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Kees van Overveld 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 -1-

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Kees van Overveld From light to Enlightenment – the 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

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Kees van Overveld ii riri d2Sd2S uu ll b v d2Rd2R retina plane pupil plane optical axis P the illumination equation -3- From light to Enlightenment – the physic of light

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Kees van Overveld ii riri d2Sd2S uu ll b v d2Rd2R retina plane pupil plane optical axis P the illumination equation d 2 E retina = P cos i cos 4 l A P d 2 R / 32 3 r i 2 b 2 cos u -4- From light to Enlightenment – the physic of light

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Kees van Overveld perceived light intensity: proportional to cos 4 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, A P no symmetry between source and detector properties of the illumination equation -5- From light to Enlightenment – the physic of light

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Kees van Overveld proportional to cos 4 l difficult to get uniform sensitivity for wide viewing angles (fish-eye lenses; endoscopes) -6- From light to Enlightenment – the physic of light

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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 : i u (condition for symmetrical condition doesn't hold) = 0: dull (Lambert surface) = : perfect (Snellius) mirror -7- From light to Enlightenment – the physic of light

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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 From light to Enlightenment – the physic of light

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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) -9- From light to Enlightenment – the physic of light

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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. -10- From light to Enlightenment – the physic of light

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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- From light to Enlightenment – the physic of light

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Kees van Overveld dependency of 1/r 2 -12- A single point source: light field is dominated by 1/r 2 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 – the physic of light

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Kees van Overveld no dependency of 1/v 2 -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 From light to Enlightenment – the physic of light

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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 From light to Enlightenment – the physic of light

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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:: L scatterred ( ) = L 0 -4 -15- From light to Enlightenment – the physic of light

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Kees van Overveld absorption and scattering -16- From light to Enlightenment – the physic of light

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Kees van Overveld absorption and scattering -17- 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 – the physic of light

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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 -18- From light to Enlightenment – the physic of light

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Kees van Overveld perspectief -19- By why is there a pojection centre? From light to Enlightenment – the physic of light

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Kees van Overveld Classical perspectivef: (Italian renaissance, Brunelleschi (1377-1446)): horizon, lines lines, points points, hence: intersections intersections Properties of central projection perspective -20- From light to Enlightenment – the physic of light

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Kees van Overveld The development in perspective in pictorial art perspective -21- Egyptian art has used more or less the same style for 30 centuries; no need for rendering of geometric perspective From light to Enlightenment – the physic of light

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Kees van Overveld The development in perspective in pictorial art perspective -22- From light to Enlightenment – the physic of light http://www.ottobw.dds.nl/filosofie/perspect.htm 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)

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Kees van Overveld perspective -23- From light to Enlightenment – the physic of light The development in perspective in pictorial art Byzantine pictorial art: inverted’ perspective for religious reasons

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Kees van Overveld perspectief -24- From light to Enlightenment – the 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 master of Flemalle (Merode altaarstuk – ca. 1427)

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Kees van Overveld perspectief -25- 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 – the physic of light The development in perspective in pictorial art http://en.wikipedia.org/wiki/Holy_Trinity_(Masaccio)

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Kees van Overveld perspectief -26- 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 – the physic of light The development in perspective in pictorial art http://en.wikipedia.org/wiki/Johannes_Vermeer

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Kees van Overveld perspectief -27- From light to Enlightenment – the 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/

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Kees van Overveld perspectief -28- 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 – the physic of light The development in perspective in pictorial art http://burgessart.files.wordpress.com/2012/09/picasso3.jpg

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Kees van Overveld perspectief -29- 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 – the physic of light The development in perspective in pictorial art http://uima.uiowa.edu/giorgio-de-chirico/

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Kees van Overveld perspectief -30- Inverting perspective: creating a pictorial illusion on a 3-D background (Julian Beaver, England) From light to Enlightenment – the physic of light The development in perspective in pictorial art http://www.s-anand.net/blog/pavement-drawings-in-perspective/

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Kees van Overveld -31- Summary; essential concepts: Point source: centre where speherical waves originate; 1/r 2 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 – the physic of light

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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 – the physic of light

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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 – the physic of light

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Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Waves and Particles The two most commonly used models describe light.

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