excited state

available states

electron falls back to lower energy state; energy surplus E is emitted as light with wavelength =hc/E

origin and nature of light

-2-

• Energy transitions in atoms between discrete electron states cause light-quanta (photons) with distinct

Kees van Overveld

i

ri

d2S

u

l b

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d2R

retina plane

pupil plane

optical axis

P

the illumination equation

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Kees van Overveld

i

ri

d2S

u

l b

v

d2R

retina plane

pupil plane

optical axis

P

the illumination equation

dd22EEretinaretina==P cos P cos i i coscos44ll A APP d d22R / 32 R / 32 33rrii22bb22 cos cos uudd22EEretinaretina==P cos P cos i i coscos44ll A APP d d22R / 32 R / 32 33rrii22bb22 cos cos uu

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Kees van Overveld

perceived light intensity:

proportional to cos4 l : at 45 degrees, mere 25% left

interpretation of : difference between dull and shiny (among other things)

proportional to i : plasticity interpreted as relief

the case where u =i : full moon

proportional to 1/cos u : bright silhouettes

no v-dependency

r – dependency: clair obscur or impressionism

the role of the pupil size, AP

no symmetry between source and detector

properties of the illumination equation

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Kees van Overveld

proportional to cos4 l

difficult to get uniform sensitivity for wide viewing angles (fish-eye lenses; endoscopes)

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Kees van Overveld

interpretation of

simple empiric bahavior of

(Phong-shading: computer graphics (1973))

=

=cos(angle between normal vector and halfway-direction)

halfway-direction = direction between incoming and reflected ray

=1: i=u (condition for symmetrical reflection)

<1: iu (condition for symmetrical condition doesn't hold)

= 0: dull (Lambert surface)

= : perfect (Snellius) mirror-7-


Kees van Overveld

interpretation of

-8-

may not only depend on the angle between incoming and outging light ray and the surface normal, but also on their directions: anisotropic reflection

color differences in a surface are often caused by varying spectral dependencies of

local variations in shininess are caused by the behavior of in dependence of reflection angles

demo


http://www.youtube.com/watch?v=hDAcp9bStRQ&NR=1

Kees van Overveld

proportional to i

if possible, the HVS gives an interpretation to brightness differences in terms of variations of i , and hence as relief (height modulations)

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Kees van Overveld

the case where u =i

in every point of the full moon, the viewing direction and the direction of the incoming rays are (almost) equal. A Lambertian-surface then gives uniform brightness.

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Kees van Overveld

proportional to 1/cos u

there are brighter zones near silhouettes of shiny surfaces. Difficult to perceive:

•high shinyness: reflection of the surrounding world interferes

•low shinyness: (u) is close to zero near the silhouette -11-


Kees van Overveld

dependency of 1/r2

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A single point source: light field is dominated by 1/r2 behavior: dramatic clair-obscur, characteristic of 17th centure indoor scenes (Rembrandt, Caravaggio).

Homogenous distribution of point sources (e.g. due to atmospheric scattering): outdoor light gives no clair obscur. Characteristic of many impressionistic landscapes.


Kees van Overveld

no dependency of 1/v2

-13-

if the illumination equation would behave symmetrically in r and v, remote surfaces would appear darker

remote and nearby surfaces with equal and equal orientation w.r.t. light source, however, apear equally bright


Kees van Overveld

the size of the pupil

-14-

small pupil:

•lower intensity

•higher aquity

•sharp over large depth range

large pupil:

•higher intensity

•lower aquity

•sharpness drops sharply over limited depth range


Kees van Overveld

absorption and scattering

Absorption: if a layer of thickness h absorbs a fraction K (K=K(); K<1) of the light intensity, intensity becomes a function of propagation distance x:

L (x)= L 0 exp (-Kx/h)

Scattering: Einstein gave a derivation of the empirical Tyndall formula::

Lscatterred () = L 0 -4

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Kees van Overveld


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Kees van Overveld


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Leonardo da Vinci’s ‘The virgin on the rocks' is an early example of the deliberate use of atmospheric perspective in pictorial art


Kees van Overveld

Geometric properties of light rays:

1. conservation of direction

2. eventually any non-parallel beam will diverge

3. mapping a point in 3-space onto a point in a (2D) image is a central projection (i.e., a projection whereby projecting rays all pass through a so-called projecting centre)

perspective

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Kees van Overveld

perspectief

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By why is there a pojection centre?


