from light to enlightenment
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From light to Enlightenment. The 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. - PowerPoint PPT PresentationTRANSCRIPT
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
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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
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• Energy transitions in atoms between discrete electron states cause light-quanta (photons) with distinct
Kees van Overveld
i
ri
d2S
u
l b
v
d2R
retina plane
pupil plane
optical axis
P
the illumination equation
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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
proportional to cos4 l
difficult to get uniform sensitivity for wide viewing angles (fish-eye lenses; endoscopes)
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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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-
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
interpretation of
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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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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-
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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.
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
no dependency of 1/v2
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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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
the size of the pupil
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small pupil:
•lower intensity
•higher aquity
•sharp over large depth range
large pupil:
•higher intensity
•lower aquity
•sharpness drops sharply over limited depth range
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
absorption and scattering
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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
absorption and scattering
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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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
perspectief
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By why is there a pojection centre?
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld
The development in perspective in pictorial art
perspective
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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
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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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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
Byzantine pictorial art: inverted’ perspective for religious
reasons
Kees van Overveld
perspectief
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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
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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).
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
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Kees van Overveld
perspectief
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From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
.. 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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
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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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
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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)
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
The development in perspective in pictorial art
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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)
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld -32-
•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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light
Kees van Overveld -33-
•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
From light to Enlightenment – From light to Enlightenment – the physic of lightthe physic of light