directional works, positional only draws 1st time through

[jmg]

I posted this in the beginner area but no luck.

Below is the complete program so you can have something to copy/paste/compile/run if you'd like to try it yourself and see if you can help.

The program draws glColor3d()'d coordinate axes, a single lit shaded triangle in the x-z plane (y is up), with a light circling around the triangle, about the (+) z axis, in the x-y plane. I draw a little glColor3d()'d GL_POINT where the light is.

The problem is, the little dot is only drawn in the first frame. After that it's not drawn, but everything else works fine. I put in an SDL_Delay( 500 ) to make this effect more obvious.

If I change the last arg in the glLightfv() call (in draw(), where the // XXX is) from light_pos to light_dir, the problem goes away. Am I using glLightfv() incorrectly?




// tiny_sdl_gl
//
// main.cpp

#include <cstdlib>
#include <iostream>

#include <gl.h>
#include <glu.h>

#include <SDL.h>
#include <SDL_video.h>

using std::exit;
using std::cout;
using std::endl;

//------------------------------
GLfloat radius = 2.5;
GLfloat light_pos[] = { 0.0, radius, 0.0, 1.0 }; // positional
GLfloat light_dir[] = { 0.0, radius, 0.0, 0.0 }; // directional
GLfloat theta = 0; // rotation of light about the z-axis.

//---------------------------------------------------------------------
void processEvents()
{
SDL_Event event;

while( SDL_PollEvent( &amp;event ) )
{
if ( event.type == SDL_QUIT )
{
cout << "quit has been requested via ctrl-c or close window"
<< endl;
SDL_Quit();
exit( 0 );
break;
}
}
}

//---------------------------------------------------------
void initSDL(int width, int height)
{
if ( SDL_Init( SDL_INIT_VIDEO ) != 0 )
{
cout << "couldn't initialize SDL: " << SDL_GetError() << endl;
SDL_Quit();
exit(1);
}

const SDL_VideoInfo * vid_info = SDL_GetVideoInfo();
int bpp = vid_info->vfmt->BitsPerPixel;
cout << "Found bpp = " << bpp << endl;

unsigned int vid_flags =
SDL_DOUBLEBUF |
SDL_OPENGL |
SDL_HWPALETTE |
SDL_HWACCEL;

if ( vid_info->hw_available )
{
cout << "Found hw_available. Setting vid_flags |= SDL_HWSURFACE." << endl;
vid_flags |= SDL_HWSURFACE;
}
else
{
cout << "Didn't find hw_available. Setting vid_flags |= SDL_SWSURFACE." << endl;
vid_flags |= SDL_SWSURFACE;
}

if ( vid_info->blit_hw )
{
cout << "Found blit_hw. Setting vid_flags |= SDL_HWACCEL." << endl;
vid_flags |= SDL_HWACCEL;
}


SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );
SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_ALPHA_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 24 ); // default anyway.

SDL_Surface * surf = SDL_SetVideoMode( width, height, bpp, vid_flags );

if ( surf == NULL )
{
cout << "unable to set video mode: " << SDL_GetError() << endl;
SDL_Quit();
exit(1);
}

SDL_WM_SetCaption( "sdl_ast", "sdl_ast" );
}

//---------------------------------------------------------
void initGL(int width, int height)
{
glPolygonMode( GL_FRONT, GL_FILL );

glShadeModel( GL_SMOOTH );

glCullFace( GL_BACK );
glEnable( GL_CULL_FACE );

glClearColor( 0.0, 0.0, 0.0, 0.0 );
glPointSize( 4.0 );
glLineWidth( 2.0 );

glEnable( GL_DEPTH_TEST );

GLfloat mat_spec[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat mat_shin = 50.0;
GLfloat mat_emerald[] = { 0.0, 0.8, 0.1, 1.0 };

GLfloat white_light[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat amb_light[] = { 0.2, 0.2, 0.2, 1.0 };

glMaterialfv( GL_FRONT, GL_SPECULAR, mat_spec );
glMaterialf( GL_FRONT, GL_SHININESS, mat_shin );
glMaterialfv( GL_FRONT, GL_AMBIENT_AND_DIFFUSE, mat_emerald );

glLightfv( GL_LIGHT0, GL_AMBIENT, amb_light );
glLightfv( GL_LIGHT0, GL_DIFFUSE, white_light );
glLightfv( GL_LIGHT0, GL_SPECULAR, white_light );

glEnable( GL_LIGHTING );
glEnable( GL_LIGHT0 );

glViewport( 0, 0, width, height );

glMatrixMode( GL_PROJECTION );
glLoadIdentity();
gluPerspective( 40.0, 1.0, 4.0, 16.0 );

glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
}

//------------------------------------------------
void draw()
{
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

glPushMatrix();
gluLookAt( 2.0, 4.0, 7.0,
0.0, 0.0, 0.0,
0.0, 1.0, 0.0 );

glPushMatrix();
glRotated( theta, 0.0, 0.0, 1.0 );
glLightfv( GL_LIGHT0, GL_POSITION, light_pos ); // XXX

// Draw the dot representing the light.
glDisable( GL_LIGHTING );
glBegin( GL_POINTS );
glColor3d( 1.0, 1.0, 1.0 );
glVertex4fv( light_pos );
glEnd();
glEnable( GL_LIGHTING );
glPopMatrix();

