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UBI 516 Advanced Computer Graphics. Visible Surface De tection. Aydın Öztürk ozturk @ ube.ege.edu.tr http://www. ube.ege.edu.tr/~ozturk. Review : Rendering Pipeline. Almost finished with the rendering pipeline: Modeling transformations Viewing transformations Projection transformations

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ubi 516 advanced computer graphics

UBI 516Advanced Computer Graphics

Visible Surface Detection

Aydın Öztürk

ozturk@ube.ege.edu.tr

http://www.ube.ege.edu.tr/~ozturk

review rendering pipeline
Review: Rendering Pipeline
  • Almost finished with the rendering pipeline:
    • Modeling transformations
    • Viewing transformations
    • Projection transformations
    • Clipping
    • Scan conversion
  • We now know everything about how to draw a polygon on the screen, except visible surface detection.
invisible primitives
Invisible Primitives
  • Why might a polygon be invisible?
    • Polygon outside the field of view
    • Polygon is backfacing
    • Polygon is occluded by object(s) nearer the viewpoint
  • For efficiency reasons, we want to avoid spending work on polygons outside field of view or backfacing
  • For efficiency and correctness reasons, we need to know when polygons are occluded
view frustum clipping
View Frustum Clipping
  • Remove polygons entirely outside frustum
    • Note that this includes polygons “behind” eye (actually behind near plane)
  • Pass through polygons entirely inside frustum
  • Modify remaining polygonsto pass through portions intersecting view frustum
view frustum clipping1
View Frustum Clipping
  • Canonical View Volumes
    • Remember how we defined cameras
      • Eye point, lookat point, v-up
      • Orthographic | Perspective
    • Remember how we define viewport
      • Width, height (or field of view, aspect ratio)
    • These two things define rendered volume of space
    • Standardize the height, length, and width of view volumes
view frustum clipping2
View Frustum Clipping
  • Canonical View Volumes
review rendering pipeline1
Review Rendering Pipeline
  • Clipping equations are simplified
  • Perspective and Orthogonal (Parallel) projections have consistent representations
perspective viewing transf ormation
Perspective Viewing Transformation
  • Remember the viewing transformation for perspective projection
    • Translate eye point to origin
    • Rotate such that projection vector matches –z axis
    • Rotate such that up vector matches y
  • Add to this a final step where we scale the volume
clipping
Clipping
  • Because both camera types are represented by same viewing volume
    • Clipping is simplified even further
visible surface de tection
Visible Surface Detection

There are many algorithms developed for the visible surface detection

● Some methods involve more processing time.

● Some methods require more memory.

● Some others apply only to special types of objects.

classification of visible surface detection algorithms
Classification of Visible-Surface Detection Algorithms

They are classified according to whether they deal with object definitions or with their projected images.

● Object space methods.

● Image-space methods.

Most visible-surface algorithms use image space method.

back face detection
Back-Face Detection
  • Most objects in scene are typically “solid”
back face detection cont

Note: backface detectionalone doesn’t solve thehidden-surface problem!

Back-Face Detection (cont.)
  • On the surface of polygons whose normals point away from the camera are always occluded:
back face detection1
Back-Face Detection

yv

  • This test is based on inside-outside test. A point (x,y,z) is inside if

N=(A,B,C)

xv

V

zv

  • We can simplify this test by considering the normal vector vector N to a polygon surface, which has Cartesian components (A,B,C).
  • If V is a vector in the viewing direction from eye then this polygon is back face if V●N > 0.
  • If the object descriptions have been converted to projection coordinates and viewing direction is parallel to zv axis then V=(0, 0, Vz) and V●N=VzC so that we only need to consider the sign of C.
depth buffer z buffer method
Depth-Buffer (z-Buffer) Method
  • This method compares surface depths at each pixel position on the projection plane.
  • Each surface is processed separetly, one point at a time across the surface.
  • Surface S1 is closest to view plane, so its surface intensity value at (x,y) is saved.

S3

S2

yv

S1

xv

(x,y)

zv

steps for depth buffer z buffer method cont
Steps for Depth-Buffer (z-Buffer) Method(Cont.)
  • Initialize the depth buffer and refresh buffer s.t. for all buffer positions (x,y)

depth(x, y) = 0,

refresh(x, y) = Ibackground

steps for depth buffer z buffer method cont1
Steps for Depth-Buffer (z-Buffer) Method(Cont.)
  • For each position on each polygon surface, compare depth values to previously stored values in depth buffer to determine visibility.

● Calculate the depth z for each (x,y) position

on the polygon.

