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Diffstat (limited to 'examples/pdf-custom-filter.cc')
-rw-r--r-- | examples/pdf-custom-filter.cc | 532 |
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diff --git a/examples/pdf-custom-filter.cc b/examples/pdf-custom-filter.cc new file mode 100644 index 00000000..35f0ee38 --- /dev/null +++ b/examples/pdf-custom-filter.cc @@ -0,0 +1,532 @@ +#include <qpdf/QPDF.hh> +#include <qpdf/QUtil.hh> +#include <qpdf/QPDFWriter.hh> +#include <qpdf/QPDFStreamFilter.hh> + +#include <cstring> +#include <exception> +#include <iostream> +#include <memory> + +// This example shows you everything you need to know to implement a +// custom stream filter for encoding and decoding as well as a stream +// data provider that modifies the stream's dictionary. This example +// uses the pattern of having the stream data provider class use a +// second QPDF instance with copies of streams from the original QPDF +// so that the stream data provider can access the original stream +// data. This is implement very efficiently inside the qpdf library as +// the second QPDF instance knows how to read the stream data from the +// original input file, so no extra copies of the original stream data +// are made. + +// This example creates an imaginary filter called /XORDecode. There +// is no such filter in PDF, so the streams created by the example +// would not be usable by any PDF reader. However, the techniques here +// would work if you were going to implement support for a filter that +// qpdf does not support natively. For example, using the techinques +// shown here, it would be possible to create an application that +// downsampled or re-encoded images or that re-compressed streams +// using a more efficient "deflate" implementation than zlib. + +// Comments appear throughout the code describing each piece of code +// and its purpose. You can read the file top to bottom, or you can +// start with main() and follow the flow. + +// Please also see the test suite, qtest/custom-filter.test, which +// contains additional comments describing how to observe the results +// of running this example on test files that are specifically crafted +// for it. + +static char const* whoami = 0; + + +class Pl_XOR: public Pipeline +{ + // This class implements a Pipeline for the made-up XOR decoder. + // It is initialized with a single-byte "key" and just XORs each + // byte with that key. This makes it reversible, so there is no + // distinction between encoding and decoding. + + public: + Pl_XOR(char const* identifier, Pipeline* next, unsigned char key); + virtual ~Pl_XOR() = default; + virtual void write(unsigned char* data, size_t len) override; + virtual void finish() override; + + private: + unsigned char key; +}; + +Pl_XOR::Pl_XOR(char const* identifier, Pipeline* next, unsigned char key) : + Pipeline(identifier, next), + key(key) +{ +} + +void +Pl_XOR::write(unsigned char* data, size_t len) +{ + for (size_t i = 0; i < len; ++i) + { + unsigned char p = data[i] ^ this->key; + getNext()->write(&p, 1); + } +} + +void +Pl_XOR::finish() +{ + getNext()->finish(); +} + +class SF_XORDecode: public QPDFStreamFilter +{ + // This class implements a QPDFStreamFilter that knows how to + // validate and interpret decode parameters (/DecodeParms) for the + // made-up /XORDecode stream filter. Since this is not a real + // stream filter, no actual PDF reader would know how to interpret + // it. This is just to illlustrate how to create a stream filter. + // In main(), we call QPDF::registerStreamFilter to tell the + // library about the filter. See comments in QPDFStreamFilter.hh + // for details on how to implement the methods. For purposes of + // example, we are calling this a "specialized" compression + // filter, which just means QPDF assumes that it should not + // "uncompress" the stream by default. + public: + virtual ~SF_XORDecode() = default; + virtual bool setDecodeParms(QPDFObjectHandle decode_parms) override; + virtual Pipeline* getDecodePipeline(Pipeline* next) override; + virtual bool isSpecializedCompression() override; + + private: + unsigned char key; + // It is the responsibility of the QPDFStreamFilter implementation + // to ensure that the pipeline returned by getDecodePipeline() is + // deleted when the class is deleted. The easiest way to do this + // is to stash the pipeline in a std::shared_ptr, which enables us + // to use the default destructor implementation. + std::shared_ptr<Pl_XOR> pipeline; +}; + +bool +SF_XORDecode::setDecodeParms(QPDFObjectHandle decode_parms) +{ + // For purposes of example, we store the key in a separate stream. + // We could just as well store the key directly in /DecodeParms, + // but