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                                                   696
       CHAPTER 8                                                               Interactive Features



       Filling in (signing) the signature field entails updating at least the V entry and
       usually also the AP entry of the associated widget annotation. Exporting a signa-
       ture field typically exports the T, V, and AP entries.

       Like any other field, a signature field may actually be described by a widget anno-
       tation dictionary containing entries pertaining to an annotation as well as a field
       (see “Widget Annotations” on page 640). The annotation rectangle (Rect) in such
       a dictionary gives the position of the field on its page. Signature fields that are not
       intended to be visible should have an annotation rectangle that has zero height
       and width.

       The appearance dictionary (AP) of a signature field’s widget annotation defines
       the field’s visual appearance on the page (see Section 8.4.4, “Appearance
       Streams”). Information about how Acrobat handles digital signature appearances
       is in the technical note Digital Signature Appearances (see the Bibliography).

                     TABLE 8.81 Additional entries specific to a signature field
KEY     TYPE          VALUE

Lock    dictionary    (Optional; must be an indirect reference; PDF 1.5) A signature field lock dictionary that
                      specifies a set of form fields to be locked when this signature field is signed. Table 8.82
                      lists the entries in this dictionary.

SV      dictionary    (Optional; must be an indirect reference; PDF 1.5) A seed value dictionary (see Table
                      8.83) containing information that constrains the properties of a signature that is ap-
                      plied to this field.

       The value of the SV entry in the field dictionary is a seed value dictionary whose
       entries (see Table 8.83) provide constraining information that is to be used at the
       time the signature is applied. Its Ff entry specifies whether the other entries in the
       dictionary are required to be honored or whether they are merely recommenda-
       tions.

       Note: The seed value dictionary may include seed values for private entries belong-
       ing to multiple handlers. A given handler should use only those entries that are per-
       tinent to itself and ignore the others.

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