Alpha channel computation in FF revisited

["Werner D. Streidt" <wstreidt@bigfoot.de>]

Hey guys,

I reviewed the manuscript of the FF-book of this german guy. Due to the
grammar and slang, it is not so easy to read. But there are few things he
found out. The most interesting one is the following:

Run PS and create an RGB with another layer for alpha computation. Create
some colored objects with the lasso/marquee tools. Create next the following
filter in FF:

R,G,B: c
A: (src(x+val(0,-20,20),y+val(0,-20,20),3)+src(x,y,3))/2

Using the default fg/bg colors black/white) will lead you to a logical
algorithm computation. Now, exchange the fg/bg colors and apply the filter.
Or use any other background color and apply the filter!


Or apply on a layer the following code:

R,G,B: c
A: 255

We are making the whole image opaque, but which information does FF take for
deleting the transparency? FF takes the bg color info!

Now that's an interesting undocumented feature!

Greetings from Germany,

Wern