Graphics Programs Reference
In-Depth Information
Figure 7-17: Boxes with opaque and translucent RGB backgrounds.
247
You can also use the percentage form of RGB color values in RGBA.
.box1
{
background
:
rgb(100%,100%,100%)
;}
.box2
{
background
:
rgba(100%,100%,100%,0.5)
;}
h e alpha value is always represented as a number between 0 and 1 inclusive, with
0
meaning
“no opacity at all” and
1
meaning “fully opaque.” So half-opaque (and thus half-transparent)
is
0.5
. You can't put a percentage in there for historical reasons that are too messy to get into
here.
If you supply a number outside the 0 to 1 range, it will (in the words of the specii cation) be
“clamped” to the allowed range. So if you give an alpha value of
4.2
, the browser will treat it
as if you'd written
1
. Also, it isn't clear what should happen when an alpha of
0
is used. Since
the color is fully transparent, what will happen to, say, invisible text? Can you select it? If it's
used on a link, is the link clickable? Both are interesting questions with no dei nitive answers.
So be careful.
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