14 Colors and Backgrounds
CSS properties allow authors to specify the foreground color and background of
an element. Backgrounds may be colors or images. Background properties allow authors to position a background image, repeat
it, and declare whether it should be fixed with respect to the viewport or scrolled along with the document.
See the section on color units for the syntax of valid color values.
This property describes the foreground color of an element's text content. There
are different ways to specify red:
Example(s):
EM { color: red }
/* predefined color name */
EM { color: rgb(255,0,0) } /* RGB range 0-255 */
Authors may specify the background of an element (i.e., its rendering surface)
as either a color or an image. In terms of the box model, "background" refers to the background of the content and the padding areas. Border colors and styles are set with the border properties. Margins are always transparent so the background of the parent box always shines
through.
Background properties are not inherited, but the parent box's background will shine
through by default because of the initial 'transparent' value on 'background-color'.
The background of the box generated by the root element covers the entire canvas.
For HTML documents, however, we recommend that authors specify the background for
the BODY element rather than the HTML element. User agents should observe the following precedence rules to fill in the background:
if the value of the 'background' property for the HTML element is different from 'transparent' then use it, else use
the value of the 'background' property for the BODY element. If the resulting value is 'transparent', the rendering
is undefined.
According to these rules, the canvas underlying the following HTML document will
have a "marble" background:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML
4.0//EN">
<HTML>
<HEAD>
<TITLE>Setting the canvas background</TITLE>
<STYLE type="text/css">
BODY { background: url("http://style.com/marble.png") }
</STYLE>
</HEAD>
<BODY>
<P>My background is marble.
</BODY>
</HTML>
This property sets the background color of an element, either a <color> value or the keyword 'transparent', to make the underlying colors shine through.
Example(s):
H1 { background-color: #F00
}
This property sets the background image of an element. When setting a background
image, authors should also specify a background color that will be used when the image is unavailable. When the image is available,
it is rendered on top of the background color. (Thus, the color is visible in the transparent parts of the image).
Values for this property are either <uri>, to specify the image, or 'none', when no image is used.
Example(s):
BODY { background-image: url("marble.gif")
}
P { background-image: none }
- 'background-repeat'
-
| Value:
| repeat | repeat-x | repeat-y | no-repeat | inherit
|
| Initial:
| repeat
|
| Applies to:
| all elements
|
| Inherited:
| no
|
| Percentages:
| N/A
|
| Media:
| visual |
If a background image is specified, this property specifies whether the image is
repeated (tiled), and how. All tiling covers the content and padding areas of a box. Values have the following meanings:
- repeat
- The image is repeated both horizontally and vertically.
- repeat-x
- The image is repeated horizontally only.
- repeat-y
- The image is repeated vertically only.
- no-repeat
- The image is not repeated: only one copy of the image is drawn.
Example(s):
BODY {
background: white url("pendant.gif");
background-repeat: repeat-y;
background-position: center;
}
If a background image is specified, this property specifies whether it is fixed
with regard to the viewport ('fixed') or scrolls along with the document ('scroll').
Even if the image is fixed, it is still only visible when it is in the background
or padding area of the element. Thus, unless the image is tiled ('background-repeat: repeat'), it may be invisible.
Example(s):
This example creates an infinite vertical band that remains "glued" to the viewport
when the element is scrolled.
BODY {
background: red url("pendant.gif");
background-repeat: repeat-y;
background-attachment: fixed;
}
User agents may treat 'fixed' as 'scroll'. However, it is recommended they interpret
'fixed' correctly, at least for the HTML and BODY elements, since there is no way for an author to provide an image only for
those browsers that support 'fixed'. See the section on conformance for details.
If a background image has been specified, this property specifies its initial position.
Values have the following meanings:
- <percentage> <percentage>
- With a value pair of '0% 0%', the upper left corner of the image is aligned with
the upper left corner of the box's padding edge. A value pair of '100% 100%' places the lower right corner of the image in the lower
right corner of padding area. With a value pair of '14% 84%', the point 14% across and 84% down the image is to be placed
at the point 14% across and 84% down the padding area.
- <length> <length>
- With a value pair of '2cm 2cm', the upper left corner of the image is placed 2cm
to the right and 2cm below the upper left corner of the padding area.
-
- top left and left top
- Same as '0% 0%'.
- top, top center, and center top
- Same as '50% 0%'.
- right top and top right
- Same as '100% 0%'.
- left, left center, and center left
- Same as '0% 50%'.
- center and center center
- Same as '50% 50%'.
- right, right center, and center right
- Same as '100% 50%'.
- bottom left and left bottom
- Same as '0% 100%'.
- bottom, bottom center, and center bottom
- Same as '50% 100%'.
- bottom right and right bottom
- Same as '100% 100%'.
If only one percentage or length value is given, it sets the horizontal position
only, the vertical position will be 50%. If two values are given, the horizontal position comes first. Combinations of length
and percentage values are allowed, (e.g., '50% 2cm'). Negative positions are allowed. Keywords cannot be combined with percentage
values or length values (all possible combinations are given above).
Example(s):
BODY { background: url("banner.jpeg")
right top } /* 100% 0% */
BODY { background: url("banner.jpeg") top center } /* 50% 0% */
BODY { background: url("banner.jpeg") center } /* 50% 50% */
BODY { background: url("banner.jpeg") bottom } /* 50% 100% */
If the background image is fixed within the viewport (see the 'background-attachment' property), the image is placed relative to the viewport instead of the element's
padding area. For example,
Example(s):
BODY {
background-image: url("logo.png");
background-attachment: fixed;
background-position: 100% 100%;
background-repeat: no-repeat;
}
In the example above, the (single) image is placed in the lower-right corner of
the viewport.
The 'background' property is a shorthand property for setting the individual background properties
(i.e., 'background-color', 'background-image', 'background-repeat', 'background-attachment' and 'background-position') at the same place in the style sheet.
The 'background' property first sets all the individual background properties to their initial values,
then assigns explicit values given in the declaration.
Example(s):
In the first rule of the following example, only a value for 'background-color' has been given and the other individual properties are set to their initial value.
In the second rule, all individual properties have been specified.
BODY
{ background: red }
P { background: url("chess.png") gray 50% repeat fixed }
For information about gamma issues, please consult the the Gamma Tutorial in
the PNG specification ([PNG10]).
In the computation of gamma correction, UAs displaying on a CRT may assume an ideal
CRT and ignore any effects on apparent gamma caused by dithering. That means the minimal handling they need to do on current
platforms is:
- PC using MS-Windows
- none
- Unix using X11
- none
- Mac using QuickDraw
- apply gamma 1.45 [ICC32] (ColorSync-savvy applications may simply pass the sRGB ICC profile to ColorSync to
perform correct color correction)
- SGI using X
- apply the gamma value from /etc/config/system.glGammaVal (the default
value being 1.70; applications running on Irix 6.2 or above may simply pass the sRGB ICC profile to the color management system)
- NeXT using NeXTStep
- apply gamma 2.22
"Applying gamma" means that each of the three R, G and B must be converted to R'=Rgamma,
G'=Ggamma, B'=Bgamma, before being handed to the OS.
This may rapidly be done by building a 256-element lookup table once per browser
invocation thus:
for i := 0 to 255 do
raw := i / 255.0;
corr := pow (raw, gamma);
table[i] := trunc (0.5 + corr * 255.0)
end
