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lowLevelIoExamples.cpp

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//-----------------------------------------------------------------------------
//
//    Code examples that show how implement custom low-level
//    file input and output for OpenEXR files:
//
//    Classes C_IStream and C_OStream are derived from IlmImf's
//    abstract IStream and OStreamd classes.  They allow OpenEXR
//    file input and output via C stdio files (FILE *).
//
//-----------------------------------------------------------------------------


#include <ImfRgbaFile.h>
#include <ImfIO.h>
#include "Iex.h"
#include <drawImage.h>

#include <iostream>
#include <stdio.h>

using namespace std;
using namespace Imf;
using namespace Imath;


class C_IStream: public IStream
{
  public:

    C_IStream (FILE *file, const char fileName[]):
      IStream (fileName), _file (file) {}

    virtual bool  read (char c[/*n*/], int n);
    virtual Int64 tellg ();
    virtual void  seekg (Int64 pos);
    virtual void  clear ();

  private:

    FILE *        _file;
};


class C_OStream: public OStream
{
  public:

    C_OStream (FILE *file, const char fileName[]):
      OStream (fileName), _file (file) {}

    virtual void  write (const char c[/*n*/], int n);
    virtual Int64 tellp ();
    virtual void  seekp (Int64 pos);

  private:

    FILE *        _file;
};


bool
C_IStream::read (char c[/*n*/], int n)
{
    if (n != fread (c, 1, n, _file))
    {
      //
      // fread() failed, but the return value does not distinguish
      // between I/O errors and end of file, so we call ferror() to
      // determine what happened.
      //

      if (ferror (_file))
          Iex::throwErrnoExc();
      else
          throw Iex::InputExc ("Unexpected end of file.");
    }

    return feof (_file);
}


Int64
C_IStream::tellg ()
{
    return ftell (_file);
}


void
C_IStream::seekg (Int64 pos)
{
    clearerr (_file);
    fseek (_file, pos, SEEK_SET);
}


void
C_IStream::clear ()
{
    clearerr (_file);
}


void
C_OStream::write (const char c[/*n*/], int n)
{
    clearerr (_file);

    if (n != fwrite (c, 1, n, _file))
      Iex::throwErrnoExc();
}


Int64
C_OStream::tellp ()
{
    return ftell (_file);
}


void
C_OStream::seekp (Int64 pos)
{
    clearerr (_file);
    fseek (_file, pos, SEEK_SET);
}


void
writeRgbaFILE (FILE *cfile,
             const char fileName[],
             const Rgba *pixels,
             int width,
             int height)
{
    //
    // Store an RGBA image in a C stdio file that has already been opened:
    //
    //      - create a C_OStream object for writing to the file
    //      - create an RgbaOutputFile object, and attach it to the C_OStream
    //      - describe the memory layout of the pixels
    //      - store the pixels in the file
    //


    C_OStream ostr (cfile, fileName);
    RgbaOutputFile file (ostr, Header (width, height), WRITE_RGBA);
    file.setFrameBuffer (pixels, 1, width);
    file.writePixels (height);
}


void
readRgbaFILE (FILE *cfile,
            const char fileName[],
            Array2D<Rgba> &pixels,
            int &width,
            int &height)
{
    //
    // Read an RGBA image from a C stdio file that has already been opened:
    //
    //      - create a C_IStream object for reading from the file
    //      - create an RgbaInputFile object, and attach it to the C_IStream
    //      - allocate memory for the pixels
    //      - describe the memory layout of the pixels
    //      - read the pixels from the file
    //

    C_IStream istr (cfile, fileName);
    RgbaInputFile file (istr);
    Box2i dw = file.dataWindow();

    width  = dw.max.x - dw.min.x + 1;
    height = dw.max.y - dw.min.y + 1;
    pixels.resizeErase (height, width);

    file.setFrameBuffer (&pixels[0][0] - dw.min.x - dw.min.y * width, 1, width);
    file.readPixels (dw.min.y, dw.max.y);
}


void
lowLevelIoExamples ()
{
    cout << "\nCustom low-level file I/O\n" << endl;
    cout << "drawing image" << endl;

    int w = 800;
    int h = 600;
    const char *fileName = "rgba4.exr";

    Array2D<Rgba> p (h, w);
    drawImage1 (p, w, h);

    //
    // The following code is somewhat complicated because
    // fopen() returns 0 on error, but writeRgbaFILE() and
    // readRgbaFILE() throw exceptions.  Also, if a FILE *
    // goes out of scope, the corresponding file is not
    // automatically closed.  In order to avoid leaking
    // file descriptors, we have to make sure fclose() is
    // called in all cases.
    //

    cout << "writing image" << endl;

    {
      FILE *file = fopen (fileName, "wb");

      if (file == 0)
      {
          THROW_ERRNO ("Cannot open file " << fileName << " (%T).");
      }
      else
      {
          try
          {
            writeRgbaFILE (file, fileName, &p[0][0], w, h);
          }
          catch (...)
          {
            fclose (file);
            throw;
          }

          fclose (file);
      }
    }

    cout << "reading image" << endl;

    {
      FILE *file = fopen (fileName, "rb");

      if (file == 0)
      {
          THROW_ERRNO ("Cannot open file " << fileName << " (%T).");
      }
      else
      {
          try
          {
            readRgbaFILE (file, fileName, p, w, h);
          }
          catch (...)
          {
            fclose (file);
            throw;
          }

          fclose (file);
      }
    }
}

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