Depth Buffers

1
Depth Buffers Transparency
ECS175 Discussion Session 6

• Hiroshi Akiba Feb 20, 2004

2
Outline

• Depth Buffer
• Overview
• OpenGL depth buffer functions
• Transparency Effect
• overview
• OpenGL transparency functions
• example

3
Depth Buffer Overview

• The most widely used visible surface detection
  method
• Easy to implement
• Compatible with pipeline architecture
• Works in image space
• It can be implemented with a small number of
  additional calculations at the scan conversion
  process
• Worst case complexity is proportional to the
  number of polygons

4
Depth Buffer Overview






5
Depth Buffer Overview






6
Depth Buffer Overview






7
Depth Buffer Functions

• glutInitDisplayMode(GLUT_SINGLEGLUT_RGBGLUT_DEPT
  H)
• Declares to use depth buffer(z buffer)
• The defaults are GLUT_SINGLE and GLUT_RGB
• glClear(GL_DEPTH_BUFFER_BIT)
• Sets all depth-buffer values to the maximum value
  (1.0 in default)
• Need to call for every frame
• glEnable(GL_DEPTH_TEST)
• Activate visibility detection routines
• glClearDepth(maxDepth)
• Change maximum depth value in the range 0.0 to
  1.0
• glDepthRange(nearDepth, farDepth)t
• The defaults are 0.0 and 1.0
• glDepthFunc(testCondition) --- GL_LESS(default),
  GL_GREATER,
• glDepthMask(writeStatus) --- GL_TRUE or GL_FALSE

Necessary in hw4
8
OpenGL Transparency Overview

• The transparency effect is simulated by color
  blending
• Ignore the path shifts due to refraction

9
Transparency Functions

• glutInitDisplayMode(GLUT_SINGLEGLUT_RGBAGLUT_DEP
  TH)
• glColor4f(R,G,B,A)
• glColor3f(R,G,B) glColor4f(R,G,B,1)
• glEnable(GL_BLEND)
• glBlendFunc(GLenum sfactor, GLenum dfactor)
• Controls how color values in the fragment being
  processed (the source) are combined with those
  already stored in the framebuffer (the
  destination).
• let the source and destination blending factors
  be (Sr, Sg, Sb, Sa) and (Dr, Dg, Db, Da) Final
  Color will be
• (RsSrRdDr, GsSgGdDg, BsSbBdDb, AsSaAdDa)
• GL_ONE_MINUS_SRC_ALPHA (1, 1, 1, 1)-(As, As, As,
  As)
• GL_SRC_ALPHA (As, As, As, As)

10
Example

• glBlendFunc(GL_ONE_MINUS_SRC_ALPHA, GL_SRC_ALPHA)
• (RsSrRdDr, GsSgGdDg, BsSbBdDb, AsSaAdDa)
• GL_ONE_MINUS_SRC_ALPHA (1, 1, 1, 1)-(As, As, As,
  As)
• GL_SRC_ALPHA (As, As, As, As)

glColor4f(0,0,1,1)
Draw blue polygon in the back fist
(Sr,Sg,Sb)(0.6, 0.6, 0.6) (Dr,Dg,Db)(0.4, 0.4,
0.4) Color (00.610.4, 00.600.4,
10.600.4) ( 0.4, 0, 0.6)
glColor4f(1,0,0,0.4)
11
Problem

• What if there are an opaque object and two
  transparent objects intersect each other?

12
Solution

• Render opaque objects first with the depth
  buffer, and render transparent objects without
  writing to the depth buffer.

glEnable(GL_DEPTH_TEST) DrawAllOpaqueObjects()
glEnable(GL_BLEND) glDepthMask(GL_FALSE) glBle
ndFunc(GL_ONE_MINUS_SRC_ALPHA, GL_SRC_ALPHA)
DrawAllTransparentObjects() glDepthMask(GL_TRUE)
glDisable(GL_BLEND)
13
References

• 1.Hearn baker, Computer Graphics width OpenGL,
  2004
• 2.Edward Angel, Interactive Computer Graphics A
  Top-Down Approach with OpenGL, 2000
• 3. Edward Angle, OpenGL A PRIMER, 2002
• 4.OpenGL Red Book, http//www.opengl.org/documenta
  tion/red_book_1.0/
• 5.Ken Joy, Online Graphics Note,
  http//graphics.cs.ucdavis.edu/GraphicsNotes/Z-Buf
  fer-Algorithm/Z-Buffer-Algorithm.html