src/common/imagpng.cpp

/////////////////////////////////////////////////////////////////////////////
// Name:        src/common/imagpng.cpp
// Purpose:     wxImage PNG handler
// Author:      Robert Roebling
// RCS-ID:      $Id$
// Copyright:   (c) Robert Roebling
// Licence:     wxWindows licence
/////////////////////////////////////////////////////////////////////////////

// ============================================================================
// declarations
// ============================================================================

// ----------------------------------------------------------------------------
// headers
// ----------------------------------------------------------------------------

// For compilers that support precompilation, includes "wx.h".
#include "wx/wxprec.h"

#ifdef __BORLANDC__
    #pragma hdrstop
#endif

#if wxUSE_IMAGE && wxUSE_LIBPNG

#include "wx/imagpng.h"
#include "wx/versioninfo.h"

#ifndef WX_PRECOMP
    #include "wx/log.h"
    #include "wx/intl.h"
    #include "wx/palette.h"
    #include "wx/stream.h"
#endif

#include "png.h"

// For memcpy
#include <string.h>

// ----------------------------------------------------------------------------
// constants
// ----------------------------------------------------------------------------

// image cannot have any transparent pixels at all, have only 100% opaque
// and/or 100% transparent pixels in which case a simple mask is enough to
// store this information in wxImage or have a real alpha channel in which case
// we need to have it in wxImage as well
enum Transparency
{
    Transparency_None,
    Transparency_Mask,
    Transparency_Alpha
};

// ----------------------------------------------------------------------------
// local functions
// ----------------------------------------------------------------------------

// return the kind of transparency needed for this image assuming that it does
// have transparent pixels, i.e. either Transparency_Alpha or Transparency_Mask
static Transparency
CheckTransparency(unsigned char **lines,
                  png_uint_32 x, png_uint_32 y, png_uint_32 w, png_uint_32 h,
                  size_t numColBytes);

// init the alpha channel for the image and fill it with 1s up to (x, y)
static unsigned char *InitAlpha(wxImage *image, png_uint_32 x, png_uint_32 y);

// find a free colour for the mask in the PNG data array
static void
FindMaskColour(unsigned char **lines, png_uint_32 width, png_uint_32 height,
               unsigned char& rMask, unsigned char& gMask, unsigned char& bMask);

// is the pixel with this value of alpha a fully opaque one?
static inline
bool IsOpaque(unsigned char a)
{
    return a == 0xff;
}

// is the pixel with this value of alpha a fully transparent one?
static inline
bool IsTransparent(unsigned char a)
{
    return !a;
}

// ============================================================================
// wxPNGHandler implementation
// ============================================================================

IMPLEMENT_DYNAMIC_CLASS(wxPNGHandler,wxImageHandler)

#if wxUSE_STREAMS

#ifndef PNGLINKAGEMODE
    #ifdef PNGAPI
        #define PNGLINKAGEMODE PNGAPI
    #elif defined(__WATCOMC__)
        // we need an explicit cdecl for Watcom, at least according to
        //
        // http://sf.net/tracker/index.php?func=detail&aid=651492&group_id=9863&atid=109863
        //
        // more testing is needed for this however, please remove this comment
        // if you can confirm that my fix works with Watcom 11
        #define PNGLINKAGEMODE cdecl
    #else
        #define PNGLINKAGEMODE LINKAGEMODE
    #endif
#endif


// VS: wxPNGInfoStruct declared below is a hack that needs some explanation.
//     First, let me describe what's the problem: libpng uses jmp_buf in
//     its png_struct structure. Unfortunately, this structure is
//     compiler-specific and may vary in size, so if you use libpng compiled
//     as DLL with another compiler than the main executable, it may not work.
//     Luckily, it is still possible to use setjmp() & longjmp() as long as the
//     structure is not part of png_struct.
//
//     Sadly, there's no clean way to attach user-defined data to png_struct.
//     There is only one customizable place, png_struct.io_ptr, which is meant
//     only for I/O routines and is set with png_set_read_fn or
//     png_set_write_fn. The hacky part is that we use io_ptr to store
//     a pointer to wxPNGInfoStruct that holds I/O structures _and_ jmp_buf.

struct wxPNGInfoStruct
{
    jmp_buf jmpbuf;
    bool verbose;

    union
    {
        wxInputStream  *in;
        wxOutputStream *out;
    } stream;
};

