Uni 4 Graficacion

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GraficacinUnidad IV. Iluminacin y Sombreado


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Iluminacin y Sombreado

Projections, required for displaying a three-dimensional stwo-dimensional plane or screen.

A projection is a special type of mapping from the three-dispace to a plane.

In this sense, a projection describes only where a point orhas to be drawn on the projection plane.


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The information where an object should be drawn on the plane, i.e., which pixels are covered by the object, is sufficient for a realistic representation of a three-dimension

Figure 8.1 shows the projections of a grey sphere and a g

both in two variants.

The first variant simply assigns the colour of the sphere anddirectly to the pixels that are occupied by the corresponding


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This leads to geometric shapes with a homogeneous colalmost the complete information about the three-distructure. The projection of the sphere is a grey circle,becomes a grey hexagon.


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Taking illumination and light reflections into account leads tlight effects on the surfaces of the three-dimensional objecnonhomogeneous shading of their projections.

In this way, even the flat images appear vivid and provid

dimensional impression as can be seen in figure 8.1

Shadingrefers to rendering an objectssurface with illuminlight reflection effects.


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4.1 Relleno de polgonos


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4.2 Modelos bsicos de iluminacin


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Light sources

In addition to information about the objects and the vdescription of a three-dimensional scene must alsinformation about illumination of the scene.

A single light source or a number of light sources can con

the illumination of a scene.

The colour and intensity of a light source are defined by suivalues.


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Light sources

The simplest form of light is ambient light.

Ambient light does not come from a specific light source adirection.

It represents the light that is more or less everywhere in originating from multiple reflections of light at various surfa


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Light sources

In a room with a lamp on a table, it will not be completely dthe table although the lamp cannot shed its light directly table.

The light is reflected by the surface of the table, the walls,

and the floor.

Ambient light is a simplification of the computations for illum


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Light sources

The correct way to take ambient light into account would bthe multiple reflections of the light completely. This woulthe computational effort enormously, so that this appro(yet) well suited for real-time computer graphics.

For ambient light it is sufficient to specify its colour.

A directional light source has in addition to a colour also a d


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Light sources

The light rays from a directional light source are parallel.

Directional light is used to model light coming from a sourceinfinite distance, for instance sunlight.

A lamp is modelled as a point light source. A point light soposition and the light rays spread in all directions from this p


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Light sources

The intensity of the light decreases with increasing disteffect is called attenuation.

The following argument shows that the intensity of decreases quadratically with the distance to the light source

If a point light source is in the centre of a sphere with radthe full energy of the light will be distributed equally on theof the surface of the sphere.


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Light sources

If the sphere is replaced by a bigger sphere with radius R, thenergy of the light will not change. But it is now distributed surface.

The ratio of the surfaces of the two spheres is


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Light sources

For a ratio of r/R = 1/2 each point on the inner part of the the larger sphere receives therefore only one quarter of thea point on the inner part of the surface of the smaller spher

The theoretical model for attenuation would then be to m

intensity of the light from a point light source by t1/2when it hits the surface of an object at distanced tosource.


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Light sources

The decrease of the intensity caused by attenuation is modgeneral quadratic polynomial in the denominator in the form

where the constants c1, c2, c3 can be chosen individually

point light source, d is the distance of an object to the light

This formula guarantees that the intensity can never exceed1.


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Light sources

The quadratic decrease of the light intensity comes distribution of the energy of the light onto a larger surfaincreasing distance.

In addition, part of the light is absorbed by dust particles

causing atmospheric opacity.

This leads obviously to a linear decrease of the inteincreasing distance.


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Light sources

Another common light source are spotlights (proyectores). to a point light source, a spotlight has a direction in whichits light in the form of a cone.

A spotlight is characterised by the colour of its light, its loc

direction in which it shines and an angular limit that desextension of the cone of light.


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Light sources

Attenuation is computed for a spotlight on the basis of equin the same way as for point light sources.

The quadratic decrease of the intensity with increasing disalso be deduced from figure 8.2 where it can be seen th

energy of the light from the spotlight is distributed ovewhose radius growth is linear with the distance.

