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me_here_me

OpenGL rendering simple polygon

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Hi I am trying to render a polygon onto the openGL screen. The polygon data (vertices and normals) are read from a file. The reading takes place fine but when I render the recieved vertices and normals onto the screen, they somehow mess and the output in not as expected. I think there is some problem with the way I am handling my normals. Foolowing is the code of my display method:
void draw( )
{
	glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
	glColor3f( 1., 0., 0. );
	glLoadIdentity();
 
	gluLookAt( 350., 250., 250., 0., 0., 0., 0., 1., 0. );
	
	glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
	glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
	glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
	glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess);

	glRotatef( xVal, 1., 0., 0. );
	glRotatef( yVal, 0., 1., 0. );
	glRotatef( zVal, 0., 0., 1. );
	
	for( unsigned int i=0; i< Mesh.size(); i++)
	{
		Vec dummy;
		glBegin( GL_TRIANGLES );
			
			dummy = Mesh->getNormal();
			glNormal3f( dummy.x, dummy.y, dummy.z );

			// vertex 1
			Mesh->getVertex( 1, dummy );
			glVertex3f( dummy.x, dummy.y, dummy.z );

			// vertex 1
			Mesh->getVertex( 2, dummy );
			glVertex3f( dummy.x, dummy.y, dummy.z );

			// vertex 1
			Mesh->getVertex( 3, dummy );
			glVertex3f( dummy.x, dummy.y, dummy.z );

		glEnd();
	}
	glutSwapBuffers();
}


getNormals and getVertices return the right values as were read from the file!! best wishes [Edited by - me_here_me on June 11, 2007 4:34:38 AM]

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I am not sure why things are not working as desired? I checked out again but cannot figure out. Is it possible that I email you the code if you have some time for that? It is not a long code!

best wishes and thanks for help?

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Ideally, you would have a screenshot demonstrating your problem. Failing that you should describe the problem more clearly. "...the output in not as expected..." leaves people guessing what the expected output is, nevermind what is actually being displayed.

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My specific problem:

Some of the triangles do not appear and therefore i can look inside the dataset, that should not happen (i think this is happening due to normals getting wrong somewhere). second, the color changes from bright to dark while rotating, as it wishes. Again this may also be caused by wrong handling of normals. But I cannot figure out the actual matter.

can i email someone the code, who has some time to look at it?

regards and best wishes

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is it possible that I need to invert the MV matrix and multiply it with the normal before setting the normal in the for loop:


something like below:


void draw( )
{
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glColor3f( 1., 0., 0. );
glLoadIdentity();

gluLookAt( 350., 250., 250., 0., 0., 0., 0., 1., 0. );

glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess);

glRotatef( xVal, 1., 0., 0. );
glRotatef( yVal, 0., 1., 0. );
glRotatef( zVal, 0., 0., 1. );

for( unsigned int i=0; i< Mesh.size(); i++)
{
Vec dummy;
glBegin( GL_TRIANGLES );

//PUSH MATRIX HERE
//INVERT MV HERE, how would i invert this ?
dummy = Mesh->getNormal();
glNormal3f( dummy.x, dummy.y, dummy.z );
//POP MATRIX HERE
// vertex 1
Mesh->getVertex( 1, dummy );
glVertex3f( dummy.x, dummy.y, dummy.z );

// vertex 1
Mesh->getVertex( 2, dummy );
glVertex3f( dummy.x, dummy.y, dummy.z );

// vertex 1
Mesh->getVertex( 3, dummy );
glVertex3f( dummy.x, dummy.y, dummy.z );

glEnd();
}
glutSwapBuffers();
}




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no its not neccessary to multiply the normals by any matrix, opengl handels this for you. Are you shure your lighting is set up correctly? And are you realy shure your model loader works fine?

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Quote:

Some of the triangles do not appear and therefore i can look inside the dataset, that should not happen (i think this is happening due to normals getting wrong somewhere).

I've never seen it described as being able to "look inside the dataset", but is your problem back face culling?

The 'normal' is not used for back face culling - the winding order of the vertices are. By default, faces with a winding order of counter clockwise are considered to be front facing. Any polygons with a clockwise winding order are therefore culled, being considered back faces.

Try glDisable (GL_CULL_FACE); to see if this temporarily solves the problem. The long term fix is to leave back face culling enabled, and choosing a winding order (preferably the default, for less confusion) and sticking to it, ensuring vertices are specified in the correct order for each polygon.

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HI
The model is fine as I loaded it on an stl viewer and it worked fine. The lighting is also set up correctly, atleast as I see that.

I tried to disable the GL_CULL_FACE and it threw all the faces onto the screeen but without any specific order. Its not going on correctly even now.

