Open GL, Fixed-Function - Introduction to Computer Graphics - Lecture Slides, Slides of Computer Graphics

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05 – OpenGL 1/

Lecture 04:

OpenGL (Fixed-Function)

05 – OpenGL 2/

Remember…

• Conceptual Graphics Framework

• Graphics Library can be Direct3D, Java3D, OpenGL, etc.

• It is a software API that controls the functions of a

piece of hardware – the graphics card.

Graphics System/ GPU Application Model/data base

Software Hardware

Application program

Graphics Library

05 – OpenGL 4/

State Variables

• Pseudo code:

SetState (LineStyle, DASHED);

SetState (LineColor, RED);

DrawLine ( PtStart = (x1,y1), PtEnd = (x2,y2) );

SetState (LineColor, BLUE);

DrawLine ( PtStart = (x2,y2), PtEnd = (x3,y3) );

SetState (LineStyle, SOLID);

DrawLine ( PtStart = (x3,y3), PtEnd = (x4,y4) );

What color and shape?

What color and shape?

What color and shape?

05 – OpenGL 5/

State Variables

• Pseudo code:

SetState (LineStyle, DASHED);

SetState (LineColor, RED);

DrawLine ( PtStart = (x1,y1), PtEnd = (x2,y2) );

SetState (LineStyle, DASHED);

SetState (LineColor, BLUE);

DrawLine ( PtStart = (x2,y2), PtEnd = (x3,y3) );

SetState (LineStyle, SOLID);

SetState (LineColor, BLUE);

DrawLine ( PtStart = (x3,y3), PtEnd = (x4,y4) );

What color and shape?

What color and shape?

What color and shape?

05 – OpenGL 7/

State Variables – Pros and Cons

• What if…?

function DrawDashedTriangle (pt1,pt2,p3) {

SetState( LineStyle, DASHED );

DrawLine( PtStart=pt1, PtStart=p2 );

DrawLine( PtStart=pt2, PtStart=p3 );

DrawLine( PtStart=pt3, PtStart=p1 );

• What color is the triangle?

– Pros: trickle down effect, caller can control the

subroutine’s behavior

– Cons: the color is undefined! Who set my color?!

05 – OpenGL 8/

State Variables – Pros and Cons

• What’s right and what’s wrong with this?

function DrawTriangle (pt1,pt2,p3,

int origColor, int curColor,

int origStyle, int curStyle ) {

SetState( LineStyle, curStyle );

SetState(LineColor, curColor);

DrawLine( PtStart=pt1, PtStart=p2 );

DrawLine( PtStart=pt2, PtStart=p3 );

DrawLine( PtStart=pt3, PtStart=p1 );

SetState( LineStyle, origStyle );

SetState(LineColor, origColor);

05 – OpenGL 10/

Forgetting the State?

• What if I forget to set some state?

DrawLine ( PtStart = (x1,y1), PtEnd = (x2,y2) );

• What if I set the wrong state?

SetState(LineStyle, 12345);

DrawLine( PtStart=pt1, PtStart=p2 );

• What if I forget the state a variable is in?

05 – OpenGL 11/

Forgetting the State?

• What if I forget to set some state?

DrawLine ( PtStart = (x1,y1), PtEnd = (x2,y2) );

• OpenGL provides some good default values. For example, for color, it’s set to (1,1,1,1) – non transparent white

• How do I know if something worked?

SetState(LineStyle, 12345); DrawLine( PtStart=pt1, PtStart=p2 );

• You don’t. Since it’s not clear if some configuration of states will send OpenGL spinning, if you suspect an error from OpenGL, call Glenum glGetError(void)

• What if I forget the state a variable is in?

• You should use this sparingly… But you can use void glGetBooleanv(Glenum paraname, Glboolean* params) void glGetFixedv(Glenum paraname, Glfixed* params) void glGetFloatv(Glenum paraname, Glfloat* params) void glGetIntegerv(Glenum paraname, Glint* params)

05 – OpenGL 13/

Typical OpenGL Application

05 – OpenGL 14/

Example

• Here we use GLUT, which is a basic GL window

implementation that is on all platforms.

