Customizing processing instructions

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I’m trying to create a custom processing
      instruction to
      insert and format a background image in a chapter abstract. I’ve
      followed the
      instructions in Chapter 9 of DocBook XSL TDG but am getting error
      messages
      about empty parameters.

    
The XML is:

    
<abstract>

                    <?dbfo bgimage="images/Box5-4.gif"?>

                    <?dbfo bgposition="50% 30%"?>

                    <?dbfo bgwidth="170mm"?>

                    <title>Chapter 1 overview</title>

                    <para>The text in … </para>

                </abstract>

            </info>

    
(I'd like to combine all three params into one
      pi if possible.)

    

    
I’ve set up variables for background-image,
      content-width
      and position:

    
<xsl:template match="d:abstract" mode="chapter.titlepage.recto.auto.mode">

            <xsl:variable name="bgimage">

              <xsl:call-template name="dbfo-attribute">

                <xsl:with-param name="pis"

                select="ancestor-or-self::d:abstract/processing-instruction('dbfo')"/>

                <xsl:with-param name="attribute" select="'background-image'"/>

              </xsl:call-template>

            </xsl:variable>

            <xsl:variable name="bgwidth">

              <xsl:call-template name="dbfo-attribute">

                <xsl:with-param name="pis"

                select="ancestor-or-self::d:abstract/processing-instruction('dbfo')"/>

                <xsl:with-param name="attribute" select="'ex:background-content-width'"/>

              </xsl:call-template>

            </xsl:variable>

            <xsl:variable name="bgposition">

              <xsl:call-template name="dbfo-attribute">

                <xsl:with-param name="pis"

                select="ancestor-or-self::d:abstract/processing-instruction('dbfo')"/>

                <xsl:with-param name="attribute" select="'background-position'"/>

              </xsl:call-template>

            </xsl:variable>

            

            <fo:block xmlns:fo="http://www.w3.org/1999/XSL/Format" 

            xmlns:ex="http://www.renderx.com/XSL/Extensions"

            xsl:use-attribute-sets="chapsummaryprops"

            background-image="{$bgimage}"

           
        ex:background-content-width="{$bgwidth}"

            background-position="{$bgposition}">

              <xsl:apply-templates select="d:title" />

              <xsl:apply-templates select="d:para" />

              <!--<xsl:apply-templates
select="."
        mode="chapter.titlepage.recto.mode"/>-->

            </fo:block>

          </xsl:template>

    
 

    
And when I process it with Saxon 6.5.5 the
      error messages
      are:

    
[warning]
        Attribute 'background-image'
        cannot have a value of "": should be either 'inherit' or a URI:
        background-image="url(...)".

            [error] Attribute
        'rx:background-content-width' cannot have a value of "".

    
I’m using DB 1.77.1, XEP 4.19.

    
Thanks

    
    
    
    
    
    
    
    
-- 

      Dave Gardiner