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# OpenGL importing .obj file into OpenGL

## 9 posts in this topic

Hi everyone, when I import an .obj file I get a model looking similar to the one I want but some triangles look distorted.

this is the model

[sharedmedia=core:attachments:13638]

and this is what i get (ignore rotation)
[sharedmedia=core:attachments:13637]

declarations:

// vertex, normal, and index related declarations
GLfloat vertices[500];
GLfloat normals[540];
int indices[1300];
int vertexIndex = 0, normalIndex = 0, elementIndex = 0;


// extracts object's vertices, indices, and normals from given file
int LoadObject( const char* filename, GLfloat vertices[], GLfloat normals[], int elements[] )
{
// local variables
const int MAX_CHARS_PER_LINE = 64;
char lineBuffer[64];
char temp[16];
char tempFloat[16];
FILE *objFile;
int count;
GLfloat coordinate; // stored in vertices
int index;
int charCount;
int isVertex = -1; // -1 for normal, 1 for vertex
int isFirstNormal = 1; // 1 for first normal, -1 for any other normal

// open file
fopen_s( &objFile, filename, "r" );

// check for errors
if ( objFile == NULL )
{
return 0;
}

// while there are lines to read
while ( fgets( lineBuffer, MAX_CHARS_PER_LINE, objFile ) != NULL )
{

// if we are reading a vector
if ( strncmp( lineBuffer, "v ", 2 ) == 0 )
{

// initialize charCount
charCount = 0;

// loop through each char in the line
for ( count = 2; count < (int)strlen( lineBuffer ); count++ )
{

// if we are reading part of a float coordinate
if ( ( lineBuffer[count] == '-' ) || ( isdigit( lineBuffer[count] ) ) || ( lineBuffer[count] == '.') )
{
// store this char into temp
temp[charCount++] = lineBuffer[count];
}

// if we are not reading part of a float and a float exists in temp
else if ( charCount != 0 )
{
// copy the float coordinate to tempFloat
strncpy( tempFloat, temp, charCount );

//
tempFloat[ charCount ] = '\n';

// store the coordinate in our vertices array
coordinate = atof( tempFloat );
vertices[vertexIndex++] = coordinate;

// re-initialize charCount
charCount = 0;
}
}
}

// if we are reading a normal
if ( strncmp( lineBuffer, "vn ", 3 ) == 0 )
{

// initialize charCount
charCount = 0;

// loop through each char in the line
for ( count = 3; count < (int)strlen( lineBuffer ); count++ )
{

// if we are reading part of a float coordinate
if ( ( lineBuffer[count] == '-' ) || ( isdigit( lineBuffer[count] ) ) || ( lineBuffer[count] == '.') )
{
// store this char into temp
temp[charCount++] = lineBuffer[count];
}

// if we are not reading part of a float and a float exists in temp
else if ( charCount != 0 )
{
// copy the float coordinate to tempFloat
strncpy( tempFloat, temp, charCount );

//
tempFloat[ charCount ] = '\n';

// store the coordinate in our normals array
coordinate = atof( tempFloat );
normals[normalIndex++] = coordinate;

// re-initialize charCount
charCount = 0;
}
}
}

// if we are reading face info
if ( strncmp( lineBuffer, "f ", 2 ) == 0 )
{

// initialize charCount, isVertex and isFirstNormal
charCount = 0;
isVertex = 1;
isFirstNormal = 1;

// loop through each char in the line
for ( count = 2; count < (int)strlen( lineBuffer ); count++ )
{

// if we are reading part of an index
if ( isdigit( lineBuffer[count] ) )
{
// store this char into temp
temp[charCount++] = lineBuffer[count];
}

// if we are not reading part of an index and an index exists in temp
else if ( charCount != 0 )
{

// if the index is a vertex
if ( isVertex == 1 )
{

// copy the float coordinate to tempFloat
strncpy( tempFloat, temp, charCount );

//
tempFloat[ charCount ] = '\n';

// store the coordinate in our indices array
index = atof( tempFloat );
elements[elementIndex++] = (int)index;
}

// if the index is a normal
else
{

// if this is the first normal in the line
if ( isFirstNormal == 1 )
{
// copy the float coordinate to tempFloat
strncpy( tempFloat, temp, charCount );

//
tempFloat[ charCount ] = '\n';

// store the coordinate in our normals array
index = atof( tempFloat );
elements[elementIndex++] = (int)index;

// switch isFirstNormal
isFirstNormal *= -1;
}
}

// switch isVertex
isVertex *= -1;

// re-initialize charCount
charCount = 0;
}
}
}

}

// close file
fclose( objFile );

// return ok
return 1;
}


my draw routine:

// draw our model
glBegin( GL_TRIANGLES );

// go through each element
for ( i = 0; i < elementIndex; i += 4 )
{
// set offsets
normalOffset  = 3;
vertexOffset = 3;

// calculate index of x coordinate of current normal
currentNormalIndex = normalOffset * ( indices[i + 1] - 1 );

// calculate index of x coordinates of the three vertices
firstVertexIndex = vertexOffset * ( indices[ i + 1 ] - 1 );
secondVertexIndex = vertexOffset * ( indices[ i + 2 ] - 1 );
thirdVertexIndex = vertexOffset * ( indices[ i + 3 ] - 1 );

// set the normal for our triangle face
glNormal3f( normals[ normalOffset ],
normals[ normalOffset + 1 ],
normals[ normalOffset + 2 ] );

