Conceptual Graphics Framework - Introduction to Computer Graphics - Lecture Slides, Slides of Computer Graphics

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Lecture 05:

OpenGL 2 (Fixed-Function)

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

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?

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?

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?!

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);

}

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?

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)

For Example

• How many parameters do you have to pass to render an

object?

void render () {

obj1->drawSelf(myMouse, myScreenSize, myTimer, ...);

obj2->drawSelf(myMouse, myScreenSize, myTimer, ...);

obj3->drawSelf(myMouse, myScreenSize, myTimer, ...);

• How do you enforce that there is only one instance of

myMouse, myScreenSize, myTimer in code?

Possible Answers?

1. Use global variables

2. Use mutual references. For example:

void setChild (CWidget* child)

{

myChild = child; child->setParent(this);

}

MyMouse* getMouse() {

return myMouse;

}

void setParent (PWidget* parent) { myParent = parent; }

void doStuff () { MyMouse mouse = myParent->getMouse(); doStuffWithMouse (mouse); }

Programming Strategies?

• What do you think is the best way to deal with

the state machine and maintain state variables?

– How to access local variables (that correspond to state

variables)?

– Efficiency considerations?

• How do you think the programming with a state

machine affects multi-threaded applications?

Multi-Threading and OpenGL

Thread A

void drawRedDashedTriangle()

pushAttributes();

setColor (RED);

setStyle (DASHED);

drawLine (pt1+d, pt2+d);

drawLine (pt2+d, pt3+d);

drawLine (pt3+d, pt1+d);

d++;

popAttributes();

Thread B

void drawBlueSolidTriangle()

pushAttributes();

setColor (BLUE);

setStyle (SOLID);

drawLine (pt4-p, pt5-p);

drawLine (pt5-p, pt6-p);

drawLine (pt6-p, pt4-p);

p--;

popAttributes();

Typical OpenGL Application

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.)