alpha concept?

alpha concept?

Post by chenkee » Sat, 29 Dec 2001 19:28:21



i tried to figure out what's the alpha means long time ago...but i still
cannot get the concept why an alpha must use to implement in some images??
anyone know?
 
 
 

alpha concept?

Post by David McCal » Sun, 30 Dec 2001 03:20:17



> i tried to figure out what's the alpha means long time ago...but i still
> cannot get the concept why an alpha must use to implement in some images??
> anyone know?

I'll give it a shot. Imagine that you had a scene of a beach and
you wanted to have a space ship land on the beach. Further,
you want to put the space ship into the beach scene using
another program like After Effects or a nonlinear editor.

If you were to render the space ship, in Lightwave, as a
32 bit Targa file, the render would would be a sequentially
numbered series of pictures of your space ship on a black
background (that uses the first 24 bits), the remaining 8bits
of the image are normally not seen, but they do contain the
alpha channel information. If you used a program that allowed
you to view the alpha channel you would see a black frame
with a white silouette of your space ship. Another program
(including Lightwave, After Effects and many others) can use
that white sillouette to cut out the space ship to allow it to be
pasted onto the beach scene.

If the space ship had a simitransparent dome (cockpit) on
top it, that area, of the alpha channel, wouldn't be completely
white, but rather shades of grey. The grey areas would allow
some of the beach scene to show through the transparent
parts of the dome.

Glows, like flames or lens flares, do not show up in the alpha
channels. These must be composited as a 2 part process.
The first stage is to composite the image by using an additive
or lighten effect using the reverse of the alpha channel. Then
you would composite the image normally using the normal
alpha channel. After Effects has a transfer mode called
"luminecent premultiplied" that can do both steps at once.
I believe, L:ightwave can do that as well.

David

 
 
 

alpha concept?

Post by Don Dempse » Sun, 30 Dec 2001 04:10:03


Alpha is an additional component (channel) stored in some image and movie
file formats. As you probably know, the three standard channels are
typically red, green, and blue (RGB) which indicate the amount of red, green
and blue values in the color of each pixel, but the alpha channel represents
the transparency (or opacity) value of each pixel. If viewed by itself, the
alpha channel would appear to be a greyscale image where black represents
completely transparent (0% opacity) and white represents no transparency
(100% opacity). The alpha channel is especially useful when compositing
multiple images together.

I heartily recommend the book "The Art and Science of Digital Compositing"
by Ron Brinkmann if you want more info on this topic.

http://www.amazon.com/exec/obidos/ASIN/0121339602/thelumisnetworks/10...
72-1935722

the don


Quote:> i tried to figure out what's the alpha means long time ago...but i still
> cannot get the concept why an alpha must use to implement in some images??
> anyone know?