openexr/IlmImf/ImfOutputFile.cpp

///////////////////////////////////////////////////////////////////////////
//
// Copyright (c) 2004, Industrial Light & Magic, a division of Lucas
// Digital Ltd. LLC
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
// *       Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// *       Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
// *       Neither the name of Industrial Light & Magic nor the names of
// its contributors may be used to endorse or promote products derived
// from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
///////////////////////////////////////////////////////////////////////////


//-----------------------------------------------------------------------------
//
//	class OutputFile
//
//-----------------------------------------------------------------------------

#include <ImfOutputFile.h>
#include <ImfInputFile.h>
#include <ImfChannelList.h>
#include <ImfMisc.h>
#include <ImfStdIO.h>
#include <ImfCompressor.h>
#include "ImathBox.h"
#include "ImathFun.h"
#include <ImfArray.h>
#include <ImfXdr.h>
#include <ImfPreviewImageAttribute.h>
#include "IlmThreadPool.h"
#include "IlmThreadSemaphore.h"
#include "IlmThreadMutex.h"
#include "Iex.h"
#include <string>
#include <vector>
#include <fstream>
#include <assert.h>
#include <algorithm> // for std::max()


namespace Imf {

using Imath::Box2i;
using Imath::divp;
using Imath::modp;
using std::string;
using std::vector;
using std::ofstream;
using std::min;
using std::max;
using IlmThread::Mutex;
using IlmThread::Lock;
using IlmThread::Semaphore;
using IlmThread::Task;
using IlmThread::TaskGroup;
using IlmThread::ThreadPool;

namespace {


struct OutSliceInfo
{
    PixelType		type;
    const char *	base;
    size_t		xStride;
    size_t		yStride;
    int			xSampling;
    int			ySampling;
    bool		zero;

    OutSliceInfo (PixelType type = HALF,
              const char *base = 0,
              size_t xStride = 0,
              size_t yStride = 0,
              int xSampling = 1,
              int ySampling = 1,
              bool zero = false);
};


OutSliceInfo::OutSliceInfo (PixelType t,
                    const char *b,
                    size_t xs, size_t ys,
                    int xsm, int ysm,
                    bool z)
:
    type (t),
    base (b),
    xStride (xs),
    yStride (ys),
    xSampling (xsm),
    ySampling (ysm),
    zero (z)
{
    // empty
}


struct LineBuffer
{
    Array<char>		buffer;
    const char *	dataPtr;
    int			dataSize;
    char *		endOfLineBufferData;
    int			minY;
    int			maxY;
    int			scanLineMin;
    int			scanLineMax;
    Compressor *	compressor;
    bool		partiallyFull;        // has incomplete data
    bool		hasException;
    string		exception;

    LineBuffer (Compressor *comp);
    ~LineBuffer ();

    void		wait () {_sem.wait();}
    void		post () {_sem.post();}

  private:

    Semaphore		_sem;
};


LineBuffer::LineBuffer (Compressor *comp) :
    dataPtr (0),
    dataSize (0),
    compressor (comp),
    partiallyFull (false),
    hasException (false),
    exception (),
    _sem (1)
{
    // empty
}


LineBuffer::~LineBuffer ()
{
    delete compressor;
}

} // namespace


struct OutputFile::Data: public Mutex
{
    Header		 header;		// the image header
    int			 version;		// file format version
    Int64		 previewPosition;       // file position for preview
    FrameBuffer		 frameBuffer;           // framebuffer to write into
    int			 currentScanLine;       // next scanline to be written
    int			 missingScanLines;      // number of lines to write
    LineOrder		 lineOrder;		// the file's lineorder
    int			 minX;			// data window's min x coord
    int			 maxX;			// data window's max x coord
    int			 minY;			// data window's min y coord
    int			 maxY;			// data window's max x coord
    vector<Int64>	 lineOffsets;		// stores offsets in file for
                        // each scanline
    vector<size_t>	 bytesPerLine;          // combined size of a line over
                                                // all channels
    vector<size_t>	 offsetInLineBuffer;    // offset for each scanline in
                                                // its linebuffer
    Compressor::Format	 format;                // compressor's data format
    vector<OutSliceInfo> slices;		// info about channels in file
    OStream *		 os;			// file stream to write to
    bool		 deleteStream;
    Int64		 lineOffsetsPosition;   // file position for line
                                                // offset table
    Int64		 currentPosition;       // current file position

    vector<LineBuffer*>  lineBuffers;           // each holds one line buffer
    int			 linesInBuffer;         // number of scanlines each
                                                // buffer holds
    size_t		 lineBufferSize;        // size of the line buffer

