Extended Converter Use Cases

One of the simplest ways to extend the converter is to make a thematic break. For this case, we’ll override the convert handler method for a thematic break, which is convert_thematic_break. The thematic break only consists of line graphics, no text. That means we can make use of graphics fill and stroke methods provided by Asciidoctor PDF or Prawn.

The return value of the convert handler method for a block node is ignored, which is why there’s no clear return value in this override. If this were a convert handler method for an inline node, a return value would be required, which becomes the text to render.

Every title page is as unique as the work itself. That’s why Asciidoctor PDF gives you the ability to customize the title page by overriding the ink_title_page method in an extended converter. The ink_title_page method is called after the title page has been created and the background applied, so it can focus on writing content. In this method, you can choose to honor the title-page settings from the theme, or go your own way. The one rule is that the ink_title_page method must not start a new page. If it tries to start a new page, that request will be ignored and a warning will be generated.

Let’s create a custom title page that shows the document title and subtitle between two lines in the top half and a logo in the bottom half.

The methods move_cursor_to and move_cursor advance the cursor on the page where the next content will be written. The method theme_font applies the font from the specified category in the theme (with hyphens in the category name replaced by underscores). The method stroke_horizontal_rule draws a horizontal line using the specified color and line width. The method ink_prose is provided by Asciidoctor PDF to make writing text to the page easier. Finally, the method convert will convert and render the Asciidoctor node that is passed to it, in this case a block image.

A common need is to add extra styling to the title page for a part in a multi-part book. Since this is a specialized section element, there’s a dedicated method named ink_part_title that you can override. The converter already allocates a dedicated page for the part title (so there’s no need to worry about doing that). The extended converter can override the method that inks the part title to add extra decoration or content to that page.

Let’s customize the part title page by making the background orange, making the font white, aligning the title to the right, adding a line below it, and switching off the running content.

The method ink_prose is provided by Asciidoctor PDF to make writing text to the page easier. If you wanted, you could just use the low-level text method provided by Prawn.

Now let’s look at how to center the part title both vertically and horizontally. For this, we first need to compute the height of the title using the height_of_typeset_text helper, taking into account the vertical padding as well. Then, we use that height to position the cursor so that the title falls in the vertical center of the page. Next, we set the text alignment to center (which alternately could be done using the theme). Finally, we delegate to the super method to handle rendering the title in the new position.

To find all the methods available to an extended converter, consult the API docs.

A similar need is to add extra styling to the title of a chapter, or to place it on a page by itself. The extended converter can override the method that inks the chapter title to add extra decoration or content to that page, then insert a page break afterwards.

As another way to customize the chapter title, you may want to add an image above the chapter title if specified. Once again, the extended converter can override the method that inks the chapter title and use it as an opportunity to insert an image.

The path to the image is controlled using the image block attribute on the chapter.

In addition to (or instead of) a TOC for the whole book, you may want to insert a TOC per chapter immediately following the chapter title. Inserting a TOC into the PDF is a two-step process. First, you need to allocate the space for the chapter TOC using the allocate_toc method. Then, you need to come back and ink the TOC after the chapter has been rendered using the ink_toc method.

The chapter TOC can is activated by setting the chapter-toc attribute and the depth of the TOC is controlled using the chapter-toclevels attribute. For example:

Let’s say you want to insert a license page into your documents, but you don’t want to have to put a block macro for it in the document source. You can use an extended converter to add new pages to the body of the document.

Let’s consider the case of reading the license text from a file and inserting it into the first page of the body.

The method start_new_page will create a new page in the document. The ink_prose method provides a normalize option. When this option is false, it will preserve the newlines in the content, which is what we want in the case of license text. You may want to take this a bit further and allow the location of the license file to be configurable.

To help with content auditing or correlation, you may want to add a number in front of each paragraph. You can do this first by assigning a number to each paragraph in the document in the init_pdf method. Then, you can add this number in the left margin at the start of each paragraph by overriding the convert_paragraph method.

If you have a preprocessor that adds change metadata to the content, you can use an extended converter to draw change bars to add a visual indicator in the rendered output.

This converter will look for paragraphs like this one:

This functionality is already provided by the converter if you set the breakable option on section title or discrete heading or you set the heading-min-height-after theme key to auto. The code is presented here both to explain how it works and show how to do it programmatically (perhaps to tune it for specific headings).

If an in-flow heading is followed by content that doesn’t fit on the current page, and the breakable option is not set on the heading, the converter will orphan the heading on the current page. You can fix this behavior by overriding the arrange_heading method in an extended converter.

