Blending and Fog

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Blending and Fog
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Creating Translucent Surfaces

• alpha value of 1 solid
• alpha value of 0 clear

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• glEnable ( GL_BLEND )
• glBlendFunc ( source pixel, destination pixel )
• source incoming pixel, destination existing
  pixel
• GL_ONE
• GL_ZERO
• GL_SRC_ALPHA
• GL_DST_ALPHA
• GL_ONE_MINUS_SRC_ALPHA
• etc
• Default is to not blend
• glBlendFunc (GL_ONE, GL_ZERO)
• Most Common
• glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)

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Example Blend

• Using glBlendFunc ( GL_SRC_ALPHA,
  GL_ONE_MINUS_SRC_ALPHA )
• first draw the color 1.0, 1.0, 0.0, .75
• followed by 0.0, 1.0, 1.0, .75
• yields color .25, 1.0, .75,
  0.9375
• 25 of dest (existing), plus 75 of src
  (incoming)
• reversing the order of the drawing colors yields
  .75, 1.0, .25, 0.9375

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Example cont
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Blending in 3D

• Suppose we have solid green and red boxes and
  clear blue and orange boxes. Where blue overlaps
  green we should blend. Where green overlaps
  orange, we should replace.

Z axis
view from above
projection plane
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How to Blend in 3D

• turn on Depth Test
• draw all opaque objects
• make depth buffer read-only
• glDepthMask(GL_FALSE)
• draw all translucent objects
• all the opaque objects are depth sorted as usual
• since the depth test is on, any translucent
  object that is behind a solid will not be drawn
• translucent pixels drawn in front of a solid will
  have the source blended into the destination

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Cool Blending EffectsShiny Glass

• render all opaque objects
• disable lighting
• enable blending
• render glass polygon in very light grey to create
  a faint appearance
• set ambient and diffuse material properties to
  zero
• turn up the specular material property
• enable lighting
• glDepthFunc (GL_EQUAL)
• render glass again

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Cool Blending EffectsTrees

• apply a texture that looks like leaves, and has
  lots of different alpha values
• so some light goes through, other parts are solid
• draw several of the polygons at different angles

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Fog

• when fog is turned on, objects in the distance
  have less color
• To turn fog on
• glEnable ( GL_FOG )
• glFogfv (GL_FOG_COLOR, color)
• probably dark gray

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Amount of Fog Effect

• glFogi ( GL_FOG_MODE, mode)
• GL_LINEAR
• GL_EXP (default)
• GL_EXP2
• Linear fog amount
• exponent fog amount e -(density z)
• glFogf ( GL_FOG_START, frontz)
• glFogf ( GL_FOG_END, backz)
• glFogf ( GL_FOG_DENSISTY, 0.3)