     Data (bool deleteStream, int numThreads);
    ~Data ();


    inline LineBuffer *	getLineBuffer (int number); // hash function from line
                                // buffer indices into our
                            // vector of line buffers
};


OutputFile::Data::Data (bool deleteStream, int numThreads):
    os (0),
    deleteStream (deleteStream),
    lineOffsetsPosition (0)
{
    //
    // We need at least one lineBuffer, but if threading is used,
    // to keep n threads busy we need 2*n lineBuffers.
    //

    lineBuffers.resize (max (1, 2 * numThreads));
}


OutputFile::Data::~Data ()
{
    if (deleteStream)
    delete os;

    for (size_t i = 0; i < lineBuffers.size(); i++)
        delete lineBuffers[i];
}


LineBuffer*
OutputFile::Data::getLineBuffer (int number)
{
    return lineBuffers[number % lineBuffers.size()];
}


namespace {


Int64
writeLineOffsets (OStream &os, const vector<Int64> &lineOffsets)
{
    Int64 pos = os.tellp();

    if (pos == -1)
    Iex::throwErrnoExc ("Cannot determine current file position (%T).");

    for (unsigned int i = 0; i < lineOffsets.size(); i++)
    Xdr::write <StreamIO> (os, lineOffsets[i]);

    return pos;
}


void
writePixelData (OutputFile::Data *ofd,
                int lineBufferMinY,
        const char pixelData[],
        int pixelDataSize)
{
    //
    // Store a block of pixel data in the output file, and try
    // to keep track of the current writing position the file
    // without calling tellp() (tellp() can be fairly expensive).
    //

    Int64 currentPosition = ofd->currentPosition;
    ofd->currentPosition = 0;

    if (currentPosition == 0)
    currentPosition = ofd->os->tellp();

    ofd->lineOffsets[(ofd->currentScanLine - ofd->minY) / ofd->linesInBuffer] =
    currentPosition;

    #ifdef DEBUG

    assert (ofd->os->tellp() == currentPosition);

    #endif

    Xdr::write <StreamIO> (*ofd->os, lineBufferMinY);
    Xdr::write <StreamIO> (*ofd->os, pixelDataSize);
    ofd->os->write (pixelData, pixelDataSize);

    ofd->currentPosition = currentPosition +
               Xdr::size<int>() +
               Xdr::size<int>() +
               pixelDataSize;
}


inline void
writePixelData (OutputFile::Data *ofd, const LineBuffer *lineBuffer)
{
    writePixelData (ofd,
            lineBuffer->minY,
                    lineBuffer->dataPtr,
            lineBuffer->dataSize);
}


void
convertToXdr (OutputFile::Data *ofd,
              Array<char> &lineBuffer,
              int lineBufferMinY,
              int lineBufferMaxY,
              int /*inSize*/)
{
    //
    // Convert the contents of a lineBuffer from the machine's native
    // representation to Xdr format.  This function is called by
    // CompressLineBuffer::execute(), below, if the compressor wanted
    // its input pixel data in the machine's native format, but then
    // failed to compress the data (most compressors will expand rather
    // than compress random input data).
    //
    // Note that this routine assumes that the machine's native
    // representation of the pixel data has the same size as the
    // Xdr representation.  This makes it possible to convert the
    // pixel data in place, without an intermediate temporary buffer.
    //

    int startY, endY;		// The first and last scanlines in
                    // the file that are in the lineBuffer.
    int step;

    if (ofd->lineOrder == INCREASING_Y)
    {
    startY = max (lineBufferMinY, ofd->minY);
    endY = min (lineBufferMaxY, ofd->maxY) + 1;
        step = 1;
    }
    else
    {
    startY = min (lineBufferMaxY, ofd->maxY);
    endY = max (lineBufferMinY, ofd->minY) - 1;
        step = -1;
    }