this example uses a stream to illustrate how one might do + // that. For example, if implementing /JBIG2Decode, one would need + // to handle the /JBIG2Globals key, which points to a stream. See + // comments in SF_XORDecode::registerStream for additional notes + // on this. + try + { + // Expect /DecodeParms to be a dictionary with a /KeyStream + // key that points to a one-byte stream whose single byte is + // the key. If we are successful at retrieving the key, return + // true, indicating that we are able to process with the given + // decode parameters. Under any other circumstances, return + // false. For other examples of QPDFStreamFilter + // implementations, look at the classes whose names start with + // SF_ in the qpdf library implementation. + auto buf = decode_parms.getKey("/KeyStream").getStreamData(); + if (buf->getSize() != 1) + { + return false; + } + this->key = buf->getBuffer()[0]; + return true; + } + catch (std::exception& e) + { + std::cerr << "Error extracting key for /XORDecode: " + << e.what() << std::endl; + } + return false; +} + +Pipeline* +SF_XORDecode::getDecodePipeline(Pipeline* next) +{ + // Return a pipeline that the qpdf library should pass the stream + // data through. The pipeline should receive encoded data and pass + // decoded data to "next". getDecodePipeline() can always count on + // setDecodeParms() having been called first. The setDecodeParms() + // method should store any parameters needed by the pipeline. To + // ensure that the pipeline we return disappears when the class + // disappears, stash it in a std::shared_ptr<Pl_XOR> and retrieve + // the raw pointer from there. + this->pipeline = std::make_shared<Pl_XOR>("xor", next, this->key); + return this->pipeline.get(); +} + +bool +SF_XORDecode::isSpecializedCompression() +{ + // The default implementation of QPDFStreamFilter would return + // false, so if you want a specialized or lossy compression + // filter, override one of the methods as described in + // QPDFStreamFilter.hh. + return true; +} + +class StreamReplacer: public QPDFObjectHandle::StreamDataProvider +{ + // This class implements a StreamDataProvider that, under specific + // conditions, replaces the stream data with data encoded with the + // made-up /XORDecode filter. + + // The flow for this class is as follows: + // + // * The main application iterates through streams that should be + // replaced and calls registerStream. registerStream in turn + // calls maybeReplace passing nullptr to pipeline and the + // address of a valid QPDFObjectHandle to dict_updates. The + // stream passed in for this call is the stream for the original + // QPDF object. It has not yet been altered, so we have access + // to its original dictionary and data. As described in the + // method, the method when called in this way makes a + // determination as to whether the stream should be replaced. If + // so, registerStream makes whatever changes are required. We + // have to do this now because we can't modify the stream during + // the writing process. + // + // * provideStreamData(), which is called by QPDFWriter during the + // write process, actually writes the modified stream data. It + // calls maybeReplace again, but this time it passes a valid + // pipeline and passes nullptr to dict_updates. In this mode, + // the stream dictionary has already been altered, and the + // original stream data is no longer directly accessible. Trying + // to retrieve the stream data would be an infinite loop because + // it would just end up calling provideStreamData again. This is + // why maybeReplace uses a stashed copy of the original stream + // from the "other" QPDF object. + + // Additional explanation can be found in the method + // implementations. + + public: + StreamReplacer(QPDF* pdf); + virtual ~StreamReplacer() = default; + virtual void provideStreamData(int objid, int generation, + Pipeline* pipeline) override; + + void registerStream( + QPDFObjectHandle stream, + PointerHolder<QPDFObjectHandle::StreamDataProvider> self); + + private: + bool maybeReplace(QPDFObjGen const& og, + QPDFObjectHandle& stream, Pipeline* pipeline, + QPDFObjectHandle* dict_updates); + + // Hang onto a reference to the QPDF object containing the streams + // we are replacing. We need this to create a new stream. + QPDF* pdf; + + // This second QPDF instance gives us a place to copy streams to + // so that we can access the original stream data of the streams + // whose data we are replacing. + QPDF other; + + // Map the object/generation in original file to the copied stream + // in "other". We use this to retrieve the original data. + std::map<QPDFObjGen, QPDFObjectHandle> copied_streams; + + // Each