Dear all,

I am trying to speed up the loading of a JPEG image by replacing the decode step during the loading with some libturbojpeg functions.
So I put together a minimal working example from several pieces which I found in forums etc:


.pro file:
Qt Code: 
  1. QT       += core gui
  2.  
  3. greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  4.  
  5. TARGET = Application
  6. TEMPLATE = app
  7.  
  8.  
  9. SOURCES += main.cpp\
  10.         mainwindow.cpp\
  11.     JpegEncoderDecoder/JpegEncoderDecoder.cpp
  12.  
  13. HEADERS  += mainwindow.h\
  14.     JpegEncoderDecoder/JpegEncoderDecoder.h
  15.  
  16. FORMS    += mainwindow.ui
  17.  
  18.  
  19. LIBS += -L$$PWD/usr/lib/  -ljpeg -lturbojpeg
  20.  
  21. INCLUDEPATH += $$PWD/usr/include
  22. DEPENDPATH += $$PWD/usr/include
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main.cpp:
Qt Code: 
  1. #include "mainwindow.h"
  2. #include <QApplication>
  3. #include <QImageReader>
  4. #include <QByteArray>
  5. #include <QFile>
  6. #include <QBuffer>
  7. #include <QString>
  8.  
  9. #include <jpeglib.h>
  10.  
  11. #include <iostream>
  12. #include <fstream>
  13. #include <string>
  14.  
  15. #include <JpegEncoderDecoder/JpegEncoderDecoder.h>
  16.  
  17.  
  18. int read_jpeg_file(unsigned char *pInputDataStream, std::string file_name)
  19. {
  20.     std::ifstream imageStream;
  21.     imageStream.open(file_name.c_str(), std::ios_base::binary);
  22.  
  23.     imageStream.seekg (0, std::ios::end);
  24.     int n = imageStream.tellg();
  25.     imageStream.seekg (0, std::ios::beg);
  26.  
  27.     pInputDataStream = new unsigned char[n];
  28.  
  29.  
  30.     if (!imageStream.is_open())
  31.     {
  32.         std::cout << stderr << "Can't open input file !\n";
  33.         exit(1);
  34.     }
  35.     else
  36.     {
  37.         imageStream.read((char *)pInputDataStream, n);
  38.     }
  39.     return 0;
  40. }
  41.  
  42.  
  43.  
  44. int main(int argc, char *argv[])
  45. {
  46.     QApplication a(argc, argv);
  47.  
  48.     QString qPath("jpeg-100.jpg");
  49.  
  50.     unsigned char *pInputDataStream;
  51.     int width = 320;
  52.     int height= 240;
  53.     int JPEG_QUALITY=75;
  54.     int COLOR_COMPONENTS=3;
  55.     int in_len = width*height*COLOR_COMPONENTS;
  56.     std::string strFileName=qPath.toStdString();
  57.  
  58.     //Step 1: Load .jpeg in memory filebuffer
  59.     int iRead = read_jpeg_file(pInputDataStream, strFileName);
  60.  
  61.     //Step 2: Use turbojpeglib to decompress filebuffer to uncompressedbuffer
  62.     JpegEncoderDecoder bla(width, height, JPEG_QUALITY, COLOR_COMPONENTS);
  63.     unsigned char*buffer = bla.decodeJpeg(pInputDataStream);
  64.  
  65.     //Step 3: Convert to QImage
  66.     QImage myDecodedJPEGimage(buffer, width, height, QImage::Format_RGB888) ;
  67.  
  68.     //Step 4: Show in main window
  69.     MainWindow w(myDecodedJPEGimage);
  70.     w.show();
  71.  
  72.     return a.exec();
  73. }
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JpegEncoderDecoder.h:
Qt Code: 
  1. #ifndef JPEGENCODERDECODER_H
  2. #define JPEGENCODERDECODER_H
  3.  
  4. class JpegEncoderDecoder
  5. {
  6. public:
  7.     JpegEncoderDecoder(int width=1920,  int height=1080, int JPEG_QUALITY=75, int COLOR_COMPONENTS=3);
  8.  
  9.     void encodeJpeg(unsigned char * pBuffer);
  10.     unsigned char * decodeJpeg(unsigned char * pcompressedImage);
  11.  
  12. private:
  13.     int m_width;
  14.     int m_height;
  15.     const int m_JPEG_QUALITY ;
  16.     const int m_COLOR_COMPONENTS;
  17. };
  18.  
  19. #endif // JPEGENCODERDECODER_H
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Qt Code: 
  1. // some includes....
  2.  
  3. JpegEncoderDecoder::JpegEncoderDecoder(int width,  int height, int JPEG_QUALITY, int COLOR_COMPONENTS) :
  4.     m_width (width)
  5. ,   m_height(height)
  6. ,   m_JPEG_QUALITY(JPEG_QUALITY)
  7. ,   m_COLOR_COMPONENTS(COLOR_COMPONENTS)
  8. {
  9. }
  10.  
  11. unsigned char * JpegEncoderDecoder::decodeJpeg(unsigned char *pcompressedImage)
  12. {
  13.     long unsigned int _jpegSize; //!< _jpegSize from above
  14.  
  15.  
  16.     int jpegSubsamp;
  17.     unsigned char buffer[m_width*m_height*m_COLOR_COMPONENTS]; //!< will contain the decompressed image
  18.  
  19.     tjhandle _jpegDecompressor = tjInitDecompress();
  20.  
  21.     tjDecompressHeader2(_jpegDecompressor, pcompressedImage, _jpegSize, &m_width, &m_height, &jpegSubsamp);
  22.  
  23.     tjDecompress2(_jpegDecompressor, pcompressedImage, _jpegSize, buffer, m_width, 0/*pitch*/, m_height, TJPF_RGB, TJFLAG_FASTDCT);
  24.  
  25.     tjDestroy(_jpegDecompressor);
  26.  
  27.     return buffer;
  28. }
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I am omitting the definition of JpegEncoderDecoder::encodeJpeg, as well as mainwindow.h and mainwindow.cpp here.


This is compiling and also running fine, but unfortunately it does not show the image as expected. Instead I just see a black rectangle with some colored pixels in it. What am I doing wrong?