    //
    // Iterate over all scanlines in the lineBuffer to convert.
    //

    for (int y = startY; y != endY; y += step)
    {
    //
        // Set these to point to the start of line y.
        // We will write to writePtr from readPtr.
    //

        char *writePtr = lineBuffer + ofd->offsetInLineBuffer[y - ofd->minY];
        const char *readPtr = writePtr;

    //
        // Iterate over all slices in the file.
    //

        for (unsigned int i = 0; i < ofd->slices.size(); ++i)
        {
            //
            // Test if scan line y of this channel is
            // contains any data (the scan line contains
            // data only if y % ySampling == 0).
            //

            const OutSliceInfo &slice = ofd->slices[i];

            if (modp (y, slice.ySampling) != 0)
                continue;

            //
            // Find the number of sampled pixels, dMaxX-dMinX+1, for
        // slice i in scan line y (i.e. pixels within the data window
            // for which x % xSampling == 0).
            //

            int dMinX = divp (ofd->minX, slice.xSampling);
            int dMaxX = divp (ofd->maxX, slice.xSampling);

        //
            // Convert the samples in place.
        //

            convertInPlace (writePtr, readPtr, slice.type, dMaxX - dMinX + 1);
        }
    }
}


//
// A LineBufferTask encapsulates the task of copying a set of scanlines
// from the user's frame buffer into a LineBuffer object, compressing
// the data if necessary.
//

class LineBufferTask: public Task
{
  public:

    LineBufferTask (TaskGroup *group,
                    OutputFile::Data *ofd,
            int number,
                    int scanLineMin,
            int scanLineMax);

    virtual ~LineBufferTask ();

    virtual void	execute ();

  private:

    OutputFile::Data *	_ofd;
    LineBuffer *	_lineBuffer;
};


LineBufferTask::LineBufferTask
    (TaskGroup *group,
     OutputFile::Data *ofd,
     int number,
     int scanLineMin,
     int scanLineMax)
:
    Task (group),
    _ofd (ofd),
    _lineBuffer (_ofd->getLineBuffer(number))
{
    //
    // Wait for the lineBuffer to become available
    //

    _lineBuffer->wait ();

    //
    // Initialize the lineBuffer data if necessary
    //

    if (!_lineBuffer->partiallyFull)
    {
        _lineBuffer->endOfLineBufferData = _lineBuffer->buffer;

        _lineBuffer->minY = _ofd->minY + number * _ofd->linesInBuffer;

        _lineBuffer->maxY = min (_lineBuffer->minY + _ofd->linesInBuffer - 1,
                 _ofd->maxY);

        _lineBuffer->partiallyFull = true;
    }

    _lineBuffer->scanLineMin = max (_lineBuffer->minY, scanLineMin);
    _lineBuffer->scanLineMax = min (_lineBuffer->maxY, scanLineMax);
}


LineBufferTask::~LineBufferTask ()
{
    //
    // Signal that the line buffer is now free
    //

    _lineBuffer->post ();
}


void
LineBufferTask::execute ()
{
    try
    {
        //
        // First copy the pixel data from the
    // frame buffer into the line buffer
        //

        int yStart, yStop, dy;

        if (_ofd->lineOrder == INCREASING_Y)
        {
            yStart = _lineBuffer->scanLineMin;
            yStop = _lineBuffer->scanLineMax + 1;
            dy = 1;
        }
        else
        {
            yStart = _lineBuffer->scanLineMax;
            yStop = _lineBuffer->scanLineMin - 1;
            dy = -1;
        }

    int y;

        for (y = yStart; y != yStop; y += dy)
        {
            //
            // Gather one scan line's worth of pixel data and store
            // them in _ofd->lineBuffer.
            //

            char *writePtr = _lineBuffer->buffer +
                             _ofd->offsetInLineBuffer[y - _ofd->minY];
            //
            // Iterate over all image channels.
            //

            for (unsigned int i = 0; i < _ofd->slices.size(); ++i)
            {
                //
                // Test if scan line y of this channel contains any data
        // (the scan line contains data only if y % ySampling == 0).
                //

                const OutSliceInfo &slice = _ofd->slices[i];

                if (modp (y, slice.ySampling) != 0)
                    continue;