• Pros: cross-platform, command line, easy to use

• Cons: no GUI support (no buttons, menus, etc.)

05 – OpenGL 16/

Example OpenGL Application

int main( int argc, char** argv ) {

glutInit( &argc, argv ); // Boilerplate initialization

glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);

#define GLUT_RGB 0 #define GLUT_RGBA GLUT_RGB #define GLUT_INDEX 1 #define GLUT_SINGLE 0 #define GLUT_DOUBLE 2 #define GLUT_ACCUM 4 #define GLUT_ALPHA 8 #define GLUT_DEPTH 16 #define GLUT_STENCIL 32 #if {GLUT_API_VERSION >= 2} #define GLUT_MULTISAMPLE 128 #define GLUT_STEREO 256 #endif #if {GLUT_API_VERSION >= 3} #define GLUT_LUMINANCE 512 #endif

05 – OpenGL 17/

Example OpenGL Application

int main( int argc, char** argv ) {

glutInit( &argc, argv ); // Boilerplate initialization

glutInitDisplayMode( GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH );

glutInitWindowPosition( 50, 50 ); //upper left

glutInitWindowSize( 640, 480 ); // width, height, in pixels

glutCreateWindow( "OpenGL Example" ); // window title

glViewport(

/* lower left corner of the viewport */ 0, 0,

/* width, height of the viewport */ 640, 480 ); //lower left

Careful!!

05 – OpenGL 19/

Example OpenGL Application

int main( int argc, char** argv ) {

glutInit( &argc, argv ); // Boilerplate initialization glutInitDisplayMode( GLUT_RGB | GLUT_DEPTH ); glutInitWindowPosition( 50, 50 ); //upper left glutInitWindowSize( 640, 480 ); // width, height, in pixels glutCreateWindow( "OpenGL Example" ); // window title glViewport( /* lower left corner of the viewport / 0, 0, / width, height of the viewport */ 640, 480 ); //lower left glShadeModel( GL_SMOOTH );

glPolygonMode( GL_FRONT, GL_FILL );

setupLighting();

setupCamera(640, 480);

registerCallBacks();

glutMainLoop();

}

Asks OpenGL to render fill the polygons but only in the front-facing side Options are GL_BACK, GL_FRONT_AND_BACK, and GL_POINT, GL_LINE, GL_FILL

05 – OpenGL 20/

Example OpenGL Application

int main( int argc, char** argv ) {

glutInit( &argc, argv ); // Boilerplate initialization glutInitDisplayMode( GLUT_RGB | GLUT_DEPTH ); glutInitWindowPosition( 50, 50 ); //upper left glutInitWindowSize( 640, 480 ); // width, height, in pixels glutCreateWindow( "OpenGL Example" ); // window title glViewport( /* lower left corner of the viewport / 0, 0, / width, height of the viewport */ 640, 480 ); //lower left glShadeModel( GL_SMOOTH );

glPolygonMode( GL_FRONT, GL_FILL );

setupLighting();

setupCamera(640, 480);

registerCallBacks();

glutMainLoop();

}

void setupLighting() { glEnable (GL_LIGHTING); glEnable (GL_LIGHT0);

GLfloat whiteColor[] = {0.25f, 0.25f, 0.25f, 1.0f}; glLightfv (GL_LIGHT0, GL_AMBIENT, whiteColor);

//directional (diffuse) light GLfloat whiteFull[] = {1.0f, 1.0f, 1.0f, 1.0f}; GLfloat lightPos[] = {-1.0f, 1.0f, 1.0f, 0.0f}; glLightfv(GL_LIGHT0, GL_DIFFUSE, whiteFull); glLightfv(GL_LIGHT0, GL_POSITION, lightPos);

GLfloat grey[] = {0.1f, 0.1f, 0.1f, 1.0f}; glMaterialfv(GL_FRONT, GL_DIFFUSE, grey); //reflect 10% diffuse glMaterialfv(GL_FRONT, GL_AMBIENT, whiteFull);}//reflect full ambient