Primitive Attributes

• Attributes are properties that you assign to a primitive.
• Color is one attribute that all primitives share. Others are dependent on the object.
• Points have a single attributes, size
  • This is set with glPointSize
  • It takes a single float parameter, which must be positive
  • And if it is not an int, it is rounded to one.
  • Remember this cannot be inside of a glBegin-glEnd pair (it really doesn't work)
  • You can turn antialiasing on to make "round" points.
  • glEnable(GL_POINT_SMOOTH)
  • or turn it off with glDisable(GL_POINT_SMOOTH)
  • This is one of those switch things, on until your turn it off.
  • It is off by default.
  • More on antialiasing when we learn to draw lines (for real).
• Lines have more attributes
  • Line Width, (glLineWidth).
  • Like points, this is a float value, that is rounded to integer.
  • And can be antialiased (GL_LINE_SMOOTH)
  • Line stipple, or pattern
  • This must be enabled/disabled
  • glEnable(GL_LINE_STIPPLE)
  • The command is glLineStipple(repeat, pattern)
  • The first parameter is an integer between 1 and 256
  • It is a scale factor.
  • The second is a int, between 0 and 0xFFFF
  • it specifies a bit mask for drawing the line.
  • a pattern of 0x1000 would draw the first pixel on and the next 15 off.
  • a pattern of 0xAAAA would be every other pixel on, starting with the first.
  • Stipples mark fragments as not drawn, and thus are not sent to the fragment processor.
  • The pattern starts over at 0 when a glBegin is called
• Polygons
  • glPolygonMode(face, mode)
  • Allows to draw polygons as (mode parameter)
    • points (GL_POINT)
    • lines (GL_LINE)
    • filled (GL_FILL)
  • And this effects
    • The front face (GL_FRONT)
    • The back face (GL_BACK)
    • or both (GL_FRONT_AND_BACK)
  • Front and rear is determined by the direction the points are specified.
  • You can determine this.
    • glFrontFace(mode)
    • GL_CCW, counter clockwise
    • GL_CW , clockwise
  • if 1/2∑ _i=0ⁱ⁼ⁿ x_iy_i+1 -x_i+1y_i > 0, it is CCW front
  • if 1/2∑ _i=0ⁱ⁼ⁿ x_iy_i+1 -x_i+1y_i < 0, it is CW front
  • Try the triangle (0,0), (1,1), (2,0)
  • Polygons can be stippled as well.
    • glPolygonStipple(const GLUubyte * mask)
    • the mask is a 32x32 bitmask.
    • GLubyet mask[] = { 0x00, 0x01, 0x04, ...};
    • This is upside down. (0,0) maps to the bottom of the picture.
    • This must be enabled
    • glEnable(GL_POLYGON_STIPPLE);
• Some example code