                //
                // Find the x coordinates of the leftmost and rightmost
                // sampled pixels (i.e. pixels within the data window
                // for which x % xSampling == 0).
                //

                int dMinX = divp (_ofd->minX, slice.xSampling);
                int dMaxX = divp (_ofd->maxX, slice.xSampling);

                //
        // Fill the line buffer with with pixel data.
                //

                if (slice.zero)
                {
                    //
                    // The frame buffer contains no data for this channel.
                    // Store zeroes in _lineBuffer->buffer.
                    //

                    fillChannelWithZeroes (writePtr, _ofd->format, slice.type,
                                           dMaxX - dMinX + 1);
                }
                else
                {
                    //
                    // If necessary, convert the pixel data to Xdr format.
            // Then store the pixel data in _ofd->lineBuffer.
                    //

                    const char *linePtr = slice.base +
                                          divp (y, slice.ySampling) *
                                          slice.yStride;

                    const char *readPtr = linePtr + dMinX * slice.xStride;
                    const char *endPtr  = linePtr + dMaxX * slice.xStride;

                    copyFromFrameBuffer (writePtr, readPtr, endPtr,
                                         slice.xStride, _ofd->format,
                                         slice.type);
                }
            }

            if (_lineBuffer->endOfLineBufferData < writePtr)
                _lineBuffer->endOfLineBufferData = writePtr;

            #ifdef DEBUG

                assert (writePtr - (_lineBuffer->buffer +
                        _ofd->offsetInLineBuffer[y - _ofd->minY]) ==
                        (int) _ofd->bytesPerLine[y - _ofd->minY]);

            #endif

        }

        //
        // If the next scanline isn't past the bounds of the lineBuffer
        // then we are done, otherwise compress the linebuffer
        //

        if (y >= _lineBuffer->minY && y <= _lineBuffer->maxY)
            return;

        _lineBuffer->dataPtr = _lineBuffer->buffer;

        _lineBuffer->dataSize = _lineBuffer->endOfLineBufferData -
                                _lineBuffer->buffer;

    //
        // Compress the data
    //

        Compressor *compressor = _lineBuffer->compressor;

        if (compressor)
        {
            const char *compPtr;

            int compSize = compressor->compress (_lineBuffer->dataPtr,
                                                 _lineBuffer->dataSize,
                                                 _lineBuffer->minY, compPtr);

            if (compSize < _lineBuffer->dataSize)
            {
                _lineBuffer->dataSize = compSize;
                _lineBuffer->dataPtr = compPtr;
            }
            else if (_ofd->format == Compressor::NATIVE)
            {
                //
                // The data did not shrink during compression, but
                // we cannot write to the file using the machine's
                // native format, so we need to convert the lineBuffer
                // to Xdr.
                //

                convertToXdr (_ofd, _lineBuffer->buffer, _lineBuffer->minY,
                              _lineBuffer->maxY, _lineBuffer->dataSize);
            }
        }

        _lineBuffer->partiallyFull = false;
    }
    catch (std::exception &e)
    {
        if (!_lineBuffer->hasException)
        {
            _lineBuffer->exception = e.what ();
            _lineBuffer->hasException = true;
        }
    }
    catch (...)
    {
        if (!_lineBuffer->hasException)
        {
            _lineBuffer->exception = "unrecognized exception";
            _lineBuffer->hasException = true;
        }
    }
}

} // namespace


OutputFile::OutputFile
    (const char fileName[],
     const Header &header,
     int numThreads)
:
    _data (new Data (true, numThreads))
{
    try
    {
    header.sanityCheck();
    _data->os = new StdOFStream (fileName);
    initialize (header);
    }
    catch (Iex::BaseExc &e)
    {
    delete _data;

