Attributes of Graphics Primitives

1
Attributes of Graphics Primitives
2
Agenda

• OpenGL state variables
• Color and gray scale
• Color functions
• Point attributes and functions
• Line attributes and functions
• Curve attributes and functions
• Character attributes
• Antialiasing

3
Learning Objectives

• Learn OpenGL state variables
• Understand what are the main attributes of
  graphic primitives
• color, antialiasing,
• point attributes,
• line attributes,
• curve attributes,
• character attributes

4
OpenGL State Variables

• State parameters are attributes of the output
  primitives.
• In OpenGL, state parameters include
• Color
• Current matrix mode
• Elements of the Modelview matrix
• Current position of frame buffer
• Lighting effects parameters.
• Changes affect next operations.

5
Color and Gray Scale

• RGB color buffer
• Store RGB color codes in frame buffer or in a
  separate table
• Color code placed at pixel location
• Minimum number of colors for 3-bits Left bit ?
  Red, Middle bit ? Green, Left bit ? Blue0 ?
  black, 1 ? blue, , 7 ? white
• More color bits increase the size of the frame
  buffer 24-bits 1024 1024 3MB

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Color and Gray Scale

• Color tables
• Color map (color look-up table)
• Each pixel in the frame buffer contains a value
  (Ex. 196) corresponding to an entry in the color
  table (Ex. 2081), which has 256 entries.
• Color table contains a 3-byte color code that
  represents an RGB color 1-byte for the Red,
  1-byte for the Green, 1-byte for the Blue.
• Pick 256 colors from a total of 17 million, and
  save storage 1024 1024 1MB

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Color and Gray Scale
(from Donald Hearn and Pauline Baker)
8
Color and Gray Scale

• Gray scale
• Shades of gray0 ? dark gray, 1 ? light gray
• Other color parameters
• Intensity (amount of light energy)
• Luminance (perceived brightness)

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Color Functions

• Set color display mode to RGBglutInitDisplayMode(
  GLUT_SINGLE GLUT_RGB)
• GLUT_SINGLE or GLUT_DOUBLE
• GLUT_RGB, GLUT_RGBA, GLUT_INDEX
• gl.glEnable(GL.GL_COLOR_RGB) // in JOGL
• Alpha value (RGBA) for color blending of
  overlapping primitives (transparent or not)
• Affects the color state from that function call
  on (not retroactive)
• gl.glColor
• means b (byte), i (int), s (short), d (double),
  f (float)
• default (1.0f, 1.0f, 1.0f, 1.0f) for white

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Color Functions

• public void triangleColor ()
• // displays a three-colored triangle
• gl.glClear (GL.GL_COLOR_BUFFER_BIT) //
  apply clear color to color buffer
• gl.glBegin(GL.GL_TRIANGLES) // Drawing
  Using Triangles
• gl.glColor3f(1.0f, 0.0f, 0.0f) // set
  color to red
• gl.glVertex2i( 100, 150) // top
• gl.glColor3f(0.0f, 1.0f, 0.0f) // set
  color to green
• gl.glVertex2i(10,10) // bottom Left
• gl.glColor3f(0.0f, 0.0f, 1.0f) // set
  color to blue
• gl.glVertex2i(180,15) // bottom
  right
• gl.glEnd() // finished
  drawing the triangle

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Color Functions
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Color Functions

• Point center new Point(100, 75)
• float CLUT 1.0f,0.0f,0.0f,
  1.0f,0.5f,0.0f,
• 1.0f,1.0f,0.0f,0.0f,1.0f,0.0f,0.0f,0.0f,1.0
  f,
• 1.0f,0.0f,1.0f, 0.5f,0.0f,0.5f,
  1.0f,0.0f,0.5f
• Point vertex new Point 8
• for (k 0 k lt 8 k)
• theta Math.PI 2 k / 8.0
• vertexk new Point()
• vertexk.setLocation(
  (int) (center.getX() 50 Math.cos (theta)),
• (int) (center.getY() 50
  Math.sin (theta)))
• gl.glBegin (GL.GL_POLYGON)
• for (k 0 k lt 8 k)
• gl.glColor3fv (CLUTk)
• gl.glVertex2d (vertexk.getX(),
  vertexk.getY())
• gl.glEnd ( ) // finished drawing The
  Triangle

