Line numbering cannot be achieved by standard XSL FO means, and requires a proprietary extension to XSL language. What follows is a description of one such extension, implemented by RenderX in XEP XSL FO formatter, version 3.2.
The following requirements had to be met by this extension:
It should be possible to switch line numbering on and off in the middle of a page;
Line numbers should restart at page breaks;
Empty space between blocks should be numbered as if it were filled by empty lines;
Implementation uses one empty extension element: <rx:ruler>. Like all other XEP extensions to XSL 1.0 recommendation, it is placed into a separate namespace: xmlns:rx="http://www.renderx.com/XSL/Extensions".
This element must be first child of a <fo:block-container> element. It creates a vertical sequence of uniformly spaced consecutive numbers attached to the start-edge side of the parent block-container (left side for Western scripts).
Line numbers are put into a special block-area that extends to the whole height of the block-container in the block progression direction; we call it line-number-area. In the inline progression direction, its extent is constrained by the respective attributes on <rx:ruler>: inline-progression-dimension or width
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The line-number-area overlays areas generated by its parent <fo:block-container> element, and does not affect formatting of the block-container contents. XSL FO formatters that don't support RenderX extensions can safely skip <rx:ruler> tags: layout of other elements should not be affected. |
The line-number-area is then filled with line areas; each line contains a single number. Numbers are generated consecutively; number format is controlled by the same properties as for page numering. Separation between lines is set by line-height property on <rx:ruler>. Horizontal alignment of numbers within the line-number-area is governed by text-align trait on the same element.
The following attributes of <rx:ruler> element can be used to control presentation of the number sequence:
Inline progression dimension of the line-number-area can be specified by standard attributes: inline-progression-dimension and its subcomponents, width, min-width, max-width, etc.
line-height sets vertical separation between two consecutive numbers.
text-align specifies horizontal alignment of numbers within the line-number-area. Supported values are "start"/"left" and "end"/"right".
The same properties that are used to set page number format can be applied to specify number progression for lines. These properties include format, grouping-separator, grouping-size, and letter-value. Their semantics is the same as on <fo:page-sequence>.
The full set of inheritable font and character properties can be specified on <rx:ruler> to control font selection for the numbers: font-family, font-size, font-weight, font-style, etc.
Here is a typical case for line numbering. Complete samples can be found in examples/linenumbering/ directory in XEP distribution.
<fo:block-container line-height="14pt"[1] line-stacking-strategy="font-height"[2]> <rx:ruler font-size="9pt" font-family="Times" [3] text-align="end" width="30pt"[4]/> <fo:block start-indent="36pt"[5]> Text in this paragraph will be numbered. … … …
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The combination of fixed-length line-height and line-stacking-strategy="font-height" ensures that separation between lines is constant, thus guaranteeing proper synchronization between text lines and line numbers regardless of font changes in the body of the text. | |
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Attention: you should not use font shorthand attribute here (font="9pt Times") because it resets line-height and is likely to disrupt line number alignment. | |
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Line numbers will be aligned to the right (text-align="end") on a line that is offset by 30 pt from the left edge of the block container area (width="30pt"). | |
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Text in the body of the <fo:block-container> is indented; otherwise, line numbers would overlap it. |
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