Visible Surface Detection

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Visible Surface Detection
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Agenda

• Visibility culling
• Hidden-surface elimination
• OpenGL visibility-detection functions

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Learning Objectives

• Understand how graphics systems perform
  visible-surface detection and hidden-surface
  elimination
• Understand the OpenGL functions for visibility
  detection

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Some Definitions

• Visible surface detection, or hidden surface
  elimination, refer to the methods and algorithms
  to determine which parts of which surfaces of a
  scene are visible from a chosen viewing position.

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Some Definitions

• Visibility culling refers to the detection of
  which surfaces are completely hidden with respect
  to the camera
• Hidden-surface eliminationrefers to the
  elimination of the parts of the surfaces which
  are hidden with respect to the camera
• Shadowsrefer to the determination of the parts
  of the surfaces which are hidden with respect to
  the light sources

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Visibility Culling

• Main methods
• View frustum culling do not draw objects
  outside of the view frustum
• Back face culling do not draw surfaces facing
  backwards from the camera
• Occlusion culling do not draw objects completed
  occluded by other objects

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Back Face Detection

• Back face detection
• Locates the back faces of a polyhedron
• Back faces should not be drawn
• Back faces are facing backwards with respect to
  camera
• Simple test Vview . N 0
• More simple test, if N (A, B, C), C

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Hidden Surface Elimination

• Main methods
• Object space by object comparison
• Depth sorting method
• Image space pixel by pixel
• Depth buffer method
• Scan line method
• Ray casting

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Depth Sorting Method

• Depth sorting method
• Also called the painters algorithm starts by
  background, then most distant objects, then
  nearest objects, and foreground is painted last
• Sorts the surfaces in order of increasing depth
• Surfaces are scan-converted in order, starting
  with the surface of greatest depth

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Depth Buffer Method

• Depth buffer method
• Also called the z-buffer method.
• Compares surface depth values throughout a scene
  for each pixel position on the projection plane.
• Only the pixel with the smallest depth value is
  shown.
• For each pixel, only the color of depth of the
  closest object is shown.
• Depths are interpolated from vertices just like
  colors.

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Scan Line Method

• Scan line method
• Scan line algorithm computes the depth values
  along the scan lines for a scene.
• For each scan line, algorithm constructs and
  sorts spans.
• For each scan line, algorithms sorts depths
  within each scan line.

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Ray Tracing Method

• Ray tracing method
• Traces a ray for every pixel
• Objects intersecting this ray are visible
• If ray intersects multiple objects, take the
  closest
• Only shades each pixel once
• Algorithm is slow

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Raytracing
From Alan Watt, 3D Computer Graphics
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OpenGL Visibility Detection Functions

• Polygon cullingglEnable (GL_CULL_FACE)glCullFa
  ce (mode) // mode GL_BACKthis removes the
  back faces GL_FRONT would remove the front
  faces, and GL_FRONT_AND_BACK removes all polygon
  surfaces.Default GL_BACKglDisable
  (GL_CULL_FACE)

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OpenGL Visibility Detection Functions

• Depth buffer functionsglutInitDisplayMode
  (GLUT_SINGLE GLUT_RGB GLUT_DEPTH)request
  depth bufferglClear(GL_DEPTH_BUFFER_BIT)initi
  alizes the depth bufferglEnable
  (GL_DEPTH_TEST)activates depth-buffer
  visibility detection routinesglDisable
  (GL_DEPTH_TEST) disables this

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OpenGL Visibility Detection Functions

• Wire-frame surface-visibility functionsglPolygon
  Mode (GL_FRONT_AND_BACK, GL.GL_LINE)request to
  generate only the edges of a polygon aka
  wireframeglEnable (GL_POLYGON_OFFSET_FILL)dep
  th-offset method to be used to eliminate the
  hidden edgesglDisable (GL_POLYGON_OFFSET_FILL)
  disables thisexample in Chap9Visible.java

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OpenGL Visibility Detection Functions