diff options
Diffstat (limited to 'examples/pdf-invert-images.cc')
| -rw-r--r-- | examples/pdf-invert-images.cc | 65 |
1 files changed, 26 insertions, 39 deletions
diff --git a/examples/pdf-invert-images.cc b/examples/pdf-invert-images.cc index 5692e7b2..0ef2169e 100644 --- a/examples/pdf-invert-images.cc +++ b/examples/pdf-invert-images.cc @@ -20,15 +20,12 @@ usage() exit(2); } -// Derive a class from StreamDataProvider to provide updated stream -// data. The main purpose of using this object is to avoid having to -// allocate memory up front for the objects. We want to replace the -// stream data with a function of the original stream data. In order -// to do this without actually holding all the images in memory, we -// create copies of the streams. Copying the streams doesn't actually -// copy the data. Internally, the qpdf library is holding onto the -// location of the original stream data, which makes it possible for -// the StreamDataProvider to access it when it needs it. +// Derive a class from StreamDataProvider to provide updated stream data. The main purpose of using +// this object is to avoid having to allocate memory up front for the objects. We want to replace +// the stream data with a function of the original stream data. In order to do this without actually +// holding all the images in memory, we create copies of the streams. Copying the streams doesn't +// actually copy the data. Internally, the qpdf library is holding onto the location of the original +// stream data, which makes it possible for the StreamDataProvider to access it when it needs it. class ImageInverter: public QPDFObjectHandle::StreamDataProvider { public: @@ -46,42 +43,35 @@ void ImageInverter::registerImage( QPDFObjectHandle image, std::shared_ptr<QPDFObjectHandle::StreamDataProvider> self) { - // replaceStreamData requires a pointer holder to the stream data - // provider, but there's no way for us to generate one ourselves, - // so we have to have it handed to us. Don't be tempted to have - // the class contain a std::shared_ptr to itself as a member. Doing - // this will prevent the class from ever being deleted since the - // reference count will never drop to zero (and std::shared_ptr - // doesn't have weak references). + // replaceStreamData requires a pointer holder to the stream data provider, but there's no way + // for us to generate one ourselves, so we have to have it handed to us. Don't be tempted to + // have the class contain a std::shared_ptr to itself as a member. Doing this will prevent the + // class from ever being deleted since the reference count will never drop to zero (and + // std::shared_ptr doesn't have weak references). QPDFObjGen og(image.getObjGen()); - // Store information about the images based on the object and - // generation number. Recall that a single image object may be - // used more than once, so no need to update the same stream - // multiple times. + // Store information about the images based on the object and generation number. Recall that a + // single image object may be used more than once, so no need to update the same stream multiple + // times. if (this->copied_images.count(og) > 0) { return; } this->copied_images[og] = image.copyStream(); - // Register our stream data provider for this stream. Future calls - // to getStreamData or pipeStreamData will use the new - // information. Provide null for both filter and decode - // parameters. Note that this does not mean the image data will be - // uncompressed when we write the file. By default, QPDFWriter - // will use /FlateDecode for anything that is uncompressed or - // filterable in the input QPDF object, so we don't have to deal - // with it explicitly here. We could explicitly use /DCTDecode and - // write through a DCT filter if we wanted. + // Register our stream data provider for this stream. Future calls to getStreamData or + // pipeStreamData will use the new information. Provide null for both filter and decode + // parameters. Note that this does not mean the image data will be uncompressed when we write + // the file. By default, QPDFWriter will use /FlateDecode for anything that is uncompressed or + // filterable in the input QPDF object, so we don't have to deal with it explicitly here. We + // could explicitly use /DCTDecode and write through a DCT filter if we wanted. image.replaceStreamData(self, QPDFObjectHandle::newNull(), QPDFObjectHandle::newNull()); } void ImageInverter::provideStreamData(QPDFObjGen const& og, Pipeline* pipeline) { - // Use the object and generation number supplied to look up the - // image data. Then invert the image data and write the inverted - // data to the pipeline. + // Use the object and generation number supplied to look up the image data. Then invert the + // image data and write the inverted data to the pipeline. std::shared_ptr<Buffer> data = this->copied_images[og].getStreamData(qpdf_dl_all); size_t size = data->getSize(); unsigned char* buf = data->getBuffer(); @@ -130,11 +120,9 @@ main(int argc, char* argv[]) QPDFObjectHandle color_space = image_dict.getKey("/ColorSpace"); QPDFObjectHandle bits_per_component = image_dict.getKey("/BitsPerComponent"); - // For our example, we can only work with images 8-bit - // grayscale images that we can fully decode. Use - // pipeStreamData with a null pipeline to determine - // whether the image is filterable. Directly inspect - // keys to determine the image type. + // For our example, we can only work with images 8-bit grayscale images that we can + // fully decode. Use pipeStreamData with a null pipeline to determine whether the + // image is filterable. Directly inspect keys to determine the image type. if (image.pipeStreamData(nullptr, qpdf_ef_compress, qpdf_dl_all) && color_space.isNameAndEquals("/DeviceGray") && bits_per_component.isInteger() && (bits_per_component.getIntValue() == 8)) { @@ -146,8 +134,7 @@ main(int argc, char* argv[]) // Write out a new file QPDFWriter w(qpdf, outfilename); if (static_id) { - // For the test suite, uncompress streams and use static - // IDs. + // For the test suite, uncompress streams and use static IDs. w.setStaticID(true); // for testing only } w.write(); |