Transparency & Alpha

Secret Maryo Chronicles by Florian Richter & the SMC Team, GPL licensed






Alpha



alpha
1.0fully opaque
0.5half transparent
0.0fully transparent





Blending

Most common method:

R = A1 * R1 + (1-A1) * R2
G = A1 * G1 + (1-A1) * G2
B = A1 * B1 + (1-A1) * B2


e.g. if alpha is 0.25:          
R = 0.25 * R1 + 0.75 * R2
G = 0.25 * G1 + 0.75 * G2
B = 0.25 * B1 + 0.75 * B2





Blending

Many different formulas for blending colors can be used

Formula is defined by glBlendFunc() function


R = A1 * R1 + (1-A1) * R2
G = A1 * G1 + (1-A1) * G2
B = A1 * B1 + (1-A1) * B2

is defined by:

  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)





Blending

Enable blending    glEnable(GL_BLEND)
 
Set the blending function    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
 
Assign alpha < 1    glColor4f(1, 0, 0, 0.5)

BlendFunc is applied to pixels as they are drawn - blending incoming color with color in the frame buffer

Example: alpha.py






Smooth-shaded Alpha

Alpha is treated like any other color component


If alpha differs at each vertex, it will be smoothly interpolated.
Can give objects soft edges.

Examples: smoothalpha.py     alphawheel.py






Time-varying Alpha

Changing alpha over time can make an object a fade in or out

Example: fade.py






Texture Alpha

Use 4 channel texture image

Can be used to "cut out" a texture image - creates complex shapes with simple geometry

RGB Alpha
        

Example: texalpha.py






Chroma Keying

Used in film & video to composite live actors with other imagery (video or digital)

e.g. TV weatherman, Star Wars virtual sets

Can be done digitally by finding a background color, and setting alpha to 0 for those pixels

Example code: chromakey.py






Other Blend Functions

General blending formula is:

R = SourceFactor * Rs + DestinationFactor * Rd
G = SourceFactor * Gs + DestinationFactor * Gd
B = SourceFactor * Bs + DestinationFactor * Bd

(Rs, Gs, Bs) is the source color (object being drawn)
(Rd, Gd, Bd) is the destination color (color already in the framebuffer)

SourceFactor and DestinationFactor are defined by glBlendFunc()






Other Blend Functions

Factor name Computed factor
GL_ZERO 0
GL_ONE 1

GL_SRC_ALPHA As
GL_ONE_MINUS_SRC_ALPHA 1 - As
 
GL_DST_ALPHA Ad
GL_ONE_MINUS_DST_ALPHA 1 - Ad
 
GL_CONSTANT_ALPHA Ac
GL_ONE_MINUS_CONSTANT_ALPHA 1 - Ac

GL_SRC_COLOR (Rs, Gs, Bs)
GL_ONE_MINUS_SRC_COLOR (1 - Rs, 1 - Gs, 1 - Bs)
 
GL_DST_COLOR (Rd, Gd, Bd)
GL_ONE_MINUS_DST_COLOR (1 - Rd, 1 - Gd, 1 - Bd)
 
GL_CONSTANT_COLOR (Rc, Gc, Bc)
GL_ONE_MINUS_CONSTANT_COLOR (1 - Rc, 1 - Gc, 1 - Bc)

GL_SRC_ALPHA_SATURATE min(As, 1 - Ad)





Filters

Blending can be used to apply a color filter to the whole scene

Draw a square that covers the entire window, with the appropriate blending function

 # Dim the scene 

glBlendFunc(GL_ZERO, GL_SRC_ALPHA)          
glColor4f(1.0, 1.0, 1.0, 0.5)

Example: filter.py






Filters

 # Apply a purple filter

glBlendFunc(GL_ZERO, GL_SRC_COLOR)
glColor4f(1.0, 0.0, 0.5, 1.0)
 # Invert all the colors

glBlendFunc(GL_ONE_MINUS_DST_COLOR, GL_ZERO)     
glColor4f(1.0, 1.0, 1.0, 1.0)


Creative Commons License
This document is by Dave Pape, and is released under a Creative Commons License.