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Graphics II Syllabus

Selection and Picking

Session 1

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Course Website:

www.cs.uml.edu/~hmasterm/graphics2.html

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Parametric Surfaces

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Shadowing Techniques

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Rendered with Gouraud Shading

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Rendered with Ray Tracing

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Rendered with Radiosity

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Environment Mapping

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Realistic Materials

Image: R. Rost: OpenGL Shading Language, Addison Wesley

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Diffraction and Chromatic Aberration

Image: R. Rost: OpenGL Shading Language, Addison Wesley

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Noise Textures

Image: R. Rost: OpenGL Shading Language, Addison Wesley

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Volume Rendering

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Volume Rendering:Medical Application

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Image Based Rendering

View Morph Demo:

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Simple Legged AnimationAnkle Rotation Script

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Simple Legged AnimationResulting Animation

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Nonlinear Global DeformationAn Example

Undeformed

Taper

Twist

Bend

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Factor Curves:An Example: the Animated Spoon

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PICKING

Direct Manipulation Demo

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Computer GraphicsConceptual Model

ApplicationModel

ApplicationProgram Graphics

System

OutputDevices

InputDevices

API

Function Callsor Protocol

Data

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Interaction with Graphics System

ChangeImage

Reactto

Change

Graphics System User

InputDevice

Display

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Logical Input Devices

0 String- Provides ASCII strings to user program- Usually implemented as physical keyboard

0 Locator- Inputs position in defined coordinate system- Absolute vs. relative- Direct vs. indirect

0 Choice- Choice among a discrete number of options- Implemented by menus or control button widget

0 Valuator (or Dial)- Provides analog input to user program- Bounded vs. unbounded- Often implemented as slidebar widget

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Logical Input Devices, contd.

0 Pick - Returns identifier of an object to user program- Can be implemented by locator and choice

0 Stroke- Returns an array of locations- Can be implemented by locator and choice

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Event Mode Input

EventQueue Program

Await

Event

TriggerProcess

MeasureProcess

Trigger Event:Device & Measure

Events

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OpenGL Geometric Pipline

ModelviewTransformation

Vertex Coordinatesx, y, z, w

Object Coordinates

Eye Coordinates

Clip Coordinates

Device Coordinates

Window Coordinates x,y

ProjectionTransformation

PerspectiveDivision

ViewportTransformation

Modelview Matrix

Projection Matrix

Part of OpenGL State

ViewportTransformation

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Object Picking by Ray Tracing

p1

COP

p2

WindowCoordinates

Object 1Object 2

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Picking is supported bySelection in OpenGL

z

x

y

Yellow Cube

1

zmin

zmax

Yellow Cube

Name Stack

Select Buffer

# Names on Stack

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OpenGL Rendering Mode

glRenderMode()GL_RENDER

GL_SELECT 1

zmin

zmax

Yellow CubeglRenderMode(GL_RENDER) returns the number of hits while in SELECT mode

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Selection in OpenGL:The Basic Steps

0 Specify the array to be used for the returned hit records using: glSelectBuffer()

0 Enter the selection mode using: glRenderMode(GL_SELECT);

0 Initialize the name stack using: glInitNames();0 Define the viewing volume you want to use for selection. This

does not have to be the same as used for viewing.0 Alternately issue primitive drawing commands and

commands to manipulate the name stack so that each primitive of interest has a name assigned.

0 Exit selection mode using: glRenderMode(GL_RENDER); and process the returned selection data in the select buffer.

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Initializing a Select Buffer

Void glSelectBuffer(Glsizei size, Gluint *buffer);

You must call glSelectBuffer() beforeentering select mode.

0

1

2

size-1

32-bitunsignedintegers

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Manipulating the Name Stack

glInitNames(void); Clears the name stack so that it’s empty

glPushName(Gluint name); Pushes name onto name stack

glPopName(void); Pops one name off the top of the name stack

glLoadName(Gluint name); Replaces the value on top of the name stackNote: returns error if stack is empty. Therefore push at least one name onstack using glPushName() first.

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Interpreting the Hit Record:Contents of the Selection Buffer

num_hits

zmin

zmax

name

glRenderMode() returns the number of hits thatcan be used to index into Selection Buffer

num_hits is an unsigned integer corresponding to the number of hits before name stack is manipulated

zmin and zmax store the maximum and minimumz-buffer value values for each hit. Z ranges in [0.0, 1.0] which is normalized as a 32 bit unsignedinteger in range [0, 232-1]

name is an unsigned integer

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Selection:Code Example

120 goto 120

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Picking in OpenGL

y

xz

z

x

y

Plane z= -1

Plane z= +1

DOP

z

x

y

Perspective View Volume Canonical View Volume

Projection

Transform

Pick MatrixTransform

Screen Coordinates

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Generating a Pick Matrix

Void gluPickMatrix(GLdouble x, Gldouble y, Gldouble width, Gldouble height, Gl int viewport[4]);

x cursor x location in window coordinatesy cursor y location in window coordinateswidth width of the picking region in screen coordinatesheight height of the picking region in screen coordinatesviewport current viewport boundaries in pixel screen coordinates

Typical usage:

glMatrixMode (GL_PROJECTION);glPushMatrix();glLoadIdentity();gluPickMatrix(. . .);gluPerspective( or glOrtho, glFrustrum, etc)/* draw the scene for picking */. . .glPopMatrix();

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Picking:Code Example

120 goto 120

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Backup Slide:OpenGL: Pick Matrix Transformation

z

x

y