I will share allmost all the code in my next post :

Someone told me that this program DOES NOT need face culling, but I have to correctly use normals in the code. I am not picking that thing out.

regards

[Edited by - me_here_me on June 12, 2007 7:01:50 AM]

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I have a header file called headin.h and it has the following classes:

CVector --> double x, double y and double z and some methods to manupilate them.
these methods work fine and you can find them as they are called in the code below

Trianlge --> long triIndex; // index number of the triangle
cVector vertex1; // first vertex
cVector vertex2; // second vertex
cVector vertex3; // third vertex
cVector normal; // the normal of the triangle

All these values are retrived from an StL file.

the code of my main is as below:


#include <iostream>
using namespace std;

#include <GL/glut.h>
#include <stdlib.h>
#include <math.h>

#include <iostream>
#include <algorithm>
#include <fstream>
#include <vector>
#include <string>

#include "headin.h"

float xVal = 0.;
float yVal = 0.;
float zVal = 0.;

GLuint aShape;


// declare variables to take in values and store them
std::ifstream reader;
std::vector<Triangle*> stlMesh;

// I hold the values to be used inside the vertex array
double* vertArr;
double* normArr;

// Create values for a number of lights

// White directional light
GLfloat white_light[] = { 1., 1., 1., 1. };
GLfloat light_posn[] = { 1000., 0., 0., 0. };


// Silvery material for the object
GLfloat mat_ambient[] = { 0.19, 0.19, 0.19, 1.0 };
GLfloat mat_diffuse[] = { 0.7, 0.2, 0.2, 1.0 };
GLfloat mat_specular[] = { 0.51, 0.51, 0.51, 1.0 };
GLfloat low_shininess[] = { 51.2 };

int readSTLFile( std::string filename )
{
if( !filename.empty() )
reader.open( filename.c_str() );
else
return 1;

// safety check to ensure that the file pointer opened is valid
if (!reader.is_open())
return 2;

// just blindly read in the entire file till eof in to a vector
std::vector<std::string> Word;
// File reading and breaking the file into white-space seperated words
// then storing it in the container object Word
std::string line;
while(!reader.eof())
{
reader >> line;
Word.push_back(line);
}

// This is the standard format of the stl file
// it starts with a keyword FACET
// then stores the normals of a given triangle with the keyword NORMAL
// the three vertices ( !! clockwise ? ) each starting with the
// keyword VERTEX
// all the normals are enclosed within words OUTER LOOP - ENDLOOP
// every triangle is ended with a ENDFACET keyword

/* *************************************
FACET NORMAL Nx Ny Nz
OUTER LOOP
VERTEX X1 Y1 Z1
VERTEX X2 Y2 Z2
VERTEX X3 Y3 Z3
ENDLOOP
ENDFACET
* ***************************************/


// a total of 4 sets of SbVec3f's 1 for the each of the vertices
// and 1 for the normals.
// also have a string to indentify the keyword
std::string name;
long counter = 0;

for(unsigned int i = 0; i < Word.size(); i++)
{
if(Word == "solid")
{
name = Word[i+1];
}
if(Word == "facet" && Word[i+1] == "normal")
{
// create a fresh pointer to triangle
Triangle* copy = new Triangle();

copy->setNormal( cVector( atof(Word[i+2].c_str()),
atof(Word[i+3].c_str()),
atof(Word[i+4].c_str())
) );

copy->setVertex( 1, cVector( atof(Word[i+8].c_str()),
atof(Word[i+9].c_str()),
atof(Word[i+10].c_str())
) );

copy->setVertex( 2, cVector( atof(Word[i+12].c_str()),
atof(Word[i+13].c_str()),
atof(Word[i+14].c_str())
) );

copy->setVertex( 3, cVector( atof(Word[i+16].c_str()),
atof(Word[i+17].c_str()),
atof(Word[i+18].c_str())
) );
counter++;
copy->setIndex(counter);

stlMesh.push_back( new Triangle(copy) );
delete copy;
}
}

// Pump all the values from the STL code in to a "double" array to store
// vertex values, then we can render the same data using vertex arrays
vertArr = new double[ stlMesh.size()*9 ]; // the value here 3 vertex per tri times 3 values per vertex.
normArr = new double[ stlMesh.size()*3 ];

unsigned int normalcounter = 3;
unsigned int vertexcounter = 9;

for( int i=0; i< stlMesh.size(); i++)
{
cVector dummy;
if( i >0 )
normalcounter = (i+1)*3;
dummy = stlMesh->getNormal();
normArr[ normalcounter - 3 ] = dummy.x;
normArr[ normalcounter - 2 ] = dummy.y;
normArr[ normalcounter - 1 ] = dummy.z;

if( i >0 )
vertexcounter = (i+1)*9;
// vertex 1
stlMesh->getVertex( 1, dummy );
vertArr[ vertexcounter - 9 ] = dummy.x;
vertArr[ vertexcounter - 8 ] = dummy.y;
vertArr[ vertexcounter - 7 ] = dummy.z;