Kees van Overveld

Classical perspectivef: (Italian renaissance, Brunelleschi (1377-1446)): •horizon, •lines lines, •points points, hence:•intersections intersections

Properties of central projection

perspective

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Kees van Overveld

The development in perspective in pictorial art

perspective

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Egyptian art has used more or less the same style for 30 centuries; no need for rendering of geometric perspective


Kees van Overveld


perspective

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Classical Greek art had partial understanding of projection and the geometry of 3D (Euclid) – in particular of ‘things’ that were small enough so that no visible size reduction occurs (=so called isometric perspective: parallel lines stay parallel)

Kees van Overveld

perspective

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Byzantine pictorial art: inverted’ perspective for religious

reasons

Kees van Overveld

perspectief

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Late Gothic art:Limited success in depicting geometric

perspective. The principle “one painting = one viewpoint”had not yet been discovered

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Kees van Overveld

perspectief

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Break through: Massacio applied single (vanishing-) point perspective (early Italian Renaissance).

The concept worked in virtue of the known location and gazing direction of the viewer



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Kees van Overveld

perspectief

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Full control of multi-vanishing point perspective of Dutch masters in 17th century (pioneered by Simon stevin, although theoretical underpinning had to wait until the 19th century: Gaspard Monge, projective geometry).



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Kees van Overveld

perspectief

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.. But is photo realistic perspective convincing?

Sometimes the eye wants to be deceived

.. But is photo realistic perspective convincing?

Sometimes the eye wants to be deceived

http://mrl.nyu.edu/projects/npr/mpr/

Kees van Overveld

perspectief

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Early 20th century: cubism - dropping the assumption of a single view point per painting

Shifting the responsibility from the painter to the viewer

Looking = sampling, i.e. a dynamic, attention-driven process



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Kees van Overveld

perspectief

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The Chirico (and others) deliberately used ‘wrong’ perspective for pictorial purposes, manipulating the view and creating an eery, dream-like atmosphere



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Kees van Overveld

perspectief

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Inverting perspective: creating a pictorial illusion on a 3-D background (Julian Beaver, England)



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Kees van Overveld -31-

Summary; essential concepts:•Point source: centre where speherical waves originate; 1/r2

behavior of intensity relative to the point source

•Power: energy per time interval

•Intensity: light power per surface area

•Radiance: transported light power per surface area per solid angle

•Irradiance: received or emitted light power per surface area

•Spectrum: distribution of light energy over the wavelengths (continuous or discrete)



•Reflection: interaction of light with a surface•Diffuse reflection (Lambert): BDR is more or less constant

•Mirroring: reflection where BDR only differs from 0 when incoming and outgoing rays have about the same angel with normal

•Scattering: interaction of light with a spatial medium where light rays no longer are straight lines

•Dispersion : velocity of light, and therefore refraction index varies in dependence of wavelength

•Diffraction and interference: deviation from the straight line-behavior of light rays due to their wave character

•Absortpion: decrease of light power due to reflection or passage through a medium



•Pupil: centre of perspective projection, where all light rays have to

pass

•Collimator: enforces light rays from different directions to fall onto

different sensory cells

•Perspective: transformation from 3D to 2D where distances are

represented by angles

•Straight lines and points stay straight lines and points due to

perspective

•Parallellism in 3D: coïncidence in perspective image

•Distances and angles are not preserved

•Vanishing point: limiting case for the projection of a point that, in

3D, moves along a straight line towards infinity

•Horizon: collection of vanishing points of all 3D directions parallel

to the ground plane


kees van overveld from light to enlightenment the physical layer origin and nature of light light as...

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