stream gets is own "key" for the XOR filter. We use a + // single instance of StreamReplacer for all streams, so stash all + // the keys here. + std::map<QPDFObjGen, unsigned char> keys; +}; + +StreamReplacer::StreamReplacer(QPDF* pdf) : + pdf(pdf) +{ + // Our "other" QPDF is just a place to stash streams. It doesn't + // have to be a valid PDF with pages, etc. We are never going to + // write this out. + this->other.emptyPDF(); +} + +bool +StreamReplacer::maybeReplace(QPDFObjGen const& og, + QPDFObjectHandle& stream, + Pipeline* pipeline, + QPDFObjectHandle* dict_updates) +{ + // As described in the class comments, this method is called + // twice. Before writing has started pipeline is nullptr, and + // dict_updates is provided. In this mode, we figure out whether + // we should replace the stream and, if so, take care of the + // necessary setup. When we are actually ready to supply the data, + // this method is called again with pipeline populated and + // dict_updates as a nullptr. In this mode, we are not allowed to + // change anything, sincing writing is already in progress. We + // must simply provide the stream data. + + // The return value indicates whether or not we should replace the + // stream. If the first call returns false, there will be no + // second call. If the second call returns false, something went + // wrong since the method should always make the same decision for + // a given stream. + + // For this example, all the determination logic could have + // appeared inside the if (dict_updates) block rather than being + // duplicated, but in some cases, there may be a reason to + // duplicate things. For example, if you wanted to write code that + // re-encoded an image if the new encoding was more efficient, + // you'd have to actually try it out. Then you would either have + // to cache the result somewhere or just repeat the calculations, + // depending on space/time constraints, etc. + + // In our contrived example, we are replacing the data for all + // streams that have /DoXOR = true in the stream dictionary. If + // this were a more realistic application, our criteria would be + // more sensible. For example, an image downsampler might choose + // to replace a stream that represented an image with a high pixel + // density. + auto dict = stream.getDict(); + auto mark = dict.getKey("/DoXOR"); + if (! (mark.isBool() && mark.getBoolValue())) + { + return false; + } + + // We can't replace the stream data if we can't get the original + // stream data for any reason. A more realistic application may + // actually look at the data here as well, or it may be able to + // make all its decisions from the stream dictionary. However, + // it's a good idea to make sure we can retrieve the filtered data + // if we are going to need it later. + PointerHolder<Buffer> out; + try + { + out = stream.getStreamData(); + } + catch (...) + { + return false; + } + + if (dict_updates) + { + // It's not safe to make any modifications to any objects + // during the writing process since the updated objects may + // have already been written. In this mode, when dict_updates + // is provided, we have not started writing. Store the + // modifications we intend to make to the stream dictionary + // here. We're just storing /OrigLength for purposes of + // example. Again, a realistic application would make other + // changes. For example, an image resampler might change the + // dimensions or other properties of the image. + dict_updates->replaceKey( + "/OrigLength", QPDFObjectHandle::newInteger( + QIntC::to_longlong(out->getSize()))); + // We are also storing the "key" that we will access when + // writing the data. + this->keys[og] = QIntC::to_uchar( + (og.getObj() * QIntC::to_int(out->getSize())) & 0xff); + } + + if (pipeline) + { + unsigned char key = this->keys[og]; + Pl_XOR p("xor", pipeline, key); + p.write(out->getBuffer(), out->getSize()); + p.finish(); + } + return true; +} + +void +StreamReplacer::registerStream( + QPDFObjectHandle stream, + PointerHolder<QPDFObjectHandle::StreamDataProvider> self) +{ + QPDFObjGen og(stream.getObjGen()); + + // We don't need to process a stream more than once. In this + // example, we are just iterating through objects, but if we were + // doing something like iterating through images on pages, we + // might realistically encounter the same stream more than once. + if (this->copied_streams.count(og) > 0) + { + return; + } + // Store something in copied_streams so that we don't + // double-process even in the negative case. This gets replaced + // later if needed. + this->copied_streams[og] = QPDFObjectHandle::newNull(); + + // Call maybeReplace with dict_updates. In this mode, it + // determines