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Color Functions
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Point Attributes and Functions

• Attributes are any parameters that affect the way
  graphic primitives are displayed.
• Basic attributes
• Color
• Size
• Point
• Color
• Size is a multiple of the pixel size (a square).
• gl.glPointSize (size) // size is the number of
  pixels

15
Line Attributes and Functions

• Line attributes
• Basic attributes color, width, style
• Widthcan be set in the line drawing algorithm as
  well as the length and point spacing.Raster
  algorithms display line attributes by plotting
  pixel spans, like plotting multiple pixels along
  the line either horizontally (mlt1)of vertically
  (mgt1).Produces lines of slightly different width
  according to the slope, and also abrupt ends
  (horizontal or vertical).Add line caps at the
  end (butt cap, round cap, projecting square cap).

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Line Attributes and Functions

• Line attributes
• Widththick polylines with previous method do not
  connect smoothly
• miter join extend the outer boundaries of each
  segment until they join
• round join cap connection with circular boundary
• bevel join display line segments with butt caps
  and fill-in triangular gap between the two butts.
  

17
Line Attributes and Functions

• Line attributes
• Style
• Solid lines, dashed lines, dotted lines can be
  generated by modifying the line drawing algorithm
  spacing between drawn pixels
• Can be also generated from the raster by using a
  pixel mask, for example 11111000 will display a
  dashed line with dash length of 5 and inter-dash
  length of 3.

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Line Attributes and Functions

• Line attributes
• Pen and brush options
• Select several pen and brush styles
• Shape, size, pattern
• Can be implemented in a pixel mask
• Change the mask to change the pen/brush

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Line Attributes and Functions

• Line attribute functions
• gl.glLineWidth (width) // width is the width
  of the line
• Default width is 1.0
• gl.glLineStipple (repeatFactor, pattern)
• pattern references a 16-bit integer that
  describes how the line should be displayed
  (default 0xFFFF, all bits set to 1, solid line)
• repeatFactor specifies how many times each bit in
  the pattern is to be repeated before the next bit
  in the pattern in applied (default 1).
• Example displays 0x00FF -------- --------
  -------- 8 solid line pixels,
  followed by 8 spaces
• Requires first glEnable (GL_LINE_STIPPLE)
  line-pattern feature can be disabled by
  glDisable (GL_LINE_STIPPLE)
• gl.glShadeModel (GL.GL_SMOOTH) // linear
  interpolation of colors

20
Line Attributes and Functions
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Curve Attributes and Functions

• Same parameters as for lines
• Width
• Color
• Dot-dash patterns
• Pen/brush options
• Raster curves produced by horizontal/vertical span

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Character Attributes

• Character attributes
• Font (typeface)Helvetica, Courier, Times Roman,
  
• Underlining stylesolid, dotted, double
• Styleboldface, italics, outline, shadow
• Size10-point, 12-point,
• Color.

23
Character Attributes
(from Donald Hearn and Pauline Baker)
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Character Attributes

• Character attributes
• Font (typeface)Helvetica, Courier, Times Roman,
  
• Underlining stylesolid, dotted, double
• Styleboldface, italics, outline, shadow
• Size10-point, 12-point,
• Color
• Direction (up vector)

25
Character Attributes

• Two methods
• With bitmap functionscreate images of each
  character
• With GLUT character generating routinesavailable
  in OpenGL from GLUT classBITMAP_HELVETICA_10BITM
  AP_HELVETICA_12BITMAP_HELVETICA_18BITMAP_TIMES_R
  OMAN_10BITMAP_TIMES_ROMAN_24glutBitmapCharacte
  r, glutBitmapString, glutBitmapLength,
  glutBitmapWidth

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Antialiasing

• Used to improve the appearance of lines.
• gl.glEnable ( primitive_type )primitive_typeGL_
  POINT_SMOOTHGL_LINE_SMOOTHGL_POLYGON_SMOOTH
• Color-blendinggl.glEnable (GL.GL_BLEND)gl.glBlen
  dFunc ( GL.GL_SRC_ALPHA, GL.GL_ONE_MINUS_SRC_ALPHA
  )