OBJ Model Loading (In Detail)

Ranger_One · 2011-08-17T15:59:39

As promised in a recent thread, here I will describe code that reads ascii OBJ files of 3d models. I will use simple C source that calls no libraries (except malloc/free, since I need memory control). Here we go... OBJ File Overview An OBJ file contains a static 3d model, and possibly some high-level curves/surfaces. Here I make no attempt to support any high-level command found in the standard. The information I used to design this simple loading code can be found here: Paul Bourke's OBJ Format Page. Each OBJ file is a collection of ascii text lines, each delimited by the standard characters. I use code that tests for character 10, the newline/linefeed, to determine the breaks. I've yet to encounter any problems with this method, both Unix and Windows include a newline character to delimit the end of line. The data contained in OBJ files is: Vertex coords, Vertex Normals, Vertex Texcoords, Faces, and Groups. There can be more information, though this is what we are interested in. All of the Vertex information becomes arrays which the Faces index. So, we have reached our first step. We need a structure to hold the OBJ file data after it is read. Here is what I use in the following code: typedef struct _ObjVertex { float X, Y, Z; } ObjVertex; typedef _ObjVertex _ObjNormal; typedef struct _ObjTexCoord { float U, V; } ObjTexCoord; typedef struct _ObjTriangle { int Vertex[3]; int Normal[3]; int TexCoord[3]; } ObjTriangle; typedef struct _ObjModel { int NumVertex, NumNormal, NumTexCoord, NumTriangle; ObjVertex *VertexArray; ObjNormal *NormalArray; ObjTexCoord *TexCoordArray; ObjTriangle *TriangleArray; } ObjModel; So, we have 5 components here. Counts of each array. 3 arrays for vertex information, and 1 more for the face indexes. I'm not going to spend more time explaining this. I assume you have some idea of how a 3d model is stored and rendered. Pitfalls when reading the file... Since the OBJ standard supplies no header, we encounter our first pitfall. There is no easy way to determine how many entries we need in each array. To overcome this pitfall we can take one of two approaches: waste memory, waste CPU. If we waste memory, we allocate and reallocate the arrays as we read the file. We'll end up with wasted memory that has to be recouped. If we waste CPU, we read the file twice. First scanning the number of entries needed, then allocating the arrays, and finally reading the data into them. I choose to waste CPU, since it feels like a more elegant solution to the problem. The code provided does the scan, allocate, and read approach. Here is the exported function from this code: ObjModel* ObjLoadModel(char *mem, int sz); It reads the model from a chunk of memory, presumably read verbatim from an .OBJ file. We also need some utility code, we is used internally to make everything clean. I could have used some of the std C library functions, but hey- I'm on a no C std lib kick. Here they are: int StrEqual(char *A, char *B, char *E, int count); int IsNumeric(char A); int IsDeadSpace(char A); int IsEOL(char A); We'll supply the C code of these utilities first: /* Return 1 if strings are equal, 0 if not */ /* Make sure we don't ever pass the given end (E) */ int StrEqual(char *A, char *B, char *E, int count) { int c; c = 0; while ((c < count) && (A != E)) { if (A[c] != B[c]) return 0; c++; } if (A == E) return 0; else return 1; } /* Return 1 if the character is dead space, 0 if not */ int IsNumeric(char A) { if (A == '.') return 1; if (A == '-') return 1; if ((A >= 0x30) && (A <= 0x39)) return 1; return 0; } /* Return 1 if the character is dead space, 0 if not */ int IsDeadSpace(char A) { if (A < 33) return 1; else return 0; } /* Return 1 if the character is a newline/linefeed, 0 if not */ int IsEOL(char A) { if (A == 10) return 1; else return 0; } Next post: The initial scan...

Graphics and GPU Programming Programming

Started by Ranger_One April 09, 2005 04:16 PM

15 comments, last by RobTheBloke 12 years, 8 months ago

sjf

130

April 11, 2005 06:31 AM

Ranger,

Your source had a couple of bugs (DL version). And a couple a nit-picks of mine.
I cleaned up the code for improved readability. Changed a few naming conventions to std notation. Increased speed (very minor in the read / scan loops) (just make sure the file ends with a blank line!!!). Reduced code (with use of std lib's). I also added free to release the objects. Hope you don't take offence and rain on your parade.

main.cpp

#include <stdlib.h>#include <stdio.h>#include "obj.h"int main(){   char* memory = NULL;   size_t bytes = ObjLoadFile("tank.obj", &memory);   ObjModel* model = ObjLoadModel(memory, bytes);   printf("Object Model has: %d faces!\n", model->nTriangle);      free(model->NormalArray);   free(model->TexCoordArray);   free(model->TriangleArray);   free(model->VertexArray);   free(model);   system("PAUSE");      return 0;}

