BODY {
background: white url("pendant.gif");
background-repeat: repeat-y;
background-position: center;
}
One copy of the background image is centered, and other copies are put above and below it to make a vertical band behind the element.
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Дата изменения: Tue May 12 12:29:30 1998 Дата индексирования: Tue Oct 2 02:14:18 2012 Кодировка: Поисковые слова: ngc 6520 |
Contents
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:
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>
| Value: | <color> | transparent | inherit |
| Initial: | transparent |
| Applies to: | all elements |
| Inherited: | no |
| Percentages: | N/A |
| Media: | visual |
This property sets the background color of an element, either a <color> value or the keyword 'transparent', to make the underlying colors shine through.
H1 { background-color: #F00 }
| Value: | <uri> | none | inherit |
| Initial: | none |
| Applies to: | all elements |
| Inherited: | no |
| Percentages: | N/A |
| Media: | visual |
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.
BODY { background-image: url("marble.gif") }
P { background-image: none }
| 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:
BODY {
background: white url("pendant.gif");
background-repeat: repeat-y;
background-position: center;
}
One copy of the background image is centered, and other copies are put above and below it to make a vertical band behind the element.
| Value: | scroll | fixed | inherit |
| Initial: | scroll |
| Applies to: | all elements |
| Inherited: | no |
| Percentages: | N/A |
| Media: | visual |
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.
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.
| Value: | [ [<percentage> | <length> ]{1,2} | [ [top | center | bottom] || [left | center | right] ] ] | inherit |
| Initial: | 0% 0% |
| Applies to: | block-level and replaced elements |
| Inherited: | no |
| Percentages: | refer to the size of the box itself |
| Media: | visual |
If a background image has been specified, this property specifies its initial position. Values have the following meanings:
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).
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 property), the image is placed relative to the viewport instead of the element's padding area. For example,
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.
| Value: | [<'background-color'> || <'background-image'> || <'background-repeat'> || || <'background-position'>] | inherit |
| Initial: | not defined for shorthand properties |
| Applies to: | all elements |
| Inherited: | no |
| Percentages: | allowed on 'background-position' |
| Media: | visual |
The 'background' property is a shorthand property for setting the individual background properties (i.e., 'background-color', 'background-image', 'background-repeat', 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.
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:
"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
which then avoids any need to do transcendental math per color attribute, far less per pixel.