● If z >depth(x,y), then

depth(x, y) = z,

refresh(x, y) = Isurf(x,y).

where

Ibackground is the value for the bacground intensity and

Isurf(x,y), is the projected intensity value for the surface at (x,y).

depth buffer z buffer calculations
Depth-Buffer (z-Buffer) Calculations.

Depth values for a surface position (x,y) are calculated from the plane equation

z-value for the horizontal next position

z-value down the edge(starting at top vertex)

Y

Y-1

X X+1

top scan line

Left edge

intersection

bottom scan line

scan line method
Scan-Line Method

yv

B

E

F

Scan Line 1

A

Scan Line 2

Scan Line 3

H

S1

S1

C

S2

D

G

xv

depth sorting algorithm painter s algorithm
Depth-Sorting Algorithm (Painter’s Algorithm)

This method performs the following basic functions:

  • Surfaces are sorted in order of decreasing order.
  • Surfaces are scan converted in order, starting with the surface of greatest.
depth sorting algorithm painter s algorithm1
Depth-Sorting Algorithm (Painter’s Algorithm)
  • Simple approach: render the polygons from back to front, “painting over” previous polygons:
painter s algorithm problems
Painter’s Algorithm: Problems
  • Intersecting polygons present a problem
  • Even non-intersecting polygons can form a cycle with no valid visibility order:
analytic visibility algorithms
Analytic Visibility Algorithms
  • Early visibility algorithms computed the set of visible polygon fragments directly, then rendered the fragments to a display:
    • Now known as analytic visibility algorithms
analytic visibility algorithms1
Analytic Visibility Algorithms
  • What is the minimum worst-case cost of computing the fragments for a scene composed of n polygons?
  • Answer: O(n2)
analytic visibility algorithms2
Analytic Visibility Algorithms
  • So, for about a decade (late 60s to late 70s) there was intense interest in finding efficient algorithms for hidden surface removal
  • We’ll talk about two:
    • Binary Space-Partition (BSP) Trees
bin ary space partition trees 1979
Binary Space Partition Trees (1979)
  • BSP tree: organize all of space (hence partition)into a binary tree
    • Preprocess: overlay a binary tree on objects in the scene
    • Runtime: correctly traversing this tree enumerates objects from back to front
    • Idea: divide space recursively into half-spaces by choosing splitting planes
      • Splitting planes can be arbitrarily oriented
rendering bsp trees
Rendering BSP Trees

renderBSP(BSPtree *T)

BSPtree *near, *far;

if (eye on left side of T->plane)

near = T->left; far = T->right;

else

near = T->right; far = T->left;

renderBSP(far);

if (T is a leaf node)

renderObject(T)

renderBSP(near);

polygons bsp tree construction
Polygons: BSP Tree Construction
  • Split along the plane containing any polygon
  • Classify all polygons into positive or negative half-space of the plane
    • If a polygon intersects plane, split it into two
  • Recurse down the negative half-space
  • Recurse down the positive half-space
notes about bsp tree s
Notes About BSP Trees
  • No bunnies were harmed in our example.
  • But what if a splitting plane passes through an object?
    • Split the object; give half to each node:

Ouch

bsp demo
BSP Demo
  • Nice demo:

http://symbolcraft.com/graphics/bsp/

summary bsp trees
Summary: BSP Trees
  • Advantages:
    • Simple, elegant scheme
    • Only writes to framebuffer (i.e., painters algorithm)
      • Thus very popular for video games (but getting less so)
  • Disadvantages:
    • Computationally intense preprocess stage restricts algorithm to static scenes
    • Worst-case time to construct tree: O(n3)
    • Splitting increases polygon count
      • Again, O(n3) worst case
ubi 516 advanced computer graphics1

UBI 516Advanced Computer Graphics

OpenGL Visibility Detection Functions

opengl backface culling
OpenGL Backface Culling
  • glEnable(GL_CULL_FACE);glCullFace(mode);// mode: GL_BACK, GL_FRONT, GL_FRONT_AND_BACK :-o
  • glDisable(GL_CULL_FACE);
opengl depth buffer functions
OpenGL Depth Buffer Functions
  • Set display ModeglutDisplayMode( GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH );
  • Clear screen and depth buffer every time in the display functionglClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
  • Enable/disable depth bufferglEnable( GL_DEPTH_TEST );glDisable( GL_DEPTH_TEST );
opengl depth cueing function
OpenGL Depth-Cueing Function
  • We can vary the brigthness of an objectglEnable ( GL_FOG );glFogi ( GL_FOG_MODE, mode);// modes: GL_LINEAR, GL_EXP or GL_EXP2. . .glDisable ( GL_FOG );