obj.cpp

#include <stdlib.h>#include <stdio.h>#include <memory.h>#include "obj.h"ObjModel* ObjLoadModel(char* memory, size_t size){	char* p = NULL, * e = NULL;   ObjModel* ret = (ObjModel*) calloc(1, sizeof(ObjModel));   memset(ret, 0, sizeof(ObjModel));	p = memory;	e = memory + size;		while (p != e)	{           if (memcmp(p, "vn", 2) == 0) ret->nNormal++;      else if (memcmp(p, "vt", 2) == 0) ret->nTexCoord++;      else if (memcmp(p, "v",  1) == 0) ret->nVertex++;      else if (memcmp(p, "f",  1) == 0) ret->nTriangle++;      while (*p++ != (char) 0x0A);   }   ret->VertexArray   = (ObjVertex*)   malloc(sizeof(ObjVertex) * ret->nVertex);   ret->NormalArray   = (ObjNormal*)   malloc(sizeof(ObjNormal) * ret->nNormal);   ret->TexCoordArray = (ObjTexCoord*) malloc(sizeof(ObjTexCoord) * ret->nTexCoord);   ret->TriangleArray = (ObjTriangle*) malloc(sizeof(ObjTriangle) * ret->nTriangle);	p = memory;	   int nV = 0, nN = 0, nT = 0, nF = 0;		while (p != e)	{      if (memcmp(p, "vn", 2) == 0)      {         sscanf(p, "vn %f %f %f", &ret->NormalArray[nN].x,                                  &ret->NormalArray[nN].y,                                  &ret->NormalArray[nN].z);         nN++;      }      else if (memcmp(p, "vt", 2) == 0)      {         sscanf(p, "vt %f %f", &ret->TexCoordArray[nT].u,                               &ret->TexCoordArray[nT].v);         nT++;      }      else if (memcmp(p, "v", 1) == 0) /* or *p == 'v' */      {         sscanf(p, "v %f %f %f", &ret->VertexArray[nV].x,                                 &ret->VertexArray[nV].y,                                 &ret->VertexArray[nV].z);         nV++;      }      else if (memcmp(p, "f", 1) == 0) /* or *p == 'f' */      {         sscanf(p, "f %d/%d/%d %d/%d/%d %d/%d/%d", &ret->TriangleArray[nF].Vertex[0],                                                   &ret->TriangleArray[nF].TexCoord[0],                                                   &ret->TriangleArray[nF].Normal[0],                                                   &ret->TriangleArray[nF].Vertex[1],                                                   &ret->TriangleArray[nF].TexCoord[1],                                                   &ret->TriangleArray[nF].Normal[1],                                                   &ret->TriangleArray[nF].Vertex[2],                                                   &ret->TriangleArray[nF].TexCoord[2],                                                   &ret->TriangleArray[nF].Normal[2]);         nF++;      }      while (*p++ != (char) 0x0A);   }        return ret;}size_t ObjLoadFile(char* szFileName, char** memory){	size_t bytes = 0;	FILE* file = fopen(szFileName, "rt");	if (file != NULL)   {	   fseek(file, 0, SEEK_END);	   size_t end = ftell(file);	   fseek(file, 0, SEEK_SET);   		   *memory = (char*) malloc(end);	   bytes = fread(*memory, sizeof(char), end, file);	   fclose(file);   }	return bytes;}

obj.h

#ifndef OBJ_H#define OBJ_Hstruct ObjVertex{   float x, y, z;};typedef ObjVertex ObjNormal;struct ObjTexCoord{   float u, v;};struct ObjTriangle{   int Vertex[3];   int Normal[3];   int TexCoord[3];};struct ObjModel{   int nVertex, nNormal, nTexCoord, nTriangle;   ObjVertex* VertexArray;   ObjNormal* NormalArray;   ObjTexCoord* TexCoordArray;   ObjTriangle* TriangleArray;};ObjModel* ObjLoadModel(char*, size_t);size_t    ObjLoadFile(char*, char**);#endif

justinrwalsh

171

April 11, 2005 01:27 PM

Ok the C++ version of the code is complete...
sjf i haven't reviewed your code yet to see what improvements need to be made.

There are somethings i noticed about the obj format that i think we should include...

Support for faces that have more than 3 vertex/textureCoord/normal sets.
I modeled something in blender and it hade a few with like 6 f v/vt/vn data sets in a row.

Well here is the link to what I got.. it has been zipped for ease of downloading, i'll include the VC.NET Project files as well.

objLoaderTest.zip

and while i'm at it... how about you just look at my simple un updated webpage...
Justin Walsh's Homepage

thanks to all who help and have helped.

Now one last question...
How would you go about drawing all this shit in OpenGL?
can i just traingle strip it up from the first face in the array, to the last?

Ranger_One

300

Author

April 11, 2005 02:07 PM

sjf,

I take no offense :) I did however write the code for clarity, and not for speed. I wanted to make sure you could see the process going on easily.

Justinrwalsh,

Yes, we need support for polygon faces. It would be quite simple to triangulate them, and we could use a couple different approaches to do so. Otherwise you could tell your modeller to triangulate the model before saving it (assuming that can be done). Groups are pretty useful too, and easy to add. All this would have made the code more unclear, and clarity was my prime objective.

Drawing the model is as simple as:

glBegin(GL_TRIANGLES);
... Iterate over faces, drawing each triangle.
glEnd()

Thanks! I appreciate the help.

Genjix

100

April 11, 2005 03:54 PM

*bump*

justinrwalsh

171

April 11, 2005 04:24 PM

giant error in my source...

Uploading fixed code...

[Edited by - justinrwalsh on April 11, 2005 4:24:36 PM]

Starfox

504

August 16, 2011 10:22 PM

What's the license for this?

Holy crap I started a blog - http://unobvious.typepad.com/

RobTheBloke

2,553

August 17, 2011 03:59 PM

How would you go about drawing all this shit in OpenGL?[/quote]

Step by step guide to obj files in C (Triangulates inputs meshes [in a simplistic way that can fail in some cases] with VertexArray and TBN generation). Starts simple, gets very complicated towards the end. All of this written in the days before shaders though, so the Dot3 example may want converting to GLSL instead,
http://nccastaff.bou...e/ObjLoader.zip

basic C++ library (with beziers, materials, groups and lots of other stuff)
http://nccastaff.bou.../www/libObj.zip

\edit.
Wow. That's a very old thread you dug up there ;)

OBJ Model Loading (In Detail)

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OBJ Model Loading (In Detail)

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