// set vertex 1
glVertex3f( vertices[ firstVertexIndex ],
vertices[ firstVertexIndex + 1 ],
vertices[ firstVertexIndex + 2 ]);

// set vertex 2
glVertex3f( vertices[ secondVertexIndex ],
vertices[ secondVertexIndex + 1 ],
vertices[ secondVertexIndex + 2 ]);

// set vertex 3
glVertex3f( vertices[ thirdVertexIndex ],
vertices[ thirdVertexIndex + 1 ],
vertices[ thirdVertexIndex + 2 ]);

}
glEnd();


The object was exported from blender using:

• selection only (current model)
• include normals
• triangulate faces
• keep vertex order

I understand that there are alternative ways to import and draw an .obj file but this will help me understand OpenGL and 3D objects better since I'm new to both 3D modelling and OpenGL.

Can someone help through this?

arigatou

EDIT: the indices are stored like this: [vector], [vector normal], [vector], [vector] for each line and I use the same vector normal for the 3 vectors to save space

Edited by hikarihe
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The indices in .OBJ files are 1-based.
What is the logic behind “( indices[i + 1] - 1 )”?

L. Spiro Edited by L. Spiro
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thanks ^^ it works. my mistake. should I delete the post?

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No. Others may gain insight from it.

L. Spiro
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thanks ^^ it works. my mistake. should I delete the post?

hi, how did you fix it pls? I have a similar issue

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thanks ^^ it works. my mistake. should I delete the post?

hi, how did you fix it pls? I have a similar issue

as lspiro said, the indices are 1 based in obj files, in C and C++ (and quite many other languages aswell) arrays start at 0, so instead of:
glVertex3f( vertices[ firstVertexIndex ],
vertices[ firstVertexIndex + 1 ],
vertices[ firstVertexIndex + 2 ]);

you do:
glVertex3f( vertices[ firstVertexIndex -1 ],
vertices[ firstVertexIndex ],
vertices[ firstVertexIndex + 1 ]);

(if you use vertex arrays or VBOs you do the equivalent operation where you initialize your array/vbo instead)
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thanks ^^ it works. my mistake. should I delete the post?

hi, how did you fix it pls? I have a similar issue

// calculate index of x coordinate of current normal
currentNormalIndex = normalOffset * ( indices[i + 1] - 1 );

// calculate index of x coordinates of the three vertices
firstVertexIndex = vertexOffset * ( indices[ i + 1 ] - 1 ); <<<<< here, replace with ( indices[ i ] - 1 )
secondVertexIndex = vertexOffset * ( indices[ i + 2 ] - 1 );
thirdVertexIndex = vertexOffset * ( indices[ i + 3 ] - 1 );

that's it. rest of the code is fine, but could be cleaned up ^ ^'

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thanks ^^ it works. my mistake. should I delete the post?

hi, how did you fix it pls? I have a similar issue

as lspiro said, the indices are 1 based in obj files, in C and C++ (and quite many other languages aswell) arrays start at 0, so instead of:
glVertex3f( vertices[ firstVertexIndex ],
vertices[ firstVertexIndex + 1 ],
vertices[ firstVertexIndex + 2 ]);

you do:
glVertex3f( vertices[ firstVertexIndex -1 ],
vertices[ firstVertexIndex ],
vertices[ firstVertexIndex + 1 ]);

(if you use vertex arrays or VBOs you do the equivalent operation where you initialize your array/vbo instead)

You can go to the root of the problem instead and do -1 when storing:

Store indices:

// if the index is a normal
else
{

// increment currentNormal
currentNormal++;

// if this is the third normal in the line
if ( currentNormal == 3 )
{
// copy the float coordinate to tempFloat
strncpy_s( tempFloat, 16, temp, charCount );

//
tempFloat[ charCount ] = '\n';

// store the coordinate in indices array
index = atoi( tempFloat );
elements[(*elementIndex)++] = index - 1;

// set current normal to 0
currentNormal = 0;
}
} 

Draw:

// calculate index of x coordinate of current normal
currentNormalIndex = normalOffset * ( indices[i + 3] );

// calculate index of x coordinates of the three vertices
firstVertexIndex = vertexOffset * ( indices[ i ] );
secondVertexIndex = vertexOffset * ( indices[ i + 1 ] );
thirdVertexIndex = vertexOffset * ( indices[ i + 2 ] );


looks much cleaner! ^w^

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Hi,

I am trying to import chess pieces from 3dsmax to opengl.

However, I am quite new to opengl and it has been a big challenge so far.

Any help will be appreciated?

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Hi,

I am trying to import chess pieces from 3dsmax to opengl.

However, I am quite new to opengl and it has been a big challenge so far.

Any help will be appreciated?

I recommend starting a new thread for this question, then it can get more specific attention.

I think the proper word you want to use is "export".  Export your 3D model to a 3D format that's well documented such as collada (.dae), object (.obj), .3ds, etc.  I don't have 3dsmax so I don't know what types of exports it supports; I use Blender.  Depending on the format you choose to export, you may have to write your own loader for the file and parse it manually, then format the vertex data into a compatible format that OpenGL can use.

The answer to your question is not a simple one.  Do you have a specific export format in mind?  And how far with OpenGL have you gotten already?  You may be biting off more than you can chew; I'm asking because I don't quite understand what is so difficult for you.

Shogun.

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