R: [Plib-users] Drawing a simple triangle

[Paolo L. <p.l...@ci...>]

Peter,

on my opinion you just missed to set up a camera for looking at scene
objects. It can be done this way:

#include <plib/sg.h>

	sgCoord camera;
	SGfloat pos_x, pos_y, pos_z, yaw, pitch, roll;

	// set pos_x, pos_y, pos_z, yaw, pitch and roll to some suitable
values
	// then, before ssgCullAndDraw, actually set the SSG camera
	sgSetCoord( camera, pos_x, pos_y, pos_z, yaw, pitch, roll );
	ssgSetCamera( &camera );

SG is the prefix for the Plib math/geometry library stuffs.

Finding a plausible position for the init view (given a scene, in
general, you could not know its extent along the three axes) it's a
question of taste - I could suggest this:

	sgSphere *sp = scene->getBSphere();
	SGfloat radius = sp->getRadius();
	SGfloat EyeDist = float( radius * 1.f / tan( float( fov_x/2 *
SG_DEGREES_TO_RADIANS ) ) );
	SGfloat Ex, Ey, Ez;

	pos_x = sp->getCenter()[0];
	pos_y = sp->getCenter()[1] + EyeDist;
	pos_z = sp->getCenter()[2];

Then, moving in 3D above or around the scene is a matter of changing
those six parameters to the sgSetCoord/ssgSetCamera (yaw is compass
heading, pitch rotates around the screen x-axis, roll around the axis
norma to the screen).

Hoping you'll also love Plib for its simplicity not payed with
completness -

Paolo


> -----Messaggio originale-----
> Da: pli...@li... 
> [mailto:pli...@li...] Per conto di 
> Peter Poulsen
> Inviato: domenica 7 marzo 2004 18.18
> A: pli...@li...
> Oggetto: Re: [Plib-users] Drawing a simple triangle
> 
> 
> Steve Baker <sjb...@ai...> writes:
> 
> > Peter Poulsen wrote:
> > > I have tried the program below, but it only shows a black screen. 
> > > Can anybody explain why? The program works if I make 
> direct gl-calls 
> > > (just comment out the first line).
> > 
> > SSG uses the 'Z-is-up' convention - so I guess your polygon is 
> > edge-on. Also, SSG defaults to perspective rendering - so it'll be 
> > near-plane clipped.
> 
> Thanks. I have now tried to modify the program, but it still 
> does not work :-) I'm sorry for bothering you with this 
> trivil matter, but if I cannot get that to work, I probably 
> cannot get a "real" program to work.
> 
> --- ssg_triangle.cc ---
> 
> #define USE_SSG 1
> 
> #include <GL/gl.h>
> #include <GL/glu.h>
> 
> #include <plib/ssg.h>
> #include <SDL/SDL.h>
> #include <iostream>
> 
> using namespace std;
> 
> ssgRoot* scene;
> #define SCREEN_WIDTH 320
> #define SCREEN_HEIGHT 240
> #define SCREEN_BPP 24
> 
> void init()
> {
> 	int videoFlags;
> 	SDL_Surface* surface;
> 
> 	if ( SDL_Init( SDL_INIT_VIDEO ) < 0 ) {
> 		fprintf( stderr, "Video initialization failed: 
> %s\n",  SDL_GetError( ) );
> 		exit(0);
> 	}
> 
> 	videoFlags  = SDL_OPENGL;
> 	videoFlags |= SDL_GL_DOUBLEBUFFER;
> 	videoFlags |= SDL_HWPALETTE;      
> 	videoFlags |= SDL_HWSURFACE;
> 	surface = SDL_SetVideoMode( SCREEN_WIDTH, 
> SCREEN_HEIGHT, SCREEN_BPP, videoFlags );
> 
> 	if ( !surface ) {
> 		fprintf( stderr,  "Video mode set failed: 
> %s\n", SDL_GetError( ) );
> 		exit(0);
> 	}
> 
> #ifdef USE_SSG
> 	ssgInit();
> 	
> 	glClearColor ( 0.0f, 0.0f, 0.0f, 0.0f ) ;
> 	glEnable ( GL_DEPTH_TEST ) ;
> 	ssgGetCurrentContext()->setFOV    (60.0f, 0.0f);   // New
> 	ssgGetCurrentContext()->setNearFar(0.1f, 70.0f);   // New
> 
> 	ssgVertexArray* vertices = new ssgVertexArray;
> 	sgVec3 x = { 0.5f, 0.5f , -0.25f};                 // Changed
> 	sgVec3 y = { -0.5f, 0.5f , -0.25f };               // Changed
> 	sgVec3 z = { 0.0f, 0.5f , 0.75f };                 // Changed
> 	vertices->add(x);
> 	vertices->add(y);
> 	vertices->add(z);
> 
> 	sgVec4 white = { 1.0f, 1.0f, 1.0f, 0.5f };
> 	ssgColourArray* colours = new ssgColourArray;
> 	colours->add(white);
> 	colours->add(white);
> 	colours->add(white);
> 	ssgNormalArray* normals = new ssgNormalArray(3);
> 	sgVec3 normal = { 0, -1.0f, 0 };                   // Changed
> 	normals->add(normal);
> 	normals->add(normal);
> 	normals->add(normal);
> 	ssgTexCoordArray* texs = new ssgTexCoordArray(3);
> 	sgVec2 tex_vec = {0,0};
> 	texs->add(tex_vec);
> 	texs->add(tex_vec);
> 	texs->add(tex_vec);
> 	
> 	ssgVtxTable* n = new ssgVtxTable(GL_TRIANGLES, 
> vertices, normals, texs, colours);
> 	ssgTransform* trans = new ssgTransform;
> 	trans->addKid(n);
> 
> 	scene = new ssgRoot;
> 	scene->addKid(trans);
> #else
> 	glClearColor ( 0.0f, 0.0f, 0.0f, 0.0f ) ;
> 	glEnable ( GL_DEPTH_TEST ) ;
> #endif
> }
> 
> void update()
> {
> 	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT) ;
> 
> #ifdef USE_SSG
> 	ssgCullAndDraw(scene);
> 
> #else
> 	glLoadIdentity();
> 	
> 	glColor3f(1.0f, 1.0f, 1.0f);
> 	glBegin(GL_TRIANGLES);
> 	glVertex2f(0.5f, -0.25f);
> 	glVertex2f(-0.5f, -0.25f);
> 	glVertex2f(0.0f, 0.75f);
> 	glEnd();
> #endif
> 	SDL_GL_SwapBuffers( );
> }
> 
> int main(int argc, char *argv[])
> {
> 	init();
> 	for(unsigned int i = 0; i < 1000; i++) {
> 		update();
> 	}
> 	return 0;
> }
> 
> -- 
> Yours 
> Peter Poulsen
> 
> 
> 
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