whether we should replace the stream data and, if + // so, supplies dictionary updates we should make. + bool should_replace = false; + QPDFObjectHandle dict_updates = QPDFObjectHandle::newDictionary(); + try + { + should_replace = maybeReplace(og, stream, nullptr, &dict_updates); + } + catch (std::exception& e) + { + stream.warnIfPossible( + std::string("exception while attempting to replace: ") + + e.what()); + } + + if (should_replace) + { + // Copy the stream to another QPDF object so we can get to the + // original data from the stream data provider. + this->copied_streams[og] = this->other.copyForeignObject(stream); + // Update the stream dictionary with any changes. + auto dict = stream.getDict(); + for (auto const& k: dict_updates.getKeys()) + { + dict.replaceKey(k, dict_updates.getKey(k)); + } + // Create the key stream that will be referenced from + // /DecodeParms. We have to do this now since you can't modify + // or create objects during write. + char p[1] = { static_cast<char>(this->keys[og]) }; + std::string p_str(p, 1); + QPDFObjectHandle dp_stream = + QPDFObjectHandle::newStream(this->pdf, p_str); + // Create /DecodeParms as expected by our fictitious + // /XORDecode filter. + QPDFObjectHandle decode_parms = + QPDFObjectHandle::newDictionary({{"/KeyStream", dp_stream}}); + stream.replaceStreamData( + self, + QPDFObjectHandle::newName("/XORDecode"), + decode_parms); + // Further, if /ProtectXOR = true, we disable filtering on write + // so that QPDFWriter will not decode the stream even though we + // have registered a stream filter for /XORDecode. + auto protect = dict.getKey("/ProtectXOR"); + if (protect.isBool() && protect.getBoolValue()) + { + stream.setFilterOnWrite(false); + } + } +} + +void +StreamReplacer::provideStreamData(int objid, int generation, + Pipeline* pipeline) +{ + QPDFObjGen og(objid, generation); + QPDFObjectHandle orig = this->copied_streams[og]; + // call maybeReplace again, this time with the pipeline and no + // dict_updates. In this mode, maybeReplace doesn't make any + // changes. We have to hand it the original stream data, which we + // get from copied_streams. + if (! maybeReplace(og, orig, pipeline, nullptr)) + { + // Since this only gets called for streams we already + // determined we are replacing, a false return would indicate + // a logic error. + throw std::logic_error( + "should_replace return false in provideStreamData"); + } +} + +static void process(char const* infilename, char const* outfilename, + bool decode_specialized) +{ + QPDF qpdf; + qpdf.processFile(infilename); + + // Create a single StreamReplacer instance. The interface requires + // a PointerHolder in various places, so allocate a StreamReplacer + // and stash it in a PointerHolder. + StreamReplacer* replacer = new StreamReplacer(&qpdf); + PointerHolder<QPDFObjectHandle::StreamDataProvider> p(replacer); + + for (auto& o: qpdf.getAllObjects()) + { + if (o.isStream()) + { + // Call registerStream for every stream. Only ones that + // registerStream decides to replace will actually be + // replaced. + replacer->registerStream(o, p); + } + } + + QPDFWriter w(qpdf, outfilename); + if (decode_specialized) + { + w.setDecodeLevel(qpdf_dl_specialized); + } + // For the test suite, use static IDs. + w.setStaticID(true); // for testing only + w.write(); + std::cout << whoami << ": new file written to " << outfilename + << std::endl; +} + +static void usage() +{ + std::cerr + << "\n" + << "Usage: " << whoami << " [ --decode-specialized ] infile outfile\n" + << std::endl; + exit(2); +} + +int main(int argc, char* argv[]) +{ + whoami = QUtil::getWhoami(argv[0]); + + // For libtool's sake.... + if (strncmp(whoami, "lt-", 3) == 0) + { + whoami += 3; + } + + char const* infilename = 0; + char const* outfilename = 0; + bool decode_specialized = false; + for (int i = 1; i < argc; ++i) + { + if (strcmp(argv[i], "--decode-specialized") == 0) + { + decode_specialized = true; + } + else if (! infilename) + { + infilename = argv[i]; + } + else if (! outfilename) + { + outfilename = argv[i]; + } + else + { + usage(); + } + } + if (! (infilename && outfilename)) + { + usage(); + } + + try + { + // Register our fictitious filter. This enables QPDFWriter to + // decode our streams. This is not a real filter, so no real + // PDF reading application would be able to interpret it. This + // is just for illustrative purposes. + QPDF::registerStreamFilter( + "/XORDecode", []{ return std::make_shared<SF_XORDecode>(); }); + // Do the actual processing. + process(infilename, outfilename, decode_specialized); + } + catch (std::exception &e) + { + std::cerr << whoami << ": exception: " << e.what() << std::endl; + exit(2); + } + + return 0; +} |