presentation govardhan

8/2/2019 Presentation Govardhan

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OpenGL

The success of GL lead to OpenGL (1992), a platform-independent API that was

Easy to use Close enough to the hardware to get excellent performance Focus on rendering Omitted windowing and input to avoid window system dependencies

OpenGL Evolution

Controlled by an Architectural Review Board (ARB) Members include SGI, Microsoft, NVIDIA, HP, 3DLabs, IBM Relatively stable (present version 2.0)

Evolution reflects new hardware capabilities

3D texture mapping and texture objects

Vertex programs Allows for platform specific features through extensions

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OpenGL Libraries

OpenGL core libraryOpenGL32 on Windows, GL on most unix/linux systems (libGL.a)

OpenGL Utility Library (GLU)Provides functionality in OpenGL core but avoids having to rewrite code

Links with window system

GLX for X window systems, WGL for Windows, AGL for Macintosh

GLUT (OpenGL Utility Toolkit)

Provides functionality common to all window systems

Open a window, Get input from mouse and keyboard, Menus,

Event-driven

Code is portable but GLUT lacks the functionality of a good toolkit for a

specific platform: No slide bars



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Software Organization

GLUT

GLU

GL

GLX, AGL

or WGL

X, Win32, Mac O/S

software and/or hardware

application program

OpenGL Motifwidget or similar



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OpenGL Architecture

Immediate Mode

Display

List

Polynomial

Evaluator

Per Vertex

Operations &

PrimitiveAssembly

RasterizationPer Fragment

Operations

Texture

Memory

CPU

Pixel

Operations

Frame

Buffer

geometry

pipeline



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OpenGL Functions Primitives

Points Line Segments

Polygons

Attributes

Transformations

Viewing

Modeling Control (GLUT)

Input (GLUT)

Query

OpenGL is a state machine

OpenGL functions are of two types

Primitive generating Can cause output if primitive is visible

How vertices are processed and appearance of primitive are controlled by the state

State changing

Transformation functions

Attribute functions12/16/2010 5I.Govardhana Rao


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Object Orientation

OpenGL is not object oriented so that there are multiple functions for a given logicalfunction

glVertex3fglVertex2iglVertex3dv

Underlying storage mode is the same

Easy to create overloaded functions in C++ but issue is efficiency

OpenGL #defines

Most constants are defined in the include files gl.h, glu.h and glut.h

Note #include should automatically include the others

Examples: glBegin(GL_POLYGON)

glClear(GL_COLOR_BUFFER_BIT)include files also define OpenGL data types: GLfloat, GLdouble



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OpenGL function format

glVertex3f(x,y,z)

belongs to GL library

function name

x,y,z are floats

glVertex3fv(p)

p is a pointer to an array

dimensions



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#include

void mydisplay(){

glClear(GL_COLOR_BUFFER_BIT);

glBegin(GL_POLYGON);

glVertex2f(-0.5, -0.5);

glVertex2f(-0.5, 0.5);

glVertex2f(0.5, 0.5);

glVertex2f(0.5, -0.5);

glEnd();

glFlush();

}

int main(int argc, char** argv){glutCreateWindow("simple");

glutDisplayFunc(mydisplay);

glutMainLoop();

}

Sample Program



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#include

int main(int argc, char** argv)

{

glutInit(&argc,argv);

glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);glutInitWindowSize(500,500);

glutInitWindowPosition(0,0);

glutCreateWindow("simple");

glutDisplayFunc(mydisplay);

init();glutMainLoop();

}

Sample Program



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OpenGL Primitives

GL_QUAD_STRIPGL_QUAD_STRIP

GL_POLYGONGL_POLYGON

GL_TRIANGLE_STRIPGL_TRIANGLE_STRIP GL_TRIANGLE_FANGL_TRIANGLE_FAN

GL_POINTSGL_POINTS

GL_LINESGL_LINES

GL_LINE_LOOPGL_LINE_LOOP

GL_LINE_STRIPGL_LINE_STRIP

GL_TRIANGLESGL_TRIANGLES


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Attributes

Attributes are part of the OpenGL state and determine the appearance

of objects

Color (points, lines, polygons)

Size and width (points, lines)

Stipple pattern (lines, polygons)

Polygon mode

Display as filled: solid color or stipple patternDisplay edges

Display vertices


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RGB color

Each color component is stored separately in the frame buffer

Usually 8 bits per component in buffer

Note in glColor3f the color values range from 0.0 (none) to 1.0 (all), whereas

in glColor3ub the values range from 0 to 255


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Indexed Color

Colors are indices into tables of RGB valuesRequires less memory

indices usually 8 bits

not as important now

Memory inexpensive

Need more colors for shading

Color and State

The color as set by glColor becomes part of thestate and will be used until changed

Colors and other attributes are not part of the object but are assigned whenthe object is rendered


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Smooth Color

Default is smooth shading

OpenGL interpolates vertex colors across

visible polygons

Alternative is flat shadingColor of first vertex

determines fill color

glShadeModel

(GL_SMOOTH)

orGL_FLAT


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Viewports

Do not have use the entire window for the image:

glViewport(x,y,w,h)

Values in pixels (screen coordinates)


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Draw one triangle

void triangle( GLfloat *a,

GLfloat *b, GLfloat *c)

/* display one triangle */

{

glVertex2fv(a);

glVertex2fv(b);

glVertex2fv(c);

}

Connect bisectors of sides and remove central triangle

Repeat


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void divide_triangle(GLfloat *a, GLfloat *b, GLfloat*c, int m){/* triangle subdivision using vertex numbers */

point2 v0, v1, v2;int j;if(m>0){

for(j=0; j


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Pixel Maps

OpenGL works with rectangular arrays of pixels called

pixel maps or images

Pixels are in one byte ( 8 bit) chunks

Luminance (gray scale) images 1 byte/pixel

RGB 3 bytes/pixel

Three functions

Draw pixels: processor memory to frame buffer

Read pixels: frame buffer to processor memory Copy pixels: frame buffer to frame buffer


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OpenGL Pixel Functions

glReadPixels(x,y,width,height,format,type,myimage)

start pixel in frame buffer size

type of image

type of pixels

pointer to processor

memory

GLubyte myimage[512][512][3];

glReadPixels(0,0, 512, 512, GL_RGB,

GL_UNSIGNED_BYTE, myimage);

glDrawPixels(width,height,format,type,myimage)

starts at raster position


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We often work with images in a standard

format (JPEG, TIFF, GIF)

How do we read/write such images with

OpenGL?

No support in OpenGL

OpenGL knows nothing of image formats

Some code available on Web

Can write readers/writers for some simple

formats in OpenGL

Image Formats


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Thank you

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