10.3.2.1. Math processing in the HTML stylesheets

The following code in the HTML stylesheets

(root
 (make sequence
;   (literal
;    (debug (node-property 'gi
;                         (node-property 'document-element (current-node)))))
;(define (docelem node)
;  (node-propety 'document-element 
;    (node-property 'grove-root node)))
   (process-children)
   (process-math)
   (with-mode manifest
     (process-children))
   (if html-index
       (with-mode htmlindex
         (process-children))
       (empty-sosofo))))

(found in the docbook.dsl file of the original DocBook stylesheet package) initiates exactly the same processing as in the standard stylesheets, with one addition: the (process-math) instruction will start an additional processing step, after the standard one. The process-math routine is further specified in the code as follows:

;; Write equation info to equation-list.sgml
(define (process-math)
  (make entity
    system-id: "equation-list.sgml"
    (make element gi: "equation-set"
          attributes: (list
                       (list "latexopt" $latexopt$)
                       (list "density" $density$)
                       (list "usepackage" $usepackage$))
          (with-mode htmlmath (process-children)))))

This will create a new SGML file, equation-list.sgml, in the current directory, that will contain an element of type "equation-set". That's simply a container of equations and some LaTeX options that may be passed to it from the stylesheet: it contains the LaTeX options in “latexopt”, “density” and “usepackage”, as well as the TeX equation code, enclosed between <texequation>/</texequation> tags[2]. Here's how the equation-list.sgml file may look like[3]:

<equation-set
latexopt="12pt"
density="96x96"
usepackage=""
><texequation
fileref="images/math/11074.png"
>\[
\sum _{n=1}^{\infty }\frac{x^{n}}{n}=\ln \left(\frac{1}{1-x}\right)\]
  </texequation
><texequation
fileref="images/math/15280.png"
>\begin{equation}
f(x)=\left\{ \begin{array}{cc}
 \log _{8}x &#38; x&#62;0\\
 0 &#38; x=0\\
 \sum _{i=1}^{5}\alpha _{i}+\sqrt{-\frac{1}{x}} &#38; x&#60;0\end{array}
\right.\label{eq3}\end{equation}
  </texequation
></equation-set
>

How is the TeX code extracted from the <alt> elements and inserted in equation-list.sgml? This is the core work and is done by the following code in the HTML stylesheets:

;; How to write out an equation into the equation listing file
(define (write-eqn nd)
  (let ((texmath (select-elements (children (current-node))
                                  (normalize "alt")))
        (graphic (select-elements (children (current-node))
                                  (normalize "graphic"))))
    (make element gi: "texequation"
          attributes:
          (list
           (list "fileref" (attribute-string (normalize "fileref") graphic)))
          (literal (data texmath)))))
;; Special processing mode to extract equations
(mode htmlmath
  (default
    (let ((infeqns (select-elements (descendants (current-node))
                                    (normalize "informalequation")))
          (eqns (select-elements (descendants (current-node))
                                 (normalize "equation")))
          (inleqns (select-elements (descendants (current-node))
                                    (normalize "inlineequation"))))
      (with-mode htmlmath
        (process-node-list
         (node-list infeqns eqns inleqns)))))
  (element equation (write-eqn (current-node)))
  (element informalequation (write-eqn (current-node)))
  (element inlineequation (write-eqn (current-node))))

Basically, what the above code does is the following: It processes only text found in the <alt> element and only code found inside <informalequation>, <equation> or <inlineequation> tags. It puts this code (the TeX code of an equation) between <texequation> tags in equation-list.sgml. It also lists the fileref attribute of the <graphic> element.

This completes the Mathematics processing done by openjade. We will use the equation-list.sgml file to create PNG and BMP images of each equation.

10.3.2.2. Math processing with texmath2pngbmp.pl

The texmath2pngbmp.pl is called after openjade has processed the SGML file with the stylesheet for one HTML file. It takes one argument, the file to process. It expects a file with the structure of equation-list.sgml (see Section 10.3.2.1). It basically does the following:

• It parses the equation-list.sgml file. It extracts “latexopt”, “density” and “usepackage” .

• For every text found between <texequation> tags, it creates a new TeX file. In the preample, it sets the options found in “latexopt”, “density” and “usepackage”. In the main part, it inserts the text, which is nothing else that the TeX code describing an equation [4].

• It then calls

  • LaTeX to process the TeX file,

  • dvips to create the the PostScript® file from the .dvi file created by LaTeX,

  • convert to convert the PS file to PNG format with the right density (the full name of the PNG file was extracted from equation-list.sgml)

  • convert (once again) to convert the PS file to BMP format (the full name was calculated from the path and basename of the PNG file[5].

  system ("latex $textmp"); system ("dvips -o $textmp.eps $textmp -E"); system ("convert -density $den $textmp.eps $figfile"); system ("convert $textmp.eps $filepath"."$filebasename.bmp");

At the end of the processing, PNG and BMP images are in images/math, while the HTML and RTF documents contain links to them for each equation. We are ready! We can enjoy Mathematics on the Web in TeX quality!

Note:

We only need to call texmath2pngbmp.pl once. The PNG and BMP equation images for the whole SGML file will be created in the right directory and need not be recreated for each HTML or RTF run, since our trick with the output.print.xxx entities will take care for each run to INCLUDE the graphic element with the right filenames in the fileref attribute (see Section 10.3.1.2, Section 7.1.4.1, Section 7.2.2).