Blending, Antialiasing, and Fog - Hallymgraphics.hallym.ac.kr/teach/2005/cga/src/prac09.pdf · Antialiasing In OpenGL, calculating a coverage value for each fragment RGBA mode : multiplying

Computer Graphics

Sun-Jeong Kim http://www.hallym.ac.kr/~sunkim/teach/2005/cga

Blending, Antialiasing, and Fog

December 12, 2005


Objectives

Blendingcombining color values from a source and a destination

Antialiasingaltering colors so that edges of lines and polygons appear smooth

Fogcreating the illusion of depth by computing the color values of an object based on distance from the viewpoint


Blending (1)

Transparency : alpha – RGBA color

source – polygon color, destination – background

GL_ZERO, GL_ONE,

GL_DST_COLOR, GL_SRC_COLOR, GL_ONE_MINUS_DST_COLOR, GL_ONE_MINUS_SRC_COLOR,

GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, GL_SRC_ALPHA_SATURATE

glEnable(GL_BLEND);glBlendFunc(GLenum source, GLenum destination);


Blending (2)

Example

source (Rs, Gs, Bs, As), destination (Rc, Bc, Gc, 1)

R: AsRs + (1-As) Rc

G: AsGs + (1-As) Gc

B: AsBs + (1-As) Bc

A: AsAs + (1-As) Ac

Using the depth buffer to keep track of whether a polygon is in front of all polygons

GL_FALSE – read only, GL_TRUE - writeable

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_ALPHA);

glDepthMask(GLboolean flag);


Display ( ) (1)


Display ( ) (2)


Result – Blending


Antialiasing

In OpenGL, calculating a coverage value for each fragment

RGBA mode : multiplying the fragment’s alpha value by its coverage

color-index mode : setting the least significant 4 bits of the color index (0000 ~ 1111)

Controlling the trade-off between image and speed

target – behavior to be controlled

hint – GL_FASTEST, GL_INCEST, GL_DONT_CARE

glHint(GLenum target, GLenum hint);


Display ( ) (1)


Display ( ) (2)


Result – Antialiasing


Fog

Fog equationsGL_LINEAR

GL_EXP

GL_EXP2

Specifying the variables

pname – GL_FOR_MODE, GL_FOG_DENSITY, GL_FOG_START, GL_FOG_END

glFog{if}{v}(GLenum pname, TYPE param);

startend

zendf

−−

=

( )( )zdensity

zdensitye

ef −−

− ==1

( )( )2

2

1 zdensity

zdensitye

ef −−

−==

z: distance from the viewpoint


Global Variables


Display ( ) – Fog (GL_EXP)


Result – Fog (GL_EXP)


Display ( ) – Fog (GL_EXP2)


Result – Fog (GL_EXP2)


Display ( ) – Fog (GL_LINEAR)


Result – Fog (GL_LINEAR)

Documents

Blending, Antialiasing, and Fog - Hallymgraphics.hallym.ac.kr/teach/2005/cga/src/prac09.pdf · Antialiasing In OpenGL, calculating a coverage value for each fragment RGBA mode : multiplying