Fix stream filter handling.

qpdf permits replacing standard stream filters with user provided filters. #1457 incorrectly removed that option.

Fix stream filter handling.
qpdf permits replacing standard stream filters with user provided filters. #1457 incorrectly removed that option.
m-holger
1 parent 38e9f9de
Showing 4 changed files with 55 additions and 13 deletions
libqpdf/QPDFWriter.cc
libqpdf/QPDF_Stream.cc
libqpdf/qpdf/QPDFObject_private.hh
manual/release-notes.rst
@@ -1294,7 +1294,8 @@ QPDFWriter::willFilterStream(
     }
     // Disable compression for empty streams to improve compatibility
-    if (stream_dict.getKey("/Length").isInteger() && stream_dict.getKey("/Length").getIntValue() == 0) {
+    if (stream_dict.getKey("/Length").isInteger() &&
+        stream_dict.getKey("/Length").getIntValue() == 0) {
         filter = true;
         compress_stream = false;
     }
@@ -85,11 +85,51 @@ namespace
     /// User defined streamfilter factories
     std::map<std::string, std::function<std::shared_ptr<QPDFStreamFilter>()>> filter_factories;
+    bool filter_factories_registered = false;
 } // namespace
+std::string
+QPDF_Stream::Members::expand_filter_name(std::string const& name) const
+{
+    // The PDF specification provides these filter abbreviations for use in inline images, but
+    // according to table H.1 in the pre-ISO versions of the PDF specification, Adobe Reader also
+    // accepts them for stream filters.
+    if (name == "/AHx") {
+        return "/ASCIIHexDecode";
+    }
+    if (name == "/A85") {
+        return "/ASCII85Decode";
+    }
+    if (name == "/LZW") {
+        return "/LZWDecode";
+    }
+    if (name == "/Fl") {
+        return "/FlateDecode";
+    }
+    if (name == "/RL") {
+        return "/RunLengthDecode";
+    }
+    if (name == "/CCF") {
+        return "/CCITTFaxDecode";
+    }
+    if (name == "/DCT") {
+        return "/DCTDecode";
+    }
+    return name;
+};
+
 std::function<std::shared_ptr<QPDFStreamFilter>()>
 QPDF_Stream::Members::filter_factory(std::string const& name) const
 {
+    if (filter_factories_registered) [[unlikely]] {
+        // We need to check user provided filters first as we allow users to replace qpdf provided
+        // default filters. This will have a performance impact if the facility to register stream
+        // filters is actually used. We can optimize this away if necessary.
+        auto ff = filter_factories.find(expand_filter_name(name));
+        if (ff != filter_factories.end()) {
+            return ff->second;
+        }
+    }
     if (name == "/FlateDecode") {
         return SF_FlateLzwDecode::flate_factory;
     }
@@ -112,8 +152,8 @@ QPDF_Stream::Members::filter_factory(std::string const&amp; name) const
         return SF_ASCIIHexDecode::factory;
     }
     // The PDF specification provides these filter abbreviations for use in inline images, but
-    // according to table H.1 in the pre-ISO versions of the PDF specification, Adobe Reader also
-    // accepts them for stream filters.
+    // according to table H.1 in the pre-ISO versions of the PDF specification, Adobe Reader
+    // also accepts them for stream filters.
     if (name == "/Fl") {
         return SF_FlateLzwDecode::flate_factory;
@@ -133,15 +173,7 @@ QPDF_Stream::Members::filter_factory(std::string const&amp; name) const
     if (name == "/DCT") {
         return SF_DCTDecode::factory;
     }
-    if (filter_factories.empty()) {
-        return nullptr;
-    }
-    auto ff =
-        name == "/CCF" ? filter_factories.find("/CCITTFaxDecode") : filter_factories.find(name);
-    if (ff == filter_factories.end()) {
-        return nullptr;
-    }
-    return ff->second;
+    return nullptr;
 }
 Stream::Stream(
@@ -159,6 +191,7 @@ Stream::registerStreamFilter(
     std::string const& filter_name, std::function<std::shared_ptr<QPDFStreamFilter>()> factory)
 {
     filter_factories[filter_name] = factory;
+    filter_factories_registered = true;
 }
 JSON
@@ -437,7 +470,7 @@ Stream::pipeStreamData(
     const bool empty_stream_data = s->stream_data && s->stream_data->getSize() == 0;
     const bool empty = empty_stream || empty_stream_data;
-    if(empty_stream || empty_stream_data) {
+    if (empty_stream || empty_stream_data) {
         filter = true;
     }
@@ -226,6 +226,7 @@ class QPDF_Stream final
         std::shared_ptr<Buffer> stream_data;
         std::shared_ptr<QPDFObjectHandle::StreamDataProvider> stream_provider;
         std::vector<std::shared_ptr<QPDFObjectHandle::TokenFilter>> token_filters;
+        std::string expand_filter_name(std::string const& name) const;
         std::function<std::shared_ptr<QPDFStreamFilter>()>
         filter_factory(std::string const& name) const;
     };
@@ -30,6 +30,13 @@ more detail.
     - More sanity checks have been added when files with damaged xref tables
       are recovered.
+  - Other changes
+
+    - There has been some refactoring of stream filtering. These are optimized
+      for the common case where no user provided stream filters  are
+      registered by calling ``QPDF::registerStreamFilter``. If you are
+      providing your own stream filters please open a ticket_.
+
 12.2.0: May 4, 2025
   - Upcoming C++ Version Change