Lecture 2/3

OpenGL: Texture Mapping

• Draw Quads in OpenGL

• Steps for texture mapping

  • Create texture object(s)

       GLuint texName[2];            // replace 2 (in the next line too) with the actual number of texture objects you want to create.
       glGenTextures(2, texName);
    

  • Initialize each texture object

       glBindTexture(GL_TEXTURE_2D, texName[0]);                     // or texName[1] to initialize the second texture object just created above.
       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);  // clamping if s texture val out range of [0,1]. The other common choise is GL_REPEAT.
       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);  // clamping if t texture val out range of [0,1]. The other common choise is GL_REPEAT.
    
       /* these are anti-aliasing related issue, you dont have to understand .*/
       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, 
                       GL_NEAREST);
       glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, 
                       GL_NEAREST);
    
       /*
          Now bind the actual image to be texture mapped for this texture object
          There are a few important things you have to be VERY careful,
             1. you must pass the correct data type of the color component in your image.
             2. the image size must be 2^m + b and >= 64, where ususally b = 0 as below,
       */
       glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, checkImageWidth,
                    checkImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE,
                    checkImage);
    

  • In you display() callback: Bind geometry to texture object, and bind vertexes to texture coordinates

       glBindTexture(GL_TEXTURE_2D, texName[0]);
       glBegin(...);
       ...
       glTexCoord2f(s, t); 
       glVertex3f(x, y, z);             // this vertex is related to the texture coordinate (s,t) just given.
    
       glEnd();
    

  • Texture mapping: Replace mode and Modulate mode

    Use this command to set to replace mode, in which case the pixel get color only from the texture image.

      glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
    

    and use this command to set to modulate mode, in which case the pixel get color from both the texture image and the color from either direct color specification or lighting computation.

      glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
    

  • Rendering text

    Text is also a geometry, exactly the same as triangles, quads, etc. The only difference is it has a different way to specify the space position of the character to be rendered.

      glRasterPos3f(-1, 0, -1);
      glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, 'A');
      glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, 'B');   // you dont have to specify space position for 'B'; it at the right side of 'A'. 
    

  • Write a string onto the bottom of the viewport

  • Write a string onto the top of the viewport

  • Write a string onto the middle of the viewport

  • Write a vertical string onto the middle of the viewport

• Use Canvas class to read in any ppm file and map to a quad

• Draw two geometries with different texture iamges

• Use a small image to repeatedly cover a quad

• Change TriangleMesh :: glDraw() to support texture mapping

  In display() we use this code to draw a texture mapped triangle.

      glBegin(GL_TRIANGLES);
      {    
        glTexCoord2f(0, 0); 
        glVertex3f( -2,  0, -2 );
        glTexCoord2f(1, 0); 
        glVertex3f(  2,  0, -2 );
        glTexCoord2f(0.5, 1); 
        glVertex3f(  0,  2, -2 );
      }
      glEnd();
  

  Now you do this, it should draw texture mapped triangles of the model.

     TriangleMesh msh("anobj.obj");
     msh.glDraw();
  

• Our triangle mesh is always at the center. Use glTranslatef() to move the triangle mesh into the viewing volume.

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