    REPLACE_EXC (e, "Cannot open image file "
            "\"" << fileName << "\". " << e);
    throw;
    }
    catch (...)
    {
    delete _data;
        throw;
    }
}


OutputFile::OutputFile
    (OStream &os,
     const Header &header,
     int numThreads)
:
    _data (new Data (false, numThreads))
{
    try
    {
    header.sanityCheck();
    _data->os = &os;
    initialize (header);
    }
    catch (Iex::BaseExc &e)
    {
    delete _data;

    REPLACE_EXC (e, "Cannot open image file "
            "\"" << os.fileName() << "\". " << e);
    throw;
    }
    catch (...)
    {
    delete _data;
        throw;
    }
}


void
OutputFile::initialize (const Header &header)
{
    _data->header = header;

    const Box2i &dataWindow = header.dataWindow();

    _data->currentScanLine = (header.lineOrder() == INCREASING_Y)?
                 dataWindow.min.y: dataWindow.max.y;

    _data->missingScanLines = dataWindow.max.y - dataWindow.min.y + 1;
    _data->lineOrder = header.lineOrder();
    _data->minX = dataWindow.min.x;
    _data->maxX = dataWindow.max.x;
    _data->minY = dataWindow.min.y;
    _data->maxY = dataWindow.max.y;

    size_t maxBytesPerLine = bytesPerLineTable (_data->header,
                        _data->bytesPerLine);

    for (size_t i = 0; i < _data->lineBuffers.size(); ++i)
    {
        _data->lineBuffers[i] =
        new LineBuffer (newCompressor (_data->header.compression(),
                       maxBytesPerLine,
                       _data->header));
    }

    LineBuffer *lineBuffer = _data->lineBuffers[0];
    _data->format = defaultFormat (lineBuffer->compressor);
    _data->linesInBuffer = numLinesInBuffer (lineBuffer->compressor);
    _data->lineBufferSize = maxBytesPerLine * _data->linesInBuffer;

    for (size_t i = 0; i < _data->lineBuffers.size(); i++)
        _data->lineBuffers[i]->buffer.resizeErase(_data->lineBufferSize);

    int lineOffsetSize = (dataWindow.max.y - dataWindow.min.y +
              _data->linesInBuffer) / _data->linesInBuffer;

    _data->lineOffsets.resize (lineOffsetSize);

    offsetInLineBufferTable (_data->bytesPerLine,
                 _data->linesInBuffer,
                 _data->offsetInLineBuffer);

    _data->previewPosition =
    _data->header.writeTo (*_data->os);

    _data->lineOffsetsPosition =
    writeLineOffsets (*_data->os, _data->lineOffsets);

    _data->currentPosition = _data->os->tellp();
}


OutputFile::~OutputFile ()
{
    if (_data)
    {
        {
            if (_data->lineOffsetsPosition > 0)
            {
                try
                {
                    _data->os->seekp (_data->lineOffsetsPosition);
                    writeLineOffsets (*_data->os, _data->lineOffsets);
                }
                catch (...)
                {
                    //
                    // We cannot safely throw any exceptions from here.
                    // This destructor may have been called because the
                    // stack is currently being unwound for another
                    // exception.
                    //
                }
            }
        }

    delete _data;
    }
}


const char *
OutputFile::fileName () const
{
    return _data->os->fileName();
}


const Header &
OutputFile::header () const
{
    return _data->header;
}


void
OutputFile::setFrameBuffer (const FrameBuffer &frameBuffer)
{
    Lock lock (*_data);