#define WX_PNG_INFO(png_ptr) ((wxPNGInfoStruct*)png_get_io_ptr(png_ptr))

// ----------------------------------------------------------------------------
// helper functions
// ----------------------------------------------------------------------------

extern "C"
{

static void PNGLINKAGEMODE wx_PNG_stream_reader( png_structp png_ptr, png_bytep data,
                                                 png_size_t length )
{
    WX_PNG_INFO(png_ptr)->stream.in->Read(data, length);
}

static void PNGLINKAGEMODE wx_PNG_stream_writer( png_structp png_ptr, png_bytep data,
                                                 png_size_t length )
{
    WX_PNG_INFO(png_ptr)->stream.out->Write(data, length);
}

static void
PNGLINKAGEMODE wx_PNG_warning(png_structp png_ptr, png_const_charp message)
{
    wxPNGInfoStruct *info = png_ptr ? WX_PNG_INFO(png_ptr) : NULL;
    if ( !info || info->verbose )
    {
        wxLogWarning( wxString::FromAscii(message) );
    }
}

// from pngerror.c
// so that the libpng doesn't send anything on stderr
static void
PNGLINKAGEMODE wx_PNG_error(png_structp png_ptr, png_const_charp message)
{
    wx_PNG_warning(NULL, message);

    // we're not using libpng built-in jump buffer (see comment before
    // wxPNGInfoStruct above) so we have to return ourselves, otherwise libpng
    // would just abort
    longjmp(WX_PNG_INFO(png_ptr)->jmpbuf, 1);
}

} // extern "C"

// ----------------------------------------------------------------------------
// LoadFile() helpers
// ----------------------------------------------------------------------------

// determine the kind of transparency we need for this image: if the only alpha
// values it has are 0 (transparent) and 0xff (opaque) then we can simply
// create a mask for it, we should be ok with a simple mask but otherwise we
// need a full blown alpha channel in wxImage
//
// parameters:
//      lines           raw PNG data
//      x, y            starting position
//      w, h            size of the image
//      numColBytes     number of colour bytes (1 for grey scale, 3 for RGB)
//                      (NB: alpha always follows the colour bytes)
Transparency
CheckTransparency(unsigned char **lines,
                  png_uint_32 x, png_uint_32 y, png_uint_32 w, png_uint_32 h,
                  size_t numColBytes)
{
    // suppose that a mask will suffice and check all the remaining alpha
    // values to see if it does
    for ( ; y < h; y++ )
    {
        // each pixel is numColBytes+1 bytes, offset into the current line by
        // the current x position
        unsigned const char *ptr = lines[y] + (x * (numColBytes + 1));

        for ( png_uint_32 x2 = x; x2 < w; x2++ )
        {
            // skip the grey or colour byte(s)
            ptr += numColBytes;

            unsigned char a2 = *ptr++;

            if ( !IsTransparent(a2) && !IsOpaque(a2) )
            {
                // not fully opaque nor fully transparent, hence need alpha
                return Transparency_Alpha;
            }
        }

        // during the next loop iteration check all the pixels in the row
        x = 0;
    }

    // mask will be enough
    return Transparency_Mask;
}

unsigned char *InitAlpha(wxImage *image, png_uint_32 x, png_uint_32 y)
{
    // create alpha channel
    image->SetAlpha();

    unsigned char *alpha = image->GetAlpha();

    // set alpha for the pixels we had so far
    png_uint_32 end = y * image->GetWidth() + x;
    for ( png_uint_32 i = 0; i < end; i++ )
    {
        // all the previous pixels were opaque
        *alpha++ = 0xff;
    }

    return alpha;
}

void
FindMaskColour(unsigned char **lines, png_uint_32 width, png_uint_32 height,
               unsigned char& rMask, unsigned char& gMask, unsigned char& bMask)
{
    // choosing the colour for the mask is more
    // difficult: we need to iterate over the entire
    // image for this in order to choose an unused
    // colour (this is not very efficient but what else
    // can we do?)
    wxImageHistogram h;
    unsigned nentries = 0;
    unsigned char r2, g2, b2;
    for ( png_uint_32 y2 = 0; y2 < height; y2++ )
    {
        const unsigned char *p = lines[y2];
        for ( png_uint_32 x2 = 0; x2 < width; x2++ )
        {
            r2 = *p++;
            g2 = *p++;
            b2 = *p++;
            ++p; // jump over alpha

            wxImageHistogramEntry&
                entry = h[wxImageHistogram:: MakeKey(r2, g2, b2)];

            if ( entry.value++ == 0 )
                entry.index = nentries++;
        }
    }

    if ( !h.FindFirstUnusedColour(&rMask, &gMask, &bMask) )
    {
        wxLogWarning(_("Too many colours in PNG, the image may be slightly blurred."));

