OpenGL Transformation

OpenGL Transformation

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Centre for Computational Technologies
CCTech Recruitment Brochure Simulation is The Future!

Outline
1. Transformation
2. Viewing Transformation
3. Projection Matrix
4. OpenGL Transformation Pipeline

OpenGL Simulation is The Future!

Geometric Objects and Operations
• Primitive types: scalars, vectors, points
• Primitive operations: dot product, cross product
• Representations: coordinate systems, frames
• Implementations: matrices, homogeneous coor.
• Transformations: rotation, scaling, translation
• Composition of transformations
• OpenGL transformation matrices


Current Transformation Matrix
• Model-view matrix (usually affine)
• Projection matrix (usually not affine)

• Manipulated separately
glMatrixMode (GL_MODELVIEW);
glMatrixMode (GL_PROJECTION);


Manipulating the Current Matrix
• Load or postmultiply
glLoadIdentity();
glLoadMatrixf(*m);
glMultMatrixf(*m);
• Library functions to compute matrices
glTranslatef (dx, dy, dz);
glRotatef (angle, vx, vy, vz);
glScalef (sx, sy, sz);
• Recall: last transformation is applied first!


Transformation Matrices in OpenGL
• Transformation matrices in OpenGl are vectors of 16 values
(column-major matrices)
m = {m1, m2, ..., m16} represents

• In glLoadMatrixf(GLfloat *m);
• Some books transpose all matrices!


Camera in Modeling Coordinates
• Camera position is identified with a frame
• Either move and rotate the objects
• Or move and rotate the camera
• Initially, pointing in negative z-direction
• Initially, camera at origin


Moving Camera and World Frame
• Move world frame relative to camera frame
• glTranslatef(0.0, 0.0, -d); moves world frame


Order of Viewing Transformations
• Think of moving the world frame
• Viewing transfn. is inverse of object transfn.
• Order opposite to object transformations

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0, 0.0, -d); /*T*/
glRotatef(-90.0, 0.0, 1.0, 0.0); /*R*/


Viewing Functions
• Roll (about z), pitch (about x), yaw (about y)


Viewing and Projection
• In OpenGL we distinguish between:
– Viewing: placing the camera
– Projection: describing the viewing frustum of the camera (and thereby
the projection transformation)
– Perspective divide: computing homogeneous points


OpenGL Transformations
• The viewing transformation V transforms a point from world
space to eye space:


Placing the Camera
• It is most natural to position the camera in world space as if it
were a real camera
• Identify the eye point where the camera is located
• Identify the look-at point that we wish to appear in the center
of our view
• Identify an up-vector vector that we wish to be oriented
upwards in our final image


Look-At Positioning
• We specify the view frame using the look-at vector a and the
camera up vector up
• The vector a points in the negative viewing direction

• In 3D, we need a third vector that is perpendicular to both up
and a to specify the view frame

Constructing a Frame
• The cross product between the up and the look-at vector a
will get a vector that points to the right.

• Finally, using the vector a and the vector r we can synthesize
a new vector u in the up direction:


gluLookAt()
• OpenGL provides a very helpful utility function that
implements the look-at viewing specification:

gluLookAt ( eyex, eyey, eyez, // eye point
atx, aty, atz, // lookat point
upx, upy, upz ); // up vector

• These parameters are expressed in world coordinates


• The projection transformation P transforms a point from eye
space to clip space:


Projection Transformations
• Projections fall into two categories:
– Parallel projections: Lines of projection are parallel to each other
– Perspective projections: Lines of projection converge at a point


Parallel Projections
• The simplest form of parallel projection is simply along lines
parallel to the z-axis onto the xy-plane
• This form of projection is called orthographic


Orthographic Frustum
• The user specifies the orthographic viewing frustum by
specifying minimum and maximum x/y coordinates
• It is necessary to indicate a range of distances along the z-
axis by specifying near and far planes


Orthographic Projection in OpenGL
• This matrix is constructed with the following OpenGL call:

glOrtho(left, right, bottom, top, near, far);

• And the 2D version (another GL utility function):

gluOrtho2D(left, right, bottom, top);

– Just a call to glOrtho() with near = -1 and far = +1


Properties of Parallel Projections
• Not realistic looking
• Good for exact measurements
• A kind of affine transformation
– Parallel lines remain parallel
– Ratios are preserved
– Angles (in general) not preserved
• Most often used in CAD, architectural drawings, etc., where
taking exact measurement is important


Isometric Games
• A special kind of parallel projection called isometric
projection is often used in games
• It’s essentially a shear and an orthographic projection
• Easier to compute than a full perspective transformation


Perspective Projections
• Artists (Donatello, Brunelleschi, and Da Vinci) during the
renaissance discovered the importance of perspective for
making images appear realistic
• Parallel lines intersect at a point


Perspective Viewing Frustum
• Just as in the orthographic case, we specify a perspective
viewing frustum

• Values for left, right, top, and bottom are specified at the
near depth


Perspective Viewing Frustum
• OpenGL provides a function to set up this perspective
transformation:

glFrustum(left, right, bottom, top, near, far);

• There is also a simpler OpenGL utility function:

gluPerspective(fov, aspect, near, far);

– fov = vertical field of view in degrees
– aspect = image width / height at near depth
• Can only specify symmetric viewing frustums where the
viewing window is centered around the –z axis.

gluPerspective()
• Here are the parameters of gluPerspective()


Properties of Perspective Projections
• The perspective projection is an example of a projective
transformation
• Here are some properties of projective transformations:
– Lines map to lines
– Parallel lines do not necessarily remain parallel
– Ratios are not preserved
• One of the advantages of perspective projection is that size
varies inversely with distance – looks realistic
• A disadvantage is that we can't judge distances as exactly
as we can with parallel projections


• The rest of the OpenGL transformation pipeline:


Clipping & Perspective Division
• The scene's objects are clipped against
the clip space bounding box
• This step eliminates any objects (and
pieces of objects) that are not visible in
the image
• Hill describes an efficient clipping
algorithm for homogeneous clip space
• Perspective division divides all
homogeneous coordinates through w
• Clip space becomes Normalized Device
Coordinate (NDC) space after the
perspective division


Viewport Transformation
• OpenGL provides a function to set up the viewport
transformation:

glViewport(x, y, width, height);


Screen Coordinate Systems


3D Graphics Pipeline
• The rasterization step scan converts the object into pixels


3D Graphics Pipeline
• A z-buffer depth test resolves visibility of the objects on a
per-pixel basis and writes the pixels to the frame buffer


Thank You

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Visit Us @ www.cctech.co.in
@ www.learncax.com
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OpenGL Transformation

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