OpenGL loading OBJ model, texture distortion

Question

I decided to import Wavefront .OBJ format to a test-scene that I'm working on. I get the model (vertices) to be in the right place and it displays fine. When I then apply a texture a lot of things looks distorted. I checked my Maya scene (there it looks good), and the object has many more uv-coordinates than vertex positions (this is what makes the scene looks weird in OpenGL, is my guess).

How would I go about loading a scene like that. Do I need to duplicate vertices and how do I store it in the vertex-buffer object?

Which language is your operating system configured for? Perhaps your .OBJ parser is using the decimal separator of that language. In english, period (.) is used as decimal separator but in other languages such as swedish and french, comma (,) is the standard. So if the .obj file is encoded using period as decimal separator but you parse it using comma, you will end up with the wrong coordinates. — Rulle

PeterT PeterT · Accepted Answer · 2011-11-19T14:53:23

You are right that you have to duplicate the vertices. In addition to that you have to sort them in draw order, meaning that you have to order the vertices with the same offsets as the texture coordinates and normals. basically you'll need this kind of structure:

float *verts = {v1_x,v1_y,v1_z,v1_w,v2_x,v2_y,v2_z,v2_w,...};
float *normals = {n1_x,n1_y,n1_z,n2_x,n2_y,n2_z,...};
float *texcoords = {t1_u,t1_v,t1_w,t2_u,t2_v,t2_w,...};

This however would mean that you have at least 108bytes per Triangle.

 3(vert,norm,tex)
*3(xyz/uvw)
*3(points in tri)
*4(bytes in a float)) 
-----------------------
= 108

You can significantly reduce that number by only duplicating the vertices that actually are duplicate (have identical texture coordinate,vertices and normals meaning: smoothed normals and no UV borders) and using an Index Buffer Object to set the draw order.

I faced the same problem recently in a small project and I just split the models along the hard-edges and UV-Shell borders therefore creating only the necessary duplicate Vertices. Then I used the glm.h and glm.cpp from Nate Robins and copied/sorted the normals and texture coordinates in the same order as the vertices.

Then setup the VBO and IBO:

//this is for Data that does not change dynamically
//GL_DYNAMIC_DRAW and others are available
GLuint mDrawMode = GL_STATIC_DRAW;

//////////////////////////////////////////////////////////
//Setup the VBO
//////////////////////////////////////////////////////////
  GLuint mId;
  glGenBuffers(1, &mId);
  glBindBuffer(GL_ARRAY_BUFFER, mId);
  glBufferData(GL_ARRAY_BUFFER,
               mMaxNumberOfVertices * (mVertexBlockSize + mNormalBlockSize + mColorBlockSize + mTexCoordBlockSize),
               0,
               mDrawMode);
  glBufferSubData(GL_ARRAY_BUFFER, mVertexOffset, numberOfVertsToStore * mVertexBlockSize, vertices);
  glBufferSubData(GL_ARRAY_BUFFER, mNormalOffset, numberOfVertsToStore * mNormalBlockSize, normals);
  glBufferSubData(GL_ARRAY_BUFFER, mColorOffset, numberOfVertsToStore * mColorBlockSize, colors);
  glBufferSubData(GL_ARRAY_BUFFER, mTexCoordOffset, numberOfVertsToStore * mTexCoordBlockSize, texCoords);

//////////////////////////////////////////////////////////
//Setup the IBO
//////////////////////////////////////////////////////////
  GLuint IBOId;
  glGenBuffers(1, &IBOId);
  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBOId);
  glBufferData(GL_ELEMENT_ARRAY_BUFFER, mMaxNumberOfIndices * sizeof(GLuint), 0, mDrawMode);
  glBufferSubData(GL_ELEMENT_ARRAY_BUFFER, 0, numberOfIndicesToStore * sizeof(GLuint), indices);

//////////////////////////////////////////////////////////
//This is how to draw the object  
//////////////////////////////////////////////////////////
  glBindBuffer(GL_ARRAY_BUFFER, mId);
  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBOId);

  //Enables and Disables are only necessary each draw
  //when they change between objects
  glEnableClientState(GL_VERTEX_ARRAY);
  glVertexPointer(mVertexComponents, GL_FLOAT, 0, (void*)mVertexOffset);
  if(mNormalBlockSize){
    glEnableClientState(GL_NORMAL_ARRAY);
    glNormalPointer(GL_FLOAT, 0, (void*)mNormalOffset);
  }
  if(mColorBlockSize){
    glEnableClientState(GL_COLOR_ARRAY);
    glColorPointer(mColorComponents, GL_FLOAT, 0, (void*)mColorOffset);
  }
  if(mTexCoordBlockSize){
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glTexCoordPointer(mTexCoordComponents, GL_FLOAT, 0, (void*)mTexCoordOffset);
  }

  glDrawRangeElements(primMode,
                      idFirstVertex,
                      idLastVertex,
                      idLastVertex - idFirstVertex + 1,
                      mAttachedIndexBuffer->getDataType(),
                      0);

  if(mTexCoordBlockSize)
    glDisableClientState(GL_TEXTURE_COORD_ARRAY);
  if(mColorBlockSize)
    glDisableClientState(GL_COLOR_ARRAY);
  if(mNormalBlockSize)
    glDisableClientState(GL_NORMAL_ARRAY);
  glDisableClientState(GL_VERTEX_ARRAY);

OpenGL loading OBJ model, texture distortion

1 Answers