        // use a fixed mask colour and we'll fudge
        // the real pixels with this colour (see
        // below)
        rMask = 0xfe;
        gMask = 0;
        bMask = 0xff;
    }
}

// ----------------------------------------------------------------------------
// reading PNGs
// ----------------------------------------------------------------------------

bool wxPNGHandler::DoCanRead( wxInputStream& stream )
{
    unsigned char hdr[4];

    if ( !stream.Read(hdr, WXSIZEOF(hdr)) )     // it's ok to modify the stream position here
        return false;

    return memcmp(hdr, "\211PNG", WXSIZEOF(hdr)) == 0;
}

// convert data from RGB to wxImage format
static
void CopyDataFromPNG(wxImage *image,
                     unsigned char **lines,
                     png_uint_32 width,
                     png_uint_32 height,
                     int color_type)
{
    Transparency transparency = Transparency_None;

    // only non NULL if transparency == Transparency_Alpha
    unsigned char *alpha = NULL;

    // RGB of the mask colour if transparency == Transparency_Mask
    // (but init them anyhow to avoid compiler warnings)
    unsigned char rMask = 0,
                  gMask = 0,
                  bMask = 0;

    unsigned char *ptrDst = image->GetData();
    if ( !(color_type & PNG_COLOR_MASK_COLOR) )
    {
        // grey image: GAGAGA... where G == grey component and A == alpha
        for ( png_uint_32 y = 0; y < height; y++ )
        {
            const unsigned char *ptrSrc = lines[y];
            for ( png_uint_32 x = 0; x < width; x++ )
            {
                unsigned char g = *ptrSrc++;
                unsigned char a = *ptrSrc++;

                // the first time we encounter a transparent pixel we must
                // decide about what to do about them
                if ( !IsOpaque(a) && transparency == Transparency_None )
                {
                    // we'll need at least the mask for this image and
                    // maybe even full alpha channel info: the former is
                    // only enough if we have alpha values of 0 and 0xff
                    // only, otherwisewe need the latter
                    transparency = CheckTransparency
                                   (
                                        lines,
                                        x, y,
                                        width, height,
                                        1
                                   );

                    if ( transparency == Transparency_Mask )
                    {
                        // let's choose this colour for the mask: this is
                        // not a problem here as all the other pixels are
                        // grey, i.e. R == G == B which is not the case for
                        // this one so no confusion is possible
                        rMask = 0xff;
                        gMask = 0;
                        bMask = 0xff;
                    }
                    else // transparency == Transparency_Alpha
                    {
                        alpha = InitAlpha(image, x, y);
                    }
                }

                switch ( transparency )
                {
                    case Transparency_Mask:
                        if ( IsTransparent(a) )
                        {
                            *ptrDst++ = rMask;
                            *ptrDst++ = gMask;
                            *ptrDst++ = bMask;
                            break;
                        }
                        // else: !transparent

                        // must be opaque then as otherwise we shouldn't be
                        // using the mask at all
                        wxASSERT_MSG( IsOpaque(a), wxT("logic error") );

                        // fall through

                    case Transparency_Alpha:
                        if ( alpha )
                            *alpha++ = a;
                        // fall through

                    case Transparency_None:
                        *ptrDst++ = g;
                        *ptrDst++ = g;
                        *ptrDst++ = g;
                        break;
                }
            }
        }
    }
    else // colour image: RGBRGB...
    {
        for ( png_uint_32 y = 0; y < height; y++ )
        {
            const unsigned char *ptrSrc = lines[y];
            for ( png_uint_32 x = 0; x < width; x++ )
            {
                unsigned char r = *ptrSrc++;
                unsigned char g = *ptrSrc++;
                unsigned char b = *ptrSrc++;
                unsigned char a = *ptrSrc++;

