visual realism in geometric modeling

Visual Realism In Geometric Modeling
Seminar topic
by
A. Sabiha khathun (16481D1501)
M. Tech (Machine Design-2/1)
Subject : Geometric Modeling
Faculty: Dr . A.Jawahar babu (Professor & HOD)
Department of Mechanical Engineering
Gudlavalleru Engineering College

Introduction
 Visual Realism is a method for interpreting picture data fed
into a computer and for creating pictures from difficult
multidimensional data sets.
 Visualization in geometric modeling is related to displaying
geometric models of object

Continued..
 What is realistic image?
 A picture that capture many of the effect of light interacting
with real physical object.
 the ultimate goal of picture is to convey information, then a
picture that is free of the complications of the shadows and
reflections.

Continued..
Creating a realistic pictures involves following stages:
1) Model of the object.
2) Viewing specifications and lighting conditions,
3) Visible surface determination.
4) Color determination of each pixel
5) Animated sequence

Application of realistic pictures
1. Simulation
2. Design of 3D object such as automobiles, airplanes etc.
3. Entertainment and advertisement
4. Research and education
5. Command and control

Classification of Visualization
 Visualization in geometric modeling- Shading the parts
with various shadows, colors and transparency
 Visualization in scientific computing- It changes the data
in numerical form into picture display,

Difficulties in visual realism
 Total visual realism is the complexity of the real world.
 Achieved sub goal of realism: to provide sufficient
information to let the viewer understand the 3D spatial
relationships among several objects.
 Most display devices are 2D therefore, 3D objects must be
projected into 2D, with considerable attendant loss of
information which can sometimes create ambiguities in the
image.

 Hidden line and surface removal methods remove the
uncertainty of the displays of 3D models and is accepted the
first step towards visual realism.
 Shaded images can only be created for surface and solid
models.
(A) (B) (C)

Rendering techniques for Realistic Images
 Hidden line and hidden surface algorithms have been
classified as object space methods, image space methods or
combination of both.
 Image space methods algorithms 1.Raster
2.Vector
 Object space algorithm uses the spatial and geometrical
relationships among the objects.

Existing visualization methods are:
1) Parallel projections
2) Perspective projection.
3) Hidden line removal
4) Hidden surface removal
5) Hidden solid removal
6) Shaded models

1. Parallel projections
• All the projection lines are orthogonal to the projection plane.

2.Perspective projections
 An object’s size is scaled in inverse proportion to its distance
from viewer.
 Cube example
Perspective projection of a cube

 Determine visible lines. Only surfaces bounded by lines
can obscure other lines. So objects that are to block
others must be modeled either as collections of surfaces
or as solids.
 Hidden-line removed views convey less depth
information so instead of removing that can be shown as
dashed lines.
3.Visible –line Determination

Priority algorithm
ABCD, ADFG, DCEF are given
higher priority-1. Hence, all lines in
this faces are visible, i.e.; AB, BC,
CD, DA, AD, DF, FG, AG, DC, CE,
EF and DF are visible.

AGHB, EFGH, BCEH are given lower
priority-2. Hence, all lines in this faces
other than priority-1 are invisible, that is
BH, EH and GH. These lines must be
eliminated.

4. Visible-Surface Determination
 Hidden surface removal
 Displaying only those parts of surfaces that are visible to
the viewer.
 If surfaces are rendered as opaque areas, then visible-
surface determination is essential for the picture to make
sense.

1. Z - buffer algorithm
when image being drawn, if its z coordinate at
a position is higher than z buffer value, it is
drawn, and new z coordinate value is stored; or
else, it is not drawn
Basic operations:
1. Compute y range of an object
2. Compute x range of a given scan line of an
object.
3. Calculate intersection point of a object with
ray through pixel position ( x, y).

3.Painters Algorithm
 The depth can be represented by the intensity of the
image. parts of the objects that are intended to appear
farther from the viewer are displayed at lower intensity,
this effect is know as depth cueing.
 Distant object appear dimmer than the closer objects

Shading
 Shading defines to describe depth perception in three
dimensioning models by different levels of darkness.
 Shading is applied in drawing for describes levels of
darkness on paper by adding media heavy densely shade
for darker regions, and less densely for lighter regions.

Continued…
 The input to a shading model is 1. Intensity and
color of light source
2. surface characteristics
3. Position & orientation of
surface and source
 Shading surface algorithms 1. Constant shading
2. Gourand shading
3. Phong shading

(a) Point light source (b) Ambient light source

1. Constant shading
 In this method a single intensity is calculated for each polygon and
all points over the surface are then displayed with the same
intensity value.

2. Gourand shading
 Linearly interpolating the intensity values across the surface of
each polygon which are matched with the values of adjacent
polygons along the common edges.

3. Phong shading
 Each vertex has its own surface normal and normal vectors are
interpolated

Transparency and reflection
 Now we consider transparent surface.
 Simple models of transparency do not include the refraction
(bending) of light through a transparent solid.
 More complex models include refraction, diffuse
transparency and the attenuation of light with respect to
distance. Also consider diffuse and specular reflection .
 It requires knowledge of other surfaces besides the surface
being shaded.

 Improved Camera Models
 So far we have consider camera model with a pinhole
lens and an infinitely fast shutter: all objects are in sharp
focus and represent the world at one instance in time.
 It is possible to model more accurately the way that we
and cameras see the world.

Other ways of realism
 Dynamics: We mean changes that spread across a sequence of
pictures, including changes in position ,size, material,
properties ,lighting and viewing specification
 Most popular kind of dynamics is motion ranging from
simple transformation to animation.
 If a series of projections of same object, from a slightly
different viewpoint around the object, is displayed in rapid
succession then the object appears to rotate.

visual realism in geometric modeling

visual realism in geometric modeling

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