Image Compositing
Often want to combine a sequence of images together
• different parts of final image can come from different sources
• TV stations have been doing this for a long time
Introduce a new alpha channel in addition to RGB channels
•the αvalue of a pixel indicates its opacity
–if α=0, pixel is totally transparent
–if α=1, pixel is totally opaque
• alternatively, can think of αas the fraction of the pixel actually
covered by the stored color
• convenient to store colors with αpremultiplied Area α
(, ,, ) ( , , , )rgb r g b
α
αααα
⇒
Image Compositing
Compositing one image over another is most common choice
• can think of each image drawn on a transparent plastic sheet
• the final image is formed by stacking layers together
Given images A& B, we can compute C= Aover B
• if we pre-multiply αvalues, this simplifies to
This is only one possible compositing operator
• there are in fact 12 possible ways of combining 2 images
()
A
AB
CA B
α
αα
=+1−
()
A
CA B
α
=+1−
Example: Image Compositing
Given RGB colors and (.,.,.) (,,); .; .
AB
AB
α
α
= 0 8 0 61 0 = 111 =0 5 =0 2
Premultiply: ' (., .,.) ' (., .,.)
AB
AA BB
α
α
= = 040305 = = 020202
over
'''
() ' ( ) '
(.,.,.)
AB
A
CA B
CFAFB
AB
α
=
=+
=1 +1−
=050406
() ( )
.(.). .
CAABB
AAB
FF
α
αα
α
αα
=+
=1 +1−
=05+ 0502=06
De-premultiply: '/ ( . , . , . )
C
CC
α
= = 08306710
Read RGB
α
values from frame buffer
Write RGB
α
values back into frame buffer
Aliasing
We are often beset by the problem of aliasing
• classic examples come up in rasterization (jaggies)
But aliasing can arise in other contexts
• when converting from continuous to discrete representations
We want to get rid of this problem
• we need methods for antialiasing