                // the logic here is the same as for the grey case except
                // where noted
                if ( !IsOpaque(a) && transparency == Transparency_None )
                {
                    transparency = CheckTransparency
                                   (
                                        lines,
                                        x, y,
                                        width, height,
                                        3
                                   );

                    if ( transparency == Transparency_Mask )
                    {
                        FindMaskColour(lines, width, height,
                                       rMask, gMask, bMask);
                    }
                    else // transparency == Transparency_Alpha
                    {
                        alpha = InitAlpha(image, x, y);
                    }

                }

                switch ( transparency )
                {
                    case Transparency_Mask:
                        if ( IsTransparent(a) )
                        {
                            *ptrDst++ = rMask;
                            *ptrDst++ = gMask;
                            *ptrDst++ = bMask;
                            break;
                        }
                        else // !transparent
                        {
                            // must be opaque then as otherwise we shouldn't be
                            // using the mask at all
                            wxASSERT_MSG( IsOpaque(a), wxT("logic error") );

                            // if we couldn't find a unique colour for the
                            // mask, we can have real pixels with the same
                            // value as the mask and it's better to slightly
                            // change their colour than to make them
                            // transparent
                            if ( r == rMask && g == gMask && b == bMask )
                            {
                                r++;
                            }
                        }

                        // fall through

                    case Transparency_Alpha:
                        if ( alpha )
                            *alpha++ = a;
                        // fall through

                    case Transparency_None:
                        *ptrDst++ = r;
                        *ptrDst++ = g;
                        *ptrDst++ = b;
                        break;
                }
            }
        }
    }

    if ( transparency == Transparency_Mask )
    {
        image->SetMaskColour(rMask, gMask, bMask);
    }
}

// temporarily disable the warning C4611 (interaction between '_setjmp' and
// C++ object destruction is non-portable) - I don't see any dtors here
#ifdef __VISUALC__
    #pragma warning(disable:4611)
#endif /* VC++ */

bool
wxPNGHandler::LoadFile(wxImage *image,
                       wxInputStream& stream,
                       bool verbose,
                       int WXUNUSED(index))
{
    // VZ: as this function uses setjmp() the only fool-proof error handling
    //     method is to use goto (setjmp is not really C++ dtors friendly...)

    unsigned char **lines = NULL;
    png_infop info_ptr = (png_infop) NULL;
    wxPNGInfoStruct wxinfo;

    png_uint_32 i, width, height = 0;
    int bit_depth, color_type, interlace_type;

    wxinfo.verbose = verbose;
    wxinfo.stream.in = &stream;

    image->Destroy();

    png_structp png_ptr = png_create_read_struct
                          (
                            PNG_LIBPNG_VER_STRING,
                            NULL,
                            wx_PNG_error,
                            wx_PNG_warning
                          );
    if (!png_ptr)
        goto error;

    // NB: please see the comment near wxPNGInfoStruct declaration for
    //     explanation why this line is mandatory
    png_set_read_fn( png_ptr, &wxinfo, wx_PNG_stream_reader);

    info_ptr = png_create_info_struct( png_ptr );
    if (!info_ptr)
        goto error;

    if (setjmp(wxinfo.jmpbuf))
        goto error;

    png_read_info( png_ptr, info_ptr );
    png_get_IHDR( png_ptr, info_ptr, &width, &height, &bit_depth, &color_type, &interlace_type, NULL, NULL );

    if (color_type == PNG_COLOR_TYPE_PALETTE)
        png_set_expand( png_ptr );

    // Fix for Bug [ 439207 ] Monochrome PNG images come up black
    if (bit_depth < 8)
        png_set_expand( png_ptr );

    png_set_strip_16( png_ptr );
    png_set_packing( png_ptr );
    if (png_get_valid( png_ptr, info_ptr, PNG_INFO_tRNS))
        png_set_expand( png_ptr );
    png_set_filler( png_ptr, 0xff, PNG_FILLER_AFTER );

    image->Create((int)width, (int)height, (bool) false /* no need to init pixels */);

    if (!image->IsOk())
        goto error;