// Draw the x y z coordinate axes.
glDisable( GL_LIGHTING );
glBegin( GL_LINES );
glColor3d( 1.0, 0.0, 0.0 );
glVertex3d( 0.0, 0.0, 0.0 );
glVertex3d( 2.0, 0.0, 0.0 );

glColor3d( 0.0, 1.0, 0.0 );
glVertex3d( 0.0, 0.0, 0.0 );
glVertex3d( 0.0, 2.0, 0.0 );

glColor3d( 0.0, 0.0, 1.0 );
glVertex3d( 0.0, 0.0, 0.0 );
glVertex3d( 0.0, 0.0, 2.0 );
glEnd();
glEnable( GL_LIGHTING );

// Draw a simple triangle facing up near the origin.
glBegin( GL_TRIANGLES );
glNormal3d( 0.0, 1.0, 0.0 ); // Normal facing up.

// The winding is correct so the "front" is facing up,
// in the same direction as the normal.
glVertex3d( 0.0, 0.0, 0.0 );
glVertex3d( 0.5, 0.0, 1.0 );
glVertex3d( 1.0, 0.0, 0.2 );
glEnd();
glPopMatrix();
glFinish();
}

//------------------------------------------------
int main( int argc, char * argv[] )
{
int width = 400;
int height = 400;
initSDL( width, height );
initGL( width, height );

while ( true )
{
SDL_Delay( 500 ); // This is just here to slow things down
// enough to see the effect I'm asking about.
processEvents();
theta += 5.0;
if ( theta > 360.0 )
{
theta -= 360.0;
}
draw();
SDL_GL_SwapBuffers();
}

SDL_Quit();
return 0;
}

//------------------------------------------------




And here's the makefile:




# makefile for tiny_sdl_gl
#

APP_NAME = tiny_sdl_gl

LDFLAGS = `sdl-config --libs` -L/usr/X11R6/lib -lGL -lGLU

CFLAGS = -c `sdl-config --cflags` -I/usr/X11R6/include/GL

#----------------------------------------------
$(APP_NAME): main.o
g++ -o $(APP_NAME) $(LDFLAGS) main.o

main.o: main.cpp
g++ $(CFLAGS) main.cpp

clean:
rm -f $(APP_NAME)
rm -f *.o




[This message has been edited by jmg (edited 11-30-2002).]

[jmg]

If I add a call to glLightModeli( GL_LIGHT_MODEL_LOCAL_VIEWER, GL_TRUE ) just before enabling the lighting, and leave the light set to positional, the GL_POINT doesn't show up at all...

Can someone please confirm whether or not they get the same strange behaviour so I can tell if this is an implementation bug on my end?

[knackered]

glVertex4fv( light_pos ); ?
Try changing that to
glVertex3fv( light_pos );

dunno why you're using 4 component vertices..

[Coriolis]

The fourth parameter to glVertex doesn't matter in this case. The default value for the fourth parameter is 1, which is the fourth value he's passing in with glVertex4fv.

[jmg]

Whoa, knackered,.. it worked. Thanks.

If anyone could offer an explanation, I'd love to hear it, because it makes no sense to me at all. I agreed with Coriolis that the default parameter is 1 anyway and it shouldn't matter.

The only reason I'd written it that way was:
A. I had light_pos hanging around looking to be reused, and
B. I just assumed that since light_pos was a 4-element array, I should pass it to glVertex4fv(). It didn't even occur to me that using glVertex3fv() would compile just fine.

[knackered]

Agreed, it should work because your w element is 1.0
So, it must be a driver bug - what drivers are you using?

[jmg]

> what drivers are you using?

I don't think I know how to answer that. I'm running Mandrake Linux 8.2. I'm using DRI/Mesa with my Matrox G400 -- had to hand-edit XF86Config-4. I installed Mesa myself (./configure, make, make install); it's 4.0.4. Maybe this is useful:



[john@localhost src]$ lspci
---[snip]---
01:00.0 VGA compatible controller: Matrox Graphics, Inc. MGA G400 AGP (rev 04)

[john@localhost src]$ glxinfo
name of display: :0.0
display: :0 screen: 0
direct rendering: Yes
server glx vendor string: SGI
server glx version string: 1.2
server glx extensions:
GLX_EXT_visual_info, GLX_EXT_visual_rating, GLX_EXT_import_context
client glx vendor string: SGI
client glx version string: 1.2
client glx extensions:
GLX_EXT_visual_info, GLX_EXT_visual_rating, GLX_EXT_import_context
GLX extensions:
GLX_EXT_visual_info, GLX_EXT_visual_rating, GLX_EXT_import_context
OpenGL vendor string: VA Linux Systems Inc.
OpenGL renderer string: Mesa DRI G400 20010622 AGP 1x x86/MMX/3DNow!
OpenGL version string: 1.2 Mesa 3.4.2
OpenGL extensions:
---[snip]---


Hmm... dunno why it says Mesa 3.4.2 there... Uh oh. I think I recall reading somewhere that when you use DRI, you're supposed to use only the Mesa libs that come with the DRI release.

I may have installed Mesa improperly. I need to study this some more.