Apache FOP | custom fonts | relative URL not working

I have got configuration file to load custom fonts for Apache FOP. I am struggling to configure embed-url on server so that font url changes as per server domain.

I have tried embed-url property value as:

Non working embed-urls:

embed-url="context:/etc/designs/projectName/clientlibs/pdffonts/Batang.ttf"
embed-url="file:/etc/designs/projectName/clientlibs/pdffonts/Batang.ttf"

Working embed-url:

embed-url="http://localhost:4503/etc/designs/projectName/clientlibs/pdffonts/Batang.ttf"

Somehow I can't seems to find proper syntax here. I am using FOP with AEM 6.0.

<?xml version="1.0"?>
<fop version="1.0">
    <renderers>
        <renderer mime="application/pdf">
            <fonts>
                <font kerning="yes"
                    embed-url="context:/etc/designs/projectName/clientlibs/pdffonts/Batang.ttf" -- this doesn't
                    embedding-mode="subset">
                    <font-triplet name="SimSun" style="normal" weight="normal" />
                </font>
                <font kerning="yes"
                    embed-url="file:/etc/designs/projectName/clientlibs/pdffonts/Batang.ttf" -- this doesn't
                    embedding-mode="subset">
                    <font-triplet name="Batang" style="normal" weight="normal" />
                </font>
                <font kerning="yes"
                    embed-url="http://localhost:4503/etc/designs/projectName/clientlibs/pdffonts/Batang.ttf" -- this works
                    embedding-mode="subset">
                    <font-triplet name="Batang" style="normal" weight="normal" />
                </font>
            </fonts>
        </renderer>
    </renderers>
</fop>

Solution

"Starting point" for relative paths:

if the configuration file has a font-base element (as a direct child of the document root element), its value is used to resolve relative font paths
otherwise, the value of the base element is used instead
the default configuration file included in the distribution has the element <base>.</base>, meaning that relative paths must be interpreted as relative to the position of the configuration file

Note that the values of font-base and base can be relative too, in which case they refer to the configuration file path.

<?xml version="1.0"?>
<fop version="1.0">

  <base>.</base>

  <font-base>/Users/lfurini/Library/Fonts</font-base>
  <!-- other possible examples:
  <font-base>.</font-base>
  <font-base>../fonts</font-base>
  --> 

  <!-- ... -->
</fop>

Relative paths syntax:

you don't need context: or file:
if the embed-url starts with / it's an absolute path, otherwise it's a relative one referring to the "starting point" defined before

relative paths can contain ../ to go back up in the folder hierarchy, if needed

<!-- directly in the base folder -->
<font kerning="yes" embed-url="font1.ttf">
  <font-triplet name="font1" style="normal" weight="normal"/>
</font>

<!-- in a "sister" folder -->
<font kerning="yes" embed-url="../otherFonts/font2.ttf">
  <font-triplet name="font2" style="normal" weight="normal"/>
</font>

<!-- in a sub-folder -->
<font kerning="yes" embed-url="specialFonts/font3.ttf">
  <font-triplet name="font3" style="normal" weight="normal"/>
</font>

<!-- absolute path -->
<font kerning="yes" embed-url="/Users/lfurini/Library/Fonts/font4.ttf" embedding-mode="subset">
  <font-triplet name="font4" style="normal" weight="normal"/>
</font>

(tested with FOP 1.1, 2.0 and 2.1)

(disclosure: I'm a FOP developer, though not very active nowadays)