用/Differences访问PoDoFo数组

Question

这个问题是关于PoDoFo库的一个特定问题。

如何访问字体资源的编码字典中的/Differences数组条目？

从Tf操作符读取字体名称之后，我可以通过PoDoFo::PdfPage::GetFromResources获取字体。但是，虽然PdfFont类有PoDoFo::PdfFont::GetEncoding，但我不知道如何从那里到达/Differences数组。

来自PDFSpec (我只担心类型1的字体)：

编码 (可选)字体字符编码的规范(如果与其内置编码不同)。编码的值应为预定义编码的名称(如附件D所述的MacRomanEncoding、MacExpertEncoding或WinAnsiEncoding )，或应指定与字体内置编码或指定预定义编码(见9.6.6，“字符编码”)不同之处的编码字典。

这是否意味着从PdfEncoding返回的PoDoFo::PdfFont::GetEncoding对象包含差异数组(如果有)？

(不久前，我在PoDoFo邮件列表上询问过，但在这里发帖是为了看看某个熟悉PoDoFo和pdfs的人是否能提供帮助)。

Answer 1

PoDoFo知道许多不同类型的编码类，cf。编码对象工厂：

if (pObject->IsName ())
{
    const PdfName & rName = pObject->GetName ();
    if (rName == PdfName ("WinAnsiEncoding"))
        return PdfEncodingFactory::GlobalWinAnsiEncodingInstance ();
    else if (rName == PdfName ("MacRomanEncoding"))
        return PdfEncodingFactory::GlobalMacRomanEncodingInstance ();
    else if (rName == PdfName ("StandardEncoding"))      // OC 13.08.2010
        return PdfEncodingFactory::GlobalStandardEncodingInstance ();
    else if (rName == PdfName ("MacExpertEncoding"))     // OC 13.08.2010 TODO solved
        return PdfEncodingFactory::GlobalMacExpertEncodingInstance ();
    else if (rName == PdfName ("SymbolEncoding"))        // OC 13.08.2010
        return PdfEncodingFactory::GlobalSymbolEncodingInstance ();
    else if (rName == PdfName ("ZapfDingbatsEncoding"))  // OC 13.08.2010
        return PdfEncodingFactory::GlobalZapfDingbatsEncodingInstance ();
    else if (rName == PdfName ("Identity-H"))
        return new PdfIdentityEncoding ();
}
else if (pObject->HasStream ())     // Code for /ToUnicode object 
{
    return new PdfCMapEncoding(pObject);
}
else if (pObject->IsDictionary ())
{
    return new PdfDifferenceEncoding (pObject);
}

(PoDoFo/src/doc/PdfEncodingObjectFactory.cpp)

你对最后一个案子感兴趣。因此，如果手头上的编码对象是PdfDifferenceEncoding的实例，则可以使用：

/** PdfDifferenceEncoding is an encoding, which is based
 *  on either the fonts encoding or a predefined encoding
 *  and defines differences to this base encoding.
 */
class PODOFO_DOC_API PdfDifferenceEncoding : public PdfEncoding, private PdfElement {
 public:
[...]
    /** 
     * Get read-only access to the object containing the actual
     * differences.
     *
     * \returns the container with the actual differences
     */
    inline const PdfEncodingDifference & GetDifferences() const;
[...]
};

(PoDoFo/src/doc/PdfDifferenceEncoding.h)

PdfDifferenceEncoding在同一个头类中声明，并提供了一些有趣的方法：

/** A helper class for PdfDifferenceEncoding that
 *  can be used to create a differences array.
 */
class PODOFO_DOC_API PdfEncodingDifference {
    struct TDifference {
        int         nCode;
        PdfName     name;
        pdf_utf16be unicodeValue;
    };

    typedef std::vector<TDifference>                 TVecDifferences;
    typedef std::vector<TDifference>::iterator       TIVecDifferences;
    typedef std::vector<TDifference>::const_iterator TCIVecDifferences;

 public: 
    /** Create a PdfEncodingDifference object.
     */
    PdfEncodingDifference();

    /** Copy a PdfEncodingDifference object.
     */
    PdfEncodingDifference( const PdfEncodingDifference & rhs );

    /** Copy a PdfEncodingDifference object.
     */
    const PdfEncodingDifference & operator=( const PdfEncodingDifference & rhs );

    /** Add a difference to the object.
     * 
     *  \param nCode unicode code point of the difference (0 to 255 are legal values)
     *
     *  \see AddDifference if you know the name of the code point
     *       use the overload below which is faster
     */
    void AddDifference( int nCode );

    /** Add a difference to the object.
     * 
     *  \param nCode unicode code point of the difference (0 to 255 are legal values)
     *  \param rName name of the different code point or .notdef if none
     */
    void AddDifference( int nCode, const PdfName & rName );

    /** Tests if the specified code is part of the 
     *  differences.
     *
     *  \param nCode test if the given code is part of the differences
     *  \param rName write the associated name into this object if the 
     *               code is part of the difference
     *  \param rValue write the associated unicode value of the name to this value 
     *
     *  \returns true if the code is part of the difference
     */
    bool Contains( int nCode, PdfName & rName, pdf_utf16be & rValue ) const;

    /** Convert the PdfEncodingDifference to an array
     *
     *  \param rArray write to this array
     */
    void ToArray( PdfArray & rArray );

    /** Get the number of differences in this object.
     *  If the user added .notdef as a difference it is 
     *  counted, even it is no real difference in the final encoding.
     *  
     *  \returns the number of differences in this object
     */
    inline size_t GetCount() const;

 private:
    struct DifferenceComparatorPredicate {
        public:
          inline bool operator()( const TDifference & rDif1, 
                                  const TDifference & rDif2 ) const { 
              return rDif1.nCode < rDif2.nCode;
          }
    };

    TVecDifferences m_vecDifferences;
};

(PoDoFo/src/doc/PdfDifferenceEncoding.h)

Answer 2

一旦有了指向PoDoFo::PdfFont的指针，就可以通过调用GetObject来访问底层对象，因为它从PoDoFo::PdfElement继承了它。然后调用GetIndirectKey("Encoding")获取一个指向编码字典的指针，其中包含差异数组，并将其传递给PoDoFo::PdfDifferenceEncoding构造函数。

PoDoFo::PdfObject* fntobj = fnt->GetObject();
if (fntobj)
{
    PoDoFo::PdfObject* fntdic = fntobj->GetIndirectKey("Encoding");
    if (fntdic)
    {
        PoDoFo::PdfDifferenceEncoding diff(fntdic);
        PoDoFo::PdfArray diffarray;
        PoDoFo::PdfEncodingDifference d(diff.GetDifferences());
        d.ToArray(diffarray);
    }
}