    // initialize all line pointers to NULL to ensure that they can be safely
    // free()d if an error occurs before all of them could be allocated
    lines = (unsigned char **)calloc(height, sizeof(unsigned char *));
    if ( !lines )
        goto error;

    for (i = 0; i < height; i++)
    {
        if ((lines[i] = (unsigned char *)malloc( (size_t)(width * 4))) == NULL)
            goto error;
    }

    png_read_image( png_ptr, lines );
    png_read_end( png_ptr, info_ptr );

#if wxUSE_PALETTE
    if (color_type == PNG_COLOR_TYPE_PALETTE)
    {
        png_colorp palette = NULL;
        int numPalette = 0;

        (void) png_get_PLTE(png_ptr, info_ptr, &palette, &numPalette);

        unsigned char* r = new unsigned char[numPalette];
        unsigned char* g = new unsigned char[numPalette];
        unsigned char* b = new unsigned char[numPalette];

        for (int j = 0; j < numPalette; j++)
        {
            r[j] = palette[j].red;
            g[j] = palette[j].green;
            b[j] = palette[j].blue;
        }

        image->SetPalette(wxPalette(numPalette, r, g, b));
        delete[] r;
        delete[] g;
        delete[] b;
    }
#endif // wxUSE_PALETTE


    // set the image resolution if it's available
    png_uint_32 resX, resY;
    int unitType;
    if (png_get_pHYs(png_ptr, info_ptr, &resX, &resY, &unitType)
        == PNG_INFO_pHYs)
    {
        wxImageResolution res = wxIMAGE_RESOLUTION_CM;

        switch (unitType)
        {
            default:
                wxLogWarning(_("Unknown PNG resolution unit %d"), unitType);
                // fall through

            case PNG_RESOLUTION_UNKNOWN:
                image->SetOption(wxIMAGE_OPTION_RESOLUTIONX, resX);
                image->SetOption(wxIMAGE_OPTION_RESOLUTIONY, resY);

                res = wxIMAGE_RESOLUTION_NONE;
                break;

            case PNG_RESOLUTION_METER:
                /*
                Convert meters to centimeters.
                Use a string to not lose precision (converting to cm and then
                to inch would result in integer rounding error).
                If an app wants an int, GetOptionInt will convert and round
                down for them.
                */
                image->SetOption(wxIMAGE_OPTION_RESOLUTIONX,
                    wxString::FromCDouble((double) resX / 100.0, 2));
                image->SetOption(wxIMAGE_OPTION_RESOLUTIONY,
                    wxString::FromCDouble((double) resY / 100.0, 2));
                break;
        }

        image->SetOption(wxIMAGE_OPTION_RESOLUTIONUNIT, res);
    }


    png_destroy_read_struct( &png_ptr, &info_ptr, (png_infopp) NULL );

    // loaded successfully, now init wxImage with this data
    CopyDataFromPNG(image, lines, width, height, color_type);

    for ( i = 0; i < height; i++ )
        free( lines[i] );
    free( lines );

    return true;

error:
    if (verbose)
    {
       wxLogError(_("Couldn't load a PNG image - file is corrupted or not enough memory."));
    }

    if ( image->IsOk() )
    {
        image->Destroy();
    }

    if ( lines )
    {
        for ( unsigned int n = 0; n < height; n++ )
            free( lines[n] );

        free( lines );
    }

    if ( png_ptr )
    {
        if ( info_ptr )
        {
            png_destroy_read_struct( &png_ptr, &info_ptr, (png_infopp) NULL );
            free(info_ptr);
        }
        else
            png_destroy_read_struct( &png_ptr, (png_infopp) NULL, (png_infopp) NULL );
    }
    return false;
}

// ----------------------------------------------------------------------------
// SaveFile() palette helpers
// ----------------------------------------------------------------------------

typedef wxLongToLongHashMap PaletteMap;

static unsigned long PaletteMakeKey(const png_color_8& clr)
{
    return (wxImageHistogram::MakeKey(clr.red, clr.green, clr.blue) << 8) | clr.alpha;
}

static long PaletteFind(const PaletteMap& palette, const png_color_8& clr)
{
    unsigned long value = PaletteMakeKey(clr);
    PaletteMap::const_iterator it = palette.find(value);

    return (it != palette.end()) ? it->second : wxNOT_FOUND;
}

static long PaletteAdd(PaletteMap *palette, const png_color_8& clr)
{
    unsigned long value = PaletteMakeKey(clr);
    PaletteMap::const_iterator it = palette->find(value);
    size_t index;

