Visibility Queries Using Graphics Hardware

1
Visibility Queries Using Graphics Hardware

• Presented by Jinzhu Gao

2
Motivation

• Visibility queries is popular used in different
  applications
• 70 of total computation time in hierarchical
  radiosity
• Graphics hardware provides fast access to the
  data computed by the graphics hardware

3
Point-based Visibility Query

• Visibility queries with the same starting point
  and different ending points
• For example
• Eye ray-tracing
• A point light source illuminating a scene

4
Visibility Methods

• Ray casting
• Z-buffer
• Using object ID image
• HP Extensions
• NV Extensions

5
Using Object ID Image

• Basic Idea
• Assign a specific color for each element in the
  scene
• Render all the elements in the scene
• The first surface in a given direction is the one
  with the color of the pixel intersected in this
  direction

6
Using Object ID Image

• Advantage
• Easy to implement
• Extremely fast
• Disadvantage
• Have aliasing errors

7
Using Object ID Image

• Aliasing errors
• Sampling the scene using a regular grid
• Visibility query is at different points than the
  original samples

8
Using Object ID Image

• Solution
• Querying neighboring pixels
• If all correspond to same surface, answer is
  correct
• Otherwise, just use ray-casting or enhance the
  image resolution

9
Performance

• Test results on a Silicon Graphics RealityEngine
  II

10
Conclusion

• The ID images gives us rapid answers in places
  where there are no problems
• Use ray-casting for small problems that concern
  few visibility queries
• Use a better ID image for larger problems that
  concern several visibility queries

11
HP Occlusion Test (1)

• Extension Name HP_occlusion_test
• Provides a visibility determination mechanism
• After rendering, query if any of the geometry
  could have or did modify the depth buffer.
• False geometry could not have affected depth
  buffer
• True it could have or did modify depth buffer

12
HP Occlusion Test (2)

• The object is not visible if the test fails
  (returns false)
• It is visible if the test passes (returns true)
• Typical usage
• Render bounding box for target geometry.
• If test fails, you can skip the geometry
  altogether.

13
HP Occlusion Test How to Use

• (Optional) Disable updates to color/depth buffers
• glDepthMask(GL_FALSE)
• glColorMask(GL_FALSE,GL_FALSE,GL_FALSE,GL_FALSE)
• Enable occlusion test
• glEnable(GL_OCCLUSION_TEST_HP)
• Render (bounding) geometry
• Disable occlusion test
• glDisable(GL_ OCCLUSION_TEST_HP)
• Read occlusion test result
• glGetBooleanv(GL_OCCLUSION_TEST_RESULT_HP,result
  )

14
HP Occlusion Test - Limitations

• Returns a simple TRUE or FALSE
• Often useful to know how many pixels were
  rendered
• Uses a stop-and-wait model for multiple tests
• Driver has to stop and wait for result of
  previous test before beginning next test
• Mediocre performance for multiple tests
• Eliminates parallelism between CPU and GPU

15
NV Occlusion Query (1)

• Extension name NV_occlusion_query
• Solves problems in HP_occlusion_test
• Returns pixel count the no. of pixels that pass
• Provides an interface to issue multiple queries
  at once before asking for the result of any one
• Applications can now overlap the time it takes
  for the queries to return with other work
  increasing the parallelism between CPU and GPU

16
NV Occlusion Query How to Use (1)

• (Optional) Disable Depth/Color Buffers
• (Optional) Disable any other irrelevant
  non-geometric state
• Generate occlusion queries
• Begin ith occlusion query
• Render ith (bounding) geometry
• End occlusion query
• Do other CPU computation while queries are being
  made
• (Optional) Enable Depth/Color Buffers
• (Optional) Re-enable other state
• Get pixel count of ith query
• If (count gt MAX_COUNT) render ith geometry

17
NV Occlusion Query How to Use (2)

• Generate occlusion queries
• Gluint queriesN
• GLuint pixelCount
• glGenOcclusionQueriesNV(N, queries)
• Loop over queries
• for (i 0 i lt N i)
• glBeginOcclusionQueryNV(queriesi)
• // render bounding box for ith
  geometry
• glEndOcclusionQueryNV()
• Get pixel counts
• for (i 0 i lt N i)
• glGetOcclusionQueryuivNV(queriesi,
  GL_PIXEL_COUNT_NV,
• 
  pixelCount)
• if (pixelCount gt MAX_COUNT)
• // render ith geometry

18
Example Incremental Object-Level Culling

• Rendering a bounding box will cost you fill
• Need to be more intelligent in how you issue
  queries!
• Good if you use query for object that you were
  going to render in any case
• Skip the occlusion queries for visible objects
  for the next few frames.

19
Incremental Object-Level Culling (2)

• Render scene from front-to-back
• Draw the big occluders first
• Issue queries for other objects in the scene
• If query returns 0 in first pass, you can skip
  the object in subsequent passes
• If object is occluded, it will get eliminated at
  worst in the second pass.

20
Conclusion

• Simple to use in applications yet very powerful
• Provides pixel count a very useful quantity
• Can be used asynchronously, more parallelism for
  you and me
• Versatile extension that can be used in a wide
  variety of algorithms/applications