Frame Buffer Operations

1
Frame Buffer Operations

• Obscure Tricks
• and Cool Visual Effects

2
Operations on Fragments

• Depth Test
• Blending
• Scissor Test
• Alpha Test
• Stencil Test
• Dithering
• Logical Operations

3
Scissor Test

• glEnable(GL_SCISSOR_TEST)
• glScissor ( x, y, width, height)
• integer window coordinates to draw into
• x,y lower left corner

? Uses ?
4
Alpha Test syntax

• glEnable (GL_ALPHATEST)
• glAlphaFunc ( test, compare_value )
• compare_value is float 0 to 1
• test is function such as
• GL_LESS - accept fragment if alpha lt value
• GL_EQUAL - accept fragment is alpha value
• GL_NOTEQUAL - accept fragment if alpha ? value
• GL_LEQUAL - lt
• GL_NEVER - never accept any fragment
• GL_ALWAYS - always accept every fragment
• etc

5
Alpha Test uses

• Given our tree made up of a texture of leaves
  with various alpha values, make the leaves slowly
  disappear by
• slowing changing the alpha values that are
  accepted.
• If you always want something to show up, but
  still need to use the depth test
• draw scene once with depth test on,
• then turn depth test off,
• turn on alpha test for a particular value,
• redraw scene

6
Alpha Test uses
Gets bigger and bigger

• A star texture that changes
• create a texture where every texel has the same
  RGB,
• but vary the Alpha values
• apply the texture (don't use blending)
• during each drawing, use GL_LESS with different
  value, so that different areas appear

Create a texture of wavy lines?
7
Stenciling uses
Isn't this just the scissor test?
http//www.uweb.ucsb.edu/crashnburnjess/For20Jes
s/Kitchen-Window.jpg

• My animation is viewed through a kitchen window,
  so the window frame and curtains should always
  show up.
• Draw the window pieces once.
• Draw the window pieces into the stencil buffer.
• Draw the backyard over and over (only the
  fragments that are on the stencil will so up)

8
Stenciling uses

• Draw Tree and Box
• Before drawing shadow, create a mask that matches
  the front of the box.

9
Example

• Draw a square in the stencil buffer.
• The wireframe pink cord is drawn in that mask, so
  only it appears in the square.
• The same pink cord is drawn filled on the scene
  with a different stencil test.
• http//www.sulaco.co.za/opengl4.htmstencil

10
How Stenciling Operates

• for each pixel being drawn into the color buffer
• compare a value to the corresponding pixel in
  stencil buffer
• only draw pixels into color buffer that pass the
  test
• perhaps change the stencil buffer pixel based on
  outcome of that test
• programmer indicates the value, the compare test,
  and what to do if the test passes or fails

11
Stencil Functions

• glEnable ( GL_STENCIL_TEST )
• glStencilFunc(test, compare_value, mask)
• glStencilOp (stencil_fail, stencil_pass_dept
  h_fail, stencil_pass_depth_pass)
• GL_KEEP, GL_REPLACE, GL_ZERO, GL_INVERT, GL_INCR,
  etc

12
Stencil Pseudo-Code

• fill stencil buffer with 0s
• draw desired shape into stencil buffer with 1s
• either use glDrawPixels to write straight into
  stencil buffer
• or draw into the color buffer with polygon
  functions, using glStencilOp to change the
  stencil bits
• set the reference value to 1 and the test
  function to equal (only draw pixels into color
  map where stencil is equal to 1)

13
Accumulation Buffer

• Motion Blur - leave an image trail
• redraw the scene over and over while moving an
  object
• between each drawing slightly dim the previous
  scene,
• glAccum(GL_MULT, decay)
• move the accumulation buffer into the color
  buffer using
• glAccum(GL_RETURN, 1.0)

14
Accumulation Buffer

• Depth of Field - make distant objects a little
  blury
• Draw the scene several times, each with a very
  slight change in projection. Hence, distant
  objects will move around a little more than near
  objects. This is "jittering".
• Add each projected scene to the accumulation
  buffer. For example, draw the scene 10 times
  with glAccum(GL_ACCUM, 0.1) so that each
  projection makes up 10 of the final image.
• Move the accumulation buffer into the color
  buffer.