    if (it == palette->end())
    {
        index = palette->size();
        (*palette)[value] = index;
    }
    else
    {
        index = it->second;
    }

    return index;
}

// ----------------------------------------------------------------------------
// writing PNGs
// ----------------------------------------------------------------------------

bool wxPNGHandler::SaveFile( wxImage *image, wxOutputStream& stream, bool verbose )
{
    wxPNGInfoStruct wxinfo;

    wxinfo.verbose = verbose;
    wxinfo.stream.out = &stream;

    png_structp png_ptr = png_create_write_struct
                          (
                            PNG_LIBPNG_VER_STRING,
                            NULL,
                            wx_PNG_error,
                            wx_PNG_warning
                          );
    if (!png_ptr)
    {
        if (verbose)
        {
           wxLogError(_("Couldn't save PNG image."));
        }
        return false;
    }

    png_infop info_ptr = png_create_info_struct(png_ptr);
    if (info_ptr == NULL)
    {
        png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
        if (verbose)
        {
           wxLogError(_("Couldn't save PNG image."));
        }
        return false;
    }

    if (setjmp(wxinfo.jmpbuf))
    {
        png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
        if (verbose)
        {
           wxLogError(_("Couldn't save PNG image."));
        }
        return false;
    }

    // NB: please see the comment near wxPNGInfoStruct declaration for
    //     explanation why this line is mandatory
    png_set_write_fn( png_ptr, &wxinfo, wx_PNG_stream_writer, NULL);

    const int iHeight = image->GetHeight();
    const int iWidth = image->GetWidth();

    const bool bHasPngFormatOption
        = image->HasOption(wxIMAGE_OPTION_PNG_FORMAT);

    int iColorType = bHasPngFormatOption
                            ? image->GetOptionInt(wxIMAGE_OPTION_PNG_FORMAT)
                            : wxPNG_TYPE_COLOUR;

    bool bHasAlpha = image->HasAlpha();
    bool bHasMask = image->HasMask();

    bool bUsePalette = iColorType == wxPNG_TYPE_PALETTE
#if wxUSE_PALETTE
        || (!bHasPngFormatOption && image->HasPalette() )
#endif
    ;

    png_color_8 mask = { 0, 0, 0, 0, 0 };

    if (bHasMask)
    {
        mask.red   = image->GetMaskRed();
        mask.green = image->GetMaskGreen();
        mask.blue  = image->GetMaskBlue();
    }

    PaletteMap palette;

    if (bUsePalette)
    {
        png_color png_rgb  [PNG_MAX_PALETTE_LENGTH];
        png_byte  png_trans[PNG_MAX_PALETTE_LENGTH];

        const unsigned char *pColors = image->GetData();
        const unsigned char* pAlpha  = image->GetAlpha();

        if (bHasMask && !pAlpha)
        {
            // Mask must be first
            PaletteAdd(&palette, mask);
        }

        for (int y = 0; y < iHeight; y++)
        {
            for (int x = 0; x < iWidth; x++)
            {
                png_color_8 rgba;

                rgba.red   = *pColors++;
                rgba.green = *pColors++;
                rgba.blue  = *pColors++;
                rgba.gray  = 0;
                rgba.alpha = (pAlpha && !bHasMask) ? *pAlpha++ : 0;

                // save in our palette
                long index = PaletteAdd(&palette, rgba);

                if (index < PNG_MAX_PALETTE_LENGTH)
                {
                    // save in libpng's palette
                    png_rgb[index].red   = rgba.red;
                    png_rgb[index].green = rgba.green;
                    png_rgb[index].blue  = rgba.blue;
                    png_trans[index]     = rgba.alpha;
                }
                else
                {
                    bUsePalette = false;
                    break;
                }
            }
        }

        if (bUsePalette)
        {
            png_set_PLTE(png_ptr, info_ptr, png_rgb, palette.size());

            if (bHasMask && !pAlpha)
            {
                wxASSERT(PaletteFind(palette, mask) == 0);
                png_trans[0] = 0;
                png_set_tRNS(png_ptr, info_ptr, png_trans, 1, NULL);
            }
            else if (pAlpha && !bHasMask)
            {
                png_set_tRNS(png_ptr, info_ptr, png_trans, palette.size(), NULL);
            }
        }
    }

