Help needed with OpenGL

gemma188

Im looking for some help with OpenGL for an exam I have. Im doing some sample questions to prepare but stuck with some parts!

Code which draws a 2-dimensional star shape from two triangles...

#include <gl/glut.h>
void drawStar() {
glBegin(GL_TRIANGLES);
glVertex2f(-0.5, -0.5);
glVertex2f(0.5, -0.5);
glVertex2f(0.0, 0.5);
glVertex2f(-0.5, 0.15);
glVertex2f(0.5, 0.15);
glVertex2f(0.0, -0.85);
glEnd();
}
void display(void) {
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluOrtho2D(-1, 1, -1, 1);
drawStar();
glFlush ();
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutCreateWindow ("Star");
glutDisplayFunc(display);
glutMainLoop();
return 0;
}

Question:
a) Modify the display() function so that a 3-dimensional (perspective) projection is used rather than an orthographic projection

(b) Modify the display() and drawStar() functions so that the star is drawn at a 3-dimensional location and colour as specified in its parameter list:
drawStar(float x, float y, float z, float r, float g, float b)

(c) Extend the program so that a 'starfield' animation is displayed using the Idle Callback approach. Multiple stars should be stored in an array, and initialised to random x, y and z co-ordinates. They should move towards the camera along the z axis, and when they pass the camera they should be moved back to the far distance again.

Part C is the bit Im stuck with so hopefully someone might be able to help. Thanks in advance

srsly78

Part (c) builds on (a) and (b) so please post your answers for those.

The "idle function" it refers to is basically a function that runs between frames, you put your update code here and move each star a little bit every frame. In a proper application you would use a high resolution timer to make animation independent from framerate but that's overkill for this.

Please post your attempt at (c), your StarObject class (or whatever you called it) and the loop where you create lots of them.

gemma188

So far I just have Part A done.

void display(void) {
//...
float fov=..., aspect=..., near=..., far = ..;
glMatrixMode(GL_PROJECTION);
gluPerspective(fov,aspect, near, far);
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0,0,-2);
drawStar();
glFlush ();
}

Not sure if this is correct?

srsly78

Not checked it exactly... but looks ok. They just want you to replace gluOrtho2D (an orthographic 2d projection) with gluPerspective (3d perspective projection).

Now go do part (b) then we can start on (c).

Btw, writing code is no good. Have you compile and run it? Does it render on screen as you expect?

(float fov=..., aspect=..., near=..., far = ..; ) I hope you actually put real numbers in there, ellipsis will not make compiler happy.

If you want to learn graphics programming you have to compile and run code, observe result on screen, tweak code, repeat etc etc. Anything else is useless.

gemma188

For part B:

void display(void) {
//...
float fov=..., aspect=..., near=..., far = ..;
glMatrixMode(GL_PROJECTION);
gluPerspective(fov,aspect, near, far);
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0,0,-2);

glTranslate3f(x,y,z);
glColor3f(r,g,b);
drawStar();

glFlush ();
}

srsly78

Please read the question properly. You have only answered part of (b). Where is your modified drawStar function? Your answer would get partial marks. Yes the end result would be correct, but it isn't what they asked for. It's important because part (c) builds on it

Are you actually running and compiling this? Take a screenshot and show please. What are your values for fov, aspect, near and far?

gemma188

No I'm not running it. Since it's a written exam that I have to do I have been writing the code.

srsly78

Start compiling and running it or you won't learn anything. I'll wait for a screenshot of part (b) in action before helping you with (c).

gemma188

srsly78 wrote: »

Start compiling and running it or you won't learn anything. I'll wait for a screenshot of part (b) in action before helping you with (c).

Im on my work computer so Im not able to run it. Is it completely necessary to run it for doing part C?

srsly78

To verify your code is correct it's completely necessary to run it and observe the result on the screen yes.

You still haven't answered (b) correctly by the way.

gemma188

To answer B correctly do I just need to put those two function calls into a drawStar()?

srsly78

Pretty much yes. And change the parameters on the drawStar function. Can you see why this is important for (c)?

gemma188

To be honest no Im not fully sure of this part. As I said before part C is completely unknown to me so I really dont know what to do with it. This is what I really need the help with!

srsly78

How to solve (c):

0. Define a StarObject with properties needed to render a star (ie position)
1. Create an initialise function that creates lots of stars
2. Create an update function that moves each star by a little bit
3. Modify existing code to hook in the new stuff (glutIdlefunc(update()) or something to register the update call). Modify render function to draw N stars rather than 1.

Now show your attempt please. I hope you realise memorising answers to old papers is not gonna help much in an opengl exam.