Lecture 4 --- 6.837 Fall '00

Alpha Compositing Details

Definition of the Over compositing operation:

a_resultc_result = a_fg c_fg + (1 - a_fg) a_bgc_bg a_result = a_fg + (1 - a_fg) a_bg
Issues with alpha:

Premultiplied (integral) alphas:

pixel contains (a, ar, ag, ab)
saves computation
a_resultc_result = a_fg c_fg + a_bgc_bg - a_fga_bgc_bg
alpha value constrains color magnitude
alpha modulates image shape
conceptually clean - multiple composites are well defined
Non-premultiplied (independent) alphas:

pixel contains (a, r, g, b)
what Photoshop does
color values are independent of alpha
transparent pixels have a color
divison required to get color component back

(An excellent reference on the subject)

Lecture 4

Slide 3

6.837 Fall '00