// vertex 2
stlMesh->getVertex( 2, dummy );
vertArr[ vertexcounter - 6 ] = dummy.x;
vertArr[ vertexcounter - 5 ] = dummy.y;
vertArr[ vertexcounter - 4 ] = dummy.z;

// vertex 3
stlMesh->getVertex( 3, dummy );
vertArr[ vertexcounter - 3 ] = dummy.x;
vertArr[ vertexcounter - 2 ] = dummy.y;
vertArr[ vertexcounter - 1 ] = dummy.z;

normalcounter = 1;
vertexcounter = 1;
}

// enable the vertex arrays
glEnableClientState( GL_VERTEX_ARRAY );
//glEnableClientState( GL_NORMAL_ARRAY );

glVertexPointer( 3, GL_DOUBLE, 0, vertArr );
//glNormalPointer( GL_DOUBLE, 0, normArr );

reader.close();
Word.clear();
return 0;
}

void drawAxis()
{
// draw the axis
glBegin( GL_LINES );
glColor3f( 1., 0., 0. );
glVertex3f( -200., 0., 0. );
glVertex3f( 200., 0., 0. );
glEnd();

glBegin( GL_LINES );
glColor3f( 0., 1., 0. );
glVertex3f( 0., -200., 0. );
glVertex3f( 0., 200., 0. );
glEnd();

glBegin( GL_LINES );
glColor3f( 0., 0., 1. );
glVertex3f( 0., 0., -200. );
glVertex3f( 0., 0., 200. );
glEnd();

}


void keys( unsigned char key, int x, int y )
{

switch( key )
{
case '1':
{
xVal += 1.;
}break;

case '2':
{
yVal += 1.;
}break;

case '3':
{
zVal += 1.;
}break;
}

glutPostRedisplay();
}

void reshape( int w, int h )
{
glViewport( 0, 0, (GLsizei)w, (GLsizei)h );

glMatrixMode( GL_PROJECTION );
glLoadIdentity();
glFrustum( -.4, .4, -.3, .3, 1., 1300. );

glMatrixMode( GL_MODELVIEW );

glutPostRedisplay();
}

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

glColor3f( 1., 0., 0. );
glLoadIdentity();

gluLookAt( 350., 250., 250., 0., 0., 0., 0., 1., 0. );

glDisable( GL_LIGHTING );
drawAxis();
glEnable( GL_LIGHTING );

// set material
glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess);

// do the drawing
glRotatef( xVal, 1., 0., 0. );
glRotatef( yVal, 0., 1., 0. );
glRotatef( zVal, 0., 0., 1. );

for( unsigned int i=0; i< stlMesh.size(); i++)
{
cVector dummy;
glBegin( GL_TRIANGLES );

dummy = stlMesh->getNormal();
glNormal3f( dummy.x, dummy.y, dummy.z );

// vertex 1
stlMesh->getVertex( 1, dummy );
glVertex3f( dummy.x, dummy.y, dummy.z );

// vertex 2
stlMesh->getVertex( 2, dummy );
glVertex3f( dummy.x, dummy.y, dummy.z );

// vertex 3
stlMesh->getVertex( 3, dummy );
glVertex3f( dummy.x, dummy.y, dummy.z );

glEnd();
}

// Draw using display list
// glCallList( aShape );

// Draw using the vertex array
/*
glBegin( GL_TRIANGLES );
for( unsigned int i=3; i< stlMesh.size()*3; i += 3)
{
glArrayElement( i - 3 );
glArrayElement( i - 2 );
glArrayElement( i - 1 );
}
glEnd();
*/

glutSwapBuffers();
}

void init( )
{
glClearColor( .46, .53, .6, 0. );

// We keep our light within the display func so that we can influence its properties
// Create the first white light
glLightfv( GL_LIGHT0, GL_POSITION, light_posn );
glLightfv( GL_LIGHT0, GL_AMBIENT, white_light );
glLightfv( GL_LIGHT0, GL_DIFFUSE, white_light );
glLightfv( GL_LIGHT0, GL_SPECULAR, white_light );

glEnable( GL_LIGHTING );
glEnable( GL_LIGHT0 );

glEnable(GL_NORMALIZE); // always keep normals normalized

// Enable depth testing
glEnable( GL_DEPTH );
// Set up a shading model
glShadeModel( GL_SMOOTH );

glEnable (GL_CULL_FACE);

// read in a file
readSTLFile("Tp09_Catia_File_2.stl" );
// create the display list

}

int main(int argc, char* argv[])
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH );
glutInitWindowPosition( 50, 50 );
glutInitWindowSize( 640, 480 );
glutCreateWindow( "window" );

glutDisplayFunc( draw );
glutReshapeFunc( reshape );
glutKeyboardFunc( keys );

init( );

glutMainLoop();

return 0;
}





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