    //
    // Check if the new frame buffer descriptor
    // is compatible with the image file header.
    //

    const ChannelList &channels = _data->header.channels();

    for (ChannelList::ConstIterator i = channels.begin();
     i != channels.end();
     ++i)
    {
    FrameBuffer::ConstIterator j = frameBuffer.find (i.name());

    if (j == frameBuffer.end())
        continue;

    if (i.channel().type != j.slice().type)
    {
        THROW (Iex::ArgExc, "Pixel type of \"" << i.name() << "\" channel "
                    "of output file \"" << fileName() << "\" is "
                    "not compatible with the frame buffer's "
                    "pixel type.");
    }

    if (i.channel().xSampling != j.slice().xSampling ||
        i.channel().ySampling != j.slice().ySampling)
    {
        THROW (Iex::ArgExc, "X and/or y subsampling factors "
                "of \"" << i.name() << "\" channel "
                "of output file \"" << fileName() << "\" are "
                "not compatible with the frame buffer's "
                "subsampling factors.");
    }
    }

    //
    // Initialize slice table for writePixels().
    //

    vector<OutSliceInfo> slices;

    for (ChannelList::ConstIterator i = channels.begin();
     i != channels.end();
     ++i)
    {
    FrameBuffer::ConstIterator j = frameBuffer.find (i.name());

    if (j == frameBuffer.end())
    {
        //
        // Channel i is not present in the frame buffer.
        // In the file, channel i will contain only zeroes.
        //

        slices.push_back (OutSliceInfo (i.channel().type,
                        0, // base
                        0, // xStride,
                        0, // yStride,
                        i.channel().xSampling,
                        i.channel().ySampling,
                        true)); // zero
    }
    else
    {
        //
        // Channel i is present in the frame buffer.
        //

        slices.push_back (OutSliceInfo (j.slice().type,
                        j.slice().base,
                        j.slice().xStride,
                        j.slice().yStride,
                        j.slice().xSampling,
                        j.slice().ySampling,
                        false)); // zero
    }
    }

    //
    // Store the new frame buffer.
    //

    _data->frameBuffer = frameBuffer;
    _data->slices = slices;
}


const FrameBuffer &
OutputFile::frameBuffer () const
{
    Lock lock (*_data);
    return _data->frameBuffer;
}


void
OutputFile::writePixels (int numScanLines)
{
    try
    {
        Lock lock (*_data);

    if (_data->slices.size() == 0)
        throw Iex::ArgExc ("No frame buffer specified "
                   "as pixel data source.");

        //
        // Maintain two iterators:
        //     nextWriteBuffer: next linebuffer to be written to the file
        //     nextCompressBuffer: next linebuffer to compress
        //

        int first = (_data->currentScanLine - _data->minY) /
                         _data->linesInBuffer;

        int nextWriteBuffer = first;
        int nextCompressBuffer;
        int stop;
        int step;
        int scanLineMin;
        int scanLineMax;

        {
            //
            // Create a task group for all line buffer tasks. When the
            // taskgroup goes out of scope, the destructor waits until
        // all tasks are complete.
            //

            TaskGroup taskGroup;

            //
            // Determine the range of lineBuffers that intersect the scan
        // line range.  Then add the initial compression tasks to the
        // thread pool.  We always add in at least one task but the
        // individual task might not do anything if numScanLines == 0.
            //

            if (_data->lineOrder == INCREASING_Y)
            {
                int last = (_data->currentScanLine + (numScanLines - 1) -
                            _data->minY) / _data->linesInBuffer;

                scanLineMin = _data->currentScanLine;
                scanLineMax = _data->currentScanLine + numScanLines - 1;

                int numTasks = max (min ((int)_data->lineBuffers.size(),
                                         last - first + 1),
                    1);

                for (int i = 0; i < numTasks; i++)
        {
                    ThreadPool::addGlobalTask
                        (new LineBufferTask (&taskGroup, _data, first + i,
                                             scanLineMin, scanLineMax));
        }

                nextCompressBuffer = first + numTasks;
                stop = last + 1;
                step = 1;
            }
            else
            {
                int last = (_data->currentScanLine - (numScanLines - 1) -
                            _data->minY) / _data->linesInBuffer;

                scanLineMax = _data->currentScanLine;
                scanLineMin = _data->currentScanLine - numScanLines + 1;

                int numTasks = max (min ((int)_data->lineBuffers.size(),
                                         first - last + 1),
                    1);

                for (int i = 0; i < numTasks; i++)
        {
                    ThreadPool::addGlobalTask
                        (new LineBufferTask (&taskGroup, _data, first - i,
                                             scanLineMin, scanLineMax));
        }

                nextCompressBuffer = first - numTasks;
                stop = last - 1;
                step = -1;
            }

            while (true)
            {
                if (_data->missingScanLines <= 0)
                {
                    throw Iex::ArgExc ("Tried to write more scan lines "
                                       "than specified by the data window.");
                }