This extended converter takes this opportunity to use dry_run to make an attempt to write content in the remaining space on the page after the heading. If no content is written, it advances to the next page before inking the heading (and its corresponding anchor).

By default, the table of contents (TOC) only includes section references. If you want to include additional entries in the TOC, or to filter the sections that are included, you can extend the converter and override the get_entries_for_toc method. This method is invoked for each parent entry in the TOC, starting from the document.

The depth of the TOC is automatically controlled by the toclevels attributes. Once this limit is reached, the converter will not call get_entries_for_toc for that parent (as none of its children will be included in the TOC).

Let’s say you want to make the content on the TOC page(s) really narrow. You can do so by overriding the ink_toc method and squeezing the margins by applying extra indentation.

If you want all (or some) block images to be indented by an amount specified in the theme, you can override the convert handler method for block images, convert_image, and call super within an indented context.

The indent DSL method adds padding to either side of the content area, delegates to the specified code block, then shaves it back off.

This converter works when a custom theme defines the image-indent key, as follows:

The built-in HTML converter inserts a source language label in the upper right corner of the code block, which appears on hover. You can use an extended converter to imprint a fixed label in the PDF output.

To add this label, you’ll need to override the arrange_block method of the converter. This method arranges content blocks that have a border and/or background or support unbreakable, such as code blocks. The override needs to filter the arguments for a node that has the source style and language attribute. If the method detects that combination, it must decorate the callback passed via the &block argument to inject the extra logic. Otherwise, the method should delegate directly to super.

When a code block is detected, the decorator should first call the block argument using instance_exec. Then, it should look to see if the extent is set and that this is not a dry run. The extent provides information about where the background and border of the code block started. The extended converter should move to that page and cursor, reapply the code block padding, and ink the label using the code font settings.

The way this extended converter is written, the label is inked on top of the inked code block. You’re free to customize where the label is placed. The float method allows you to move the cursor around in absolute space without impacting the flow of the content. The extent gives you the information about the location of the code block.

Asciidoctor PDF provides basic support for image floats. It will wrap paragraph text on the opposing side of the float. However, if it encounters a non-paragraph, the converter will clear the float and continue positioning content below the image.

As a companion to this basics support, the converter provides a framework for broadening support for float wrapping. We can take advantage of this framework in an extended converter. By extending the converter and overriding the supports_float_wrapping? as well as the convert handler for the block you want to enlist (e.g., convert_code), you can arrange additional content into the empty space adjacent to the floated image. In the following example, code (listing and literal) blocks are included in the float wrapping.

You can configure the gap next to and below the image using the image-float-gap key in the theme.

The converter only supports custom roles on paragraphs and phrases. You can use an extended converter to add this capability to tables.

This extended converter allows you to specify any theme key on the custom role that’s supported for tables. The role must be defined under a special role name <table> (to avoid clashing with other role names). Here’s an example of a custom table role named thick that increases the width of the table border and grid lines.

You apply this role to a table by prepending .thick to the first positional attribute in the block attribute line above the table.

As written, the extended converter only supports the first role on the table. It could be enhanced to support an arbitrary number of roles, with each successive role cascading (like CSS).

Similarly to the custom table role, we can use an extended converter to add support for theme keys per admonition type.

This converter temporarily promotes keys under the admonition-<type> theme category to the admonition theme category, overriding any existing keys. The placeholder <type> represents the admonition type; caution, important, note, tip, or warning.

Here’s an example that shows how you’d use the theme to apply a border to the important admonition type when using this extended converter:

Starting with Asciidoctor PDF 2.1, this converter provides built-in support for multiple columns. This feature is available when the doctype is article or manpage, but not book. The columns get applied to the body of the document, which excludes the document title and TOC, if present.

The Asciidoctor PDF converter also provides the framework for making multi-column layouts in an extended converter. This framework is accessible via the helper method column_box. To make a multi-column layout, you put statements that ink content inside a code block and pass it to the column_box method as follows:

If you want a multi-column layout for a specific chapter or section, you can override the traverse method, look for the section you want to arrange, and wrap the call to super in a column_box enclosure.

Although not an extended converter, this use case uses information from the converter in much the same way. In this case, we’re interested in retrieving the page number and inserting it into the content.

Let’s create an inline macro named pagenum that inserts the current page number into the document when the macro is converted.

Here’s how this macro would be used.

To find even more examples of how to override the behavior of the converter, refer to the extended converter in the InfoQ Mini-Book template.