Therefore, the surface grows quadratically with the distance


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Light sources

As

For a more realistic model of a spotlight, it should be account that the intensity of the light is smaller close to theof the cone of light than at the centre.


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Light sources

Then the intensity of light at the point on the surface cominspotlight is computed in the Warn model by

I = IS fatt (cos )

[Eq. 8.2]

ISis the intensity of the spotlight,fattis the distance-dependfor attenuation as in equation (8.1) and is the angle betw

IS. The value p controls how much the spotlight is focussed.

For p = 0 the spotlight behaves in the same way as a source.


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Light sources

The larger p is chosen, the more the light concentrated aaxis of the cone and the smaller is the intensity at the bothe cone.

The cosine in equation (8.2) can be computed as the dot p

the vectors land lS

if they are normalised, i.e., if both of tthe length one.


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Light sources

The


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4.3 Tcnicas de sombreado

To make objects appear to have more volume, it can hshading, i.e., the surface is paintedwith light.


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Diffuse Shading

Many objects in the world have a surface appearancdescribed as matte, indicating that the object is not atExamples include paper and unfinished wood.

Such objects do not have a color change with a change in vi

Such matte objects can be considered as behaving as Lobjects.


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Lambertian Shading Model

Ssdsd


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Thus, the color on the surface will vary according to the coangle between the surface normal and the light direction.

Note that the vector l is typically assumed not to depelocation of the object. That assumption is equivalent to asslight is distantrelative to object size.

Such a distant light is often called a directional light, bposition is specified only by a direction.


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A surface can be made lighter or darker by changing the ithe light source or the reflectance of the surface.

The diffuse reflectance cr is the fraction of light reflectsurface. This fraction will be different for different color com

For example, a surface is red if it reflects a higher fractincident light than blue incident light.


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If we assume surface color is proportional to the light reflecsurface, then the diffuse reflectance cr( an RGB color ) muincluded:

The right-hand side of Equation (10.1) is an RGB color wicomponents in the range[0,1].


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We would like to add the effects of light intensity while keRGB components in the range [0,1].

This suggests adding an RGB intensity term cl which components in the range [0,1]:


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This is a very convenient form, but it can produce RGB co

for c that are outside the range[0,1], because the dot prodnegative.

The max function can be added to Equation (10.2) to tecase:


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Another way to deal with the negative light is to use a

value:


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Ambient Shading

One problem with the diffuse shading of Equation (10.3) i

point whose normal faces away from the light will be black.

In real life, light is reflected all over, and some light is incievery direction.

In addition, there is often skylight giving ambientlighting.

Libro: Fundamentals Computer Graphics, pp 235, pdf, 3ed, Peter Shirley


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Ambient Shading

One way to handle this is to use several light sources.

Another way is to use two-sided lighting as described by(10.4).

A more common approach is to add an ambient term 1971).



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Ambient Shading

This is just a constant color term added to Equation (10.3):

c = cr( ca+ clmax (0, n l ) )

Intuitively, you can think of the ambient color caas the avera

all surfaces in the scene.



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Phong Shading

Some surfaces are essentially like matte surfaces, but

highlights

Examples of such surfaces include polished tile flwhiteboards.

Highlights move across a surface as the viewpoint moves.



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Phong Shading

This means that we must add a unit vector e toward the eye

equations.

If you look carefully at highlights, you will see that they are reflections of the light.

The color of these highlights is the color of the light



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Phong Shading

Nosotros buscamos agregar un brillo difuso del mismo color

fuente de luz en el sitio correcto.

El centro del brillo debe colocarse donde la direccin del veunitarioe se ponga en lnea con la direccin del reflejo reprpor el vector unitario r.

Alinear los vectores ry e, equivale a que el ngulo entre ryrepresentado por sea cero.


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Phong Shading


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Referencias

Introduction to computer graphics using Java 2D and 3D, K

Frank, Springer, 2008, pp 216

Libro: Fundamentals Computer Graphics, pp 235, pdf, 3ed, PShirley

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