        //
                // Wait until the next line buffer is ready to be written
        //

                LineBuffer *writeBuffer =
            _data->getLineBuffer (nextWriteBuffer);

                writeBuffer->wait();

                int numLines = writeBuffer->scanLineMax -
                               writeBuffer->scanLineMin + 1;

                _data->missingScanLines -= numLines;

        //
                // If the line buffer is only partially full, then it is
        // not complete and we cannot write it to disk yet.
        //

                if (writeBuffer->partiallyFull)
                {
                    _data->currentScanLine = _data->currentScanLine +
                                             step * numLines;
                    writeBuffer->post();

                    return;
                }

        //
                // Write the line buffer
        //

                writePixelData (_data, writeBuffer);
                nextWriteBuffer += step;

                _data->currentScanLine = _data->currentScanLine +
                                         step * numLines;

                #ifdef DEBUG

                    assert (_data->currentScanLine ==
                            ((_data->lineOrder == INCREASING_Y) ?
                             writeBuffer->scanLineMax + 1:
                             writeBuffer->scanLineMin - 1));

                #endif

        //
                // Release the lock on the line buffer
        //

                writeBuffer->post();

        //
                // If this was the last line buffer in the scanline range
        //

                if (nextWriteBuffer == stop)
                    break;

        //
                // If there are no more line buffers to compress,
                // then only continue to write out remaining lineBuffers
        //

                if (nextCompressBuffer == stop)
                    continue;

        //
                // Add nextCompressBuffer as a compression task
        //

                ThreadPool::addGlobalTask
                    (new LineBufferTask (&taskGroup, _data, nextCompressBuffer,
                                         scanLineMin, scanLineMax));

        //
                // Update the next line buffer we need to compress
        //

                nextCompressBuffer += step;
            }

        //
            // Finish all tasks
        //
        }

    //
    // Exeption handling:
    //
    // LineBufferTask::execute() may have encountered exceptions, but
    // those exceptions occurred in another thread, not in the thread
    // that is executing this call to OutputFile::writePixels().
    // LineBufferTask::execute() has caught all exceptions and stored
    // the exceptions' what() strings in the line buffers.
    // Now we check if any line buffer contains a stored exception; if
    // this is the case then we re-throw the exception in this thread.
    // (It is possible that multiple line buffers contain stored
    // exceptions.  We re-throw the first exception we find and
    // ignore all others.)
    //

    const string *exception = 0;

        for (int i = 0; i < _data->lineBuffers.size(); ++i)
    {
            LineBuffer *lineBuffer = _data->lineBuffers[i];

        if (lineBuffer->hasException && !exception)
        exception = &lineBuffer->exception;

        lineBuffer->hasException = false;
    }

    if (exception)
        throw Iex::IoExc (*exception);
    }
    catch (Iex::BaseExc &e)
    {
    REPLACE_EXC (e, "Failed to write pixel data to image "
                "file \"" << fileName() << "\". " << e);
    throw;
    }
}


int
OutputFile::currentScanLine () const
{
    Lock lock (*_data);
    return _data->currentScanLine;
}


void
OutputFile::copyPixels (InputFile &in)
{
    Lock lock (*_data);

