Drawing objects from a vertex/edge/polygon list

• We will frequently need have data in the form of an vertex/XXX list
• Consider a cube:
  • There are 8 verticies.
  • There are 12 lines.
  • We probably can not construct an Euler cycle for most objects we are going to draw.
  • Or even something less, but as useful.
• So we tend to produce a vertex list
  • Each vertex is listed once.
  • The index for a given vertex is noted.
• And an edge list or polygon list
  • An edge is specified by two indexes into the vertex list
  • Or a number of sides, and the indexes to the corresponding verticies for a polygon.
  • Again, we like triangles
• This probably matches with you produce with your animation software (downstairs).
• Just Two new openGL things, one really.
  • glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, buffer[1]);
  • void glDrawElements(GLenum mode, GLsizei count, GLenum type, const GLvoid * indices);
    • Mode - how you want the primitive drawn
    • count - how many elements (not shapes or lines, but indicies)
    • type - type of the inidicies
      • GL_UNSIGNED_BYTE
      • GL_UNSIGNED_SHORT
      • GL_UNSIGNED_INT
    • indicies : offset to the beginning of the data.

  •      glBindBuffer(GL_ARRAY_BUFFER, buffer[0] );
         glBufferData(GL_ARRAY_BUFFER, verticies.size()*sizeof(vec3),
             verticies.data(), GL_STATIC_DRAW);
    
         loc = glGetAttribLocation( program, "in_Position");
         glVertexAttribPointer(loc, 3, GL_FLOAT, GL_FALSE, 0, 0);
         glEnableVertexAttribArray(0);
    
         glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, buffer[1]);
         glBufferData(GL_ELEMENT_ARRAY_BUFFER, triangles.size()*sizeof(int),
                      triangles.data(), GL_STATIC_DRAW);
    	  

  • And to draw:

         glBindVertexArray(myVAO);
         glDrawElements(GL_TRIANGLES, triangles.size(),
                     GL_UNSIGNED_SHORT, BUFFER_OFFSET(0));
         glBindVertexArray(0);
          

  Transformations

  • Some OpenGL Glue first
  • glUniform*
    • A uniform variable, in the shader language, does not change value across a set of primitives.
    • This call lets us set a value in the shader for all verticies, fragments, ...
  • GLint glGetUniformLocation(GLuint program, const GLchar *name);