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qpdf/examples/pdf-custom-filter.cc

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#include <qpdf/QPDF.hh>
#include <qpdf/QPDFStreamFilter.hh>
#include <qpdf/QPDFWriter.hh>
#include <qpdf/QUtil.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 implemented 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 techniques
// 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 illustrate 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 cause an infinite loop because
// it would just end up calling provideStreamData again. This is
// why maybeReplace uses a stashed copy of the original stream.
// 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,
std::shared_ptr<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;
// 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)
{
}
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, since 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.
std::shared_ptr<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,
std::shared_ptr<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 so we can get to the original data from the
// stream data provider. This doesn't actually copy any data,
// but the copy retains the original stream data after the
// original one is modified.
this->copied_streams[og] = stream.copyStream();
// 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 std::shared_ptr in various places, so allocate a StreamReplacer
// and stash it in a std::shared_ptr.
StreamReplacer* replacer = new StreamReplacer(&qpdf);
std::shared_ptr<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]);
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;
}
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