    //
    // Check if this file's and and the InputFile's
    // headers are compatible.
    //

    const Header &hdr = _data->header;
    const Header &inHdr = in.header();

    if (inHdr.find("tiles") != inHdr.end())
    THROW (Iex::ArgExc, "Cannot copy pixels from image "
                "file \"" << in.fileName() << "\" to image "
                "file \"" << fileName() << "\". "
                            "The input file is tiled, but the output file is "
                            "not. Try using TiledOutputFile::copyPixels "
                            "instead.");

    if (!(hdr.dataWindow() == inHdr.dataWindow()))
    THROW (Iex::ArgExc, "Cannot copy pixels from image "
                "file \"" << in.fileName() << "\" to image "
                "file \"" << fileName() << "\". "
                            "The files have different data windows.");

    if (!(hdr.lineOrder() == inHdr.lineOrder()))
    THROW (Iex::ArgExc, "Quick pixel copy from image "
                "file \"" << in.fileName() << "\" to image "
                "file \"" << fileName() << "\" failed. "
                "The files have different line orders.");

    if (!(hdr.compression() == inHdr.compression()))
    THROW (Iex::ArgExc, "Quick pixel copy from image "
                "file \"" << in.fileName() << "\" to image "
                "file \"" << fileName() << "\" failed. "
                "The files use different compression methods.");

    if (!(hdr.channels() == inHdr.channels()))
    THROW (Iex::ArgExc, "Quick pixel copy from image "
                "file \"" << in.fileName() << "\" to image "
                "file \"" << fileName() << "\" failed.  "
                "The files have different channel lists.");

    //
    // Verify that no pixel data have been written to this file yet.
    //

    const Box2i &dataWindow = hdr.dataWindow();

    if (_data->missingScanLines != dataWindow.max.y - dataWindow.min.y + 1)
    THROW (Iex::LogicExc, "Quick pixel copy from image "
                  "file \"" << in.fileName() << "\" to image "
                  "file \"" << fileName() << "\" failed. "
                  "\"" << fileName() << "\" already contains "
                  "pixel data.");

    //
    // Copy the pixel data.
    //

    while (_data->missingScanLines > 0)
    {
    const char *pixelData;
    int pixelDataSize;

    in.rawPixelData (_data->currentScanLine, pixelData, pixelDataSize);

    writePixelData (_data, lineBufferMinY (_data->currentScanLine,
                               _data->minY,
                               _data->linesInBuffer),
                        pixelData, pixelDataSize);

    _data->currentScanLine += (_data->lineOrder == INCREASING_Y)?
                   _data->linesInBuffer: -_data->linesInBuffer;

    _data->missingScanLines -= _data->linesInBuffer;
    }
}


void
OutputFile::updatePreviewImage (const PreviewRgba newPixels[])
{
    Lock lock (*_data);

    if (_data->previewPosition <= 0)
    THROW (Iex::LogicExc, "Cannot update preview image pixels. "
                  "File \"" << fileName() << "\" does not "
                  "contain a preview image.");

    //
    // Store the new pixels in the header's preview image attribute.
    //

    PreviewImageAttribute &pia =
    _data->header.typedAttribute <PreviewImageAttribute> ("preview");

    PreviewImage &pi = pia.value();
    PreviewRgba *pixels = pi.pixels();
    int numPixels = pi.width() * pi.height();

    for (int i = 0; i < numPixels; ++i)
    pixels[i] = newPixels[i];

    //
    // Save the current file position, jump to the position in
    // the file where the preview image starts, store the new
    // preview image, and jump back to the saved file position.
    //

    Int64 savedPosition = _data->os->tellp();

    try
    {
    _data->os->seekp (_data->previewPosition);
    pia.writeValueTo (*_data->os, _data->version);
    _data->os->seekp (savedPosition);
    }
    catch (Iex::BaseExc &e)
    {
    REPLACE_EXC (e, "Cannot update preview image pixels for "
            "file \"" << fileName() << "\". " << e);
    throw;
    }
}


void
OutputFile::breakScanLine  (int y, int offset, int length, char c)
{
    Lock lock (*_data);

    Int64 position =
    _data->lineOffsets[(y - _data->minY) / _data->linesInBuffer];

    if (!position)
    THROW (Iex::ArgExc, "Cannot overwrite scan line " << y << ". "
                "The scan line has not yet been stored in "
                "file \"" << fileName() << "\".");

    _data->currentPosition = 0;
    _data->os->seekp (position + offset);

    for (int i = 0; i < length; ++i)
    _data->os->write (&c, 1);
}


} // namespace Imf