More drawing techniques

• The loop

  for(i=0;i <Verticies.size(); i++) {
       glDrawElements(GL_TRIANGLE_FAN, Verticies[i].size,
                      GL_UNSIGNED_INT, Verticies[i].verticies);
  }
  
       

• Can be replaced with glMultiDrawElements
  • void glMultiDrawElements( GLenum mode, const GLsizei * count, GLenum type, const GLvoid ** indices, GLsizei primcount);
    • mode: as discussed.
    • count : An array of polygon sizes.
    • type: as before.
    • const GLvoid** indicies: an array of arrys of indicies.
    • primcount: the number of polygons.
  • My code is somewhat of a mess, I allocate memory for the object twice, but this could be avoided by two passes through the file.
  • Look at multiDrawElements.C
• vbo0.C draws a cardioid in the old way.
  • this gives us a familiar starting point.
  • hsv2rgb is the only really interesting part here.
    • The color space we are familiar with is RGB
    • But this has no way to transistion nicely between colors
    • HSV (Hue, Saturation, value) is a nice mode for this.
    • 
    • Hue is an angle along the outside of this cone.
    • Saturation goes from washed out to vibrant colors.
    • Value goes from dark to light.
    • I stole an HSV to RGB conversion routine (hsv.C)
    • Just allow an angle to move around the circle with fixed vaules for s and v.
    • Nice transitions between colors.
    • More on this later.
• vbo1.C draws the cardioid using VBOs, simple example
  • We could specify the data with glBufferData, but it is not all in the same array.
  • void glBufferSubData(GLenum target, GLintptr offset, GLsizeiptr size, const GLvoid * data);
    • target: GL_ARRAY_BUFFER or other.
    • offset: where in the destination array to start writing the data
    • size: the size of the data (in bytes) we will be writing.
    • data: a pointer to the base of the array where the data is stored.
  • glVertexPointer and glColorPointer
    • This is using the VBO since it is the currently bound buffer
    • This is due to the previous call to glBindBuffer
  • This routine also demostrates how to change the value in a buffer.
    • In the keypress routine.
    • void * glMapBuffer((GLenum target, GLenum access)
      • target GL_ARRAY_BUFFER, or others
      • access : GL_READ_ONLY, GL_WRITE_ONLY, GL_READ_WRITE
      • Returns a pointer or NULL on error
    • After you have changed the values
    • glUnmapBuffer(GLenum target)
    • Note, both of these are deprecated, but we will deal with that later.
• I will present more of this later.