    /*
    If saving palettised was requested but it was decided we can't use a
    palette then reset the colour type to RGB.
    */
    if (!bUsePalette && iColorType == wxPNG_TYPE_PALETTE)
    {
        iColorType = wxPNG_TYPE_COLOUR;
    }

    bool bUseAlpha = !bUsePalette && (bHasAlpha || bHasMask);

    int iPngColorType;

    if (bUsePalette)
    {
        iPngColorType = PNG_COLOR_TYPE_PALETTE;
        iColorType = wxPNG_TYPE_PALETTE;
    }
    else if ( iColorType==wxPNG_TYPE_COLOUR )
    {
        iPngColorType = bUseAlpha ? PNG_COLOR_TYPE_RGB_ALPHA
                                  : PNG_COLOR_TYPE_RGB;
    }
    else
    {
        iPngColorType = bUseAlpha ? PNG_COLOR_TYPE_GRAY_ALPHA
                                  : PNG_COLOR_TYPE_GRAY;
    }

    if (image->HasOption(wxIMAGE_OPTION_PNG_FILTER))
        png_set_filter( png_ptr, PNG_FILTER_TYPE_BASE, image->GetOptionInt(wxIMAGE_OPTION_PNG_FILTER) );

    if (image->HasOption(wxIMAGE_OPTION_PNG_COMPRESSION_LEVEL))
        png_set_compression_level( png_ptr, image->GetOptionInt(wxIMAGE_OPTION_PNG_COMPRESSION_LEVEL) );

    if (image->HasOption(wxIMAGE_OPTION_PNG_COMPRESSION_MEM_LEVEL))
        png_set_compression_mem_level( png_ptr, image->GetOptionInt(wxIMAGE_OPTION_PNG_COMPRESSION_MEM_LEVEL) );

    if (image->HasOption(wxIMAGE_OPTION_PNG_COMPRESSION_STRATEGY))
        png_set_compression_strategy( png_ptr, image->GetOptionInt(wxIMAGE_OPTION_PNG_COMPRESSION_STRATEGY) );

    if (image->HasOption(wxIMAGE_OPTION_PNG_COMPRESSION_BUFFER_SIZE))
        png_set_compression_buffer_size( png_ptr, image->GetOptionInt(wxIMAGE_OPTION_PNG_COMPRESSION_BUFFER_SIZE) );

    int iBitDepth = !bUsePalette && image->HasOption(wxIMAGE_OPTION_PNG_BITDEPTH)
                            ? image->GetOptionInt(wxIMAGE_OPTION_PNG_BITDEPTH)
                            : 8;

    png_set_IHDR( png_ptr, info_ptr, image->GetWidth(), image->GetHeight(),
                  iBitDepth, iPngColorType,
                  PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE,
                  PNG_FILTER_TYPE_BASE);

    int iElements;
    png_color_8 sig_bit;

    if ( iPngColorType & PNG_COLOR_MASK_COLOR )
    {
        sig_bit.red =
        sig_bit.green =
        sig_bit.blue = (png_byte)iBitDepth;
        iElements = 3;
    }
    else // grey
    {
        sig_bit.gray = (png_byte)iBitDepth;
        iElements = 1;
    }

    if ( bUseAlpha )
    {
        sig_bit.alpha = (png_byte)iBitDepth;
        iElements++;
    }

    if ( iBitDepth == 16 )
        iElements *= 2;

    // save the image resolution if we have it
    int resX, resY;
    switch ( GetResolutionFromOptions(*image, &resX, &resY) )
    {
        case wxIMAGE_RESOLUTION_INCHES:
            {
                const double INCHES_IN_METER = 10000.0 / 254;
                resX = int(resX * INCHES_IN_METER);
                resY = int(resY * INCHES_IN_METER);
            }
            break;

        case wxIMAGE_RESOLUTION_CM:
            resX *= 100;
            resY *= 100;
            break;

        case wxIMAGE_RESOLUTION_NONE:
            break;

        default:
            wxFAIL_MSG( wxT("unsupported image resolution units") );
    }

    if ( resX && resY )
        png_set_pHYs( png_ptr, info_ptr, resX, resY, PNG_RESOLUTION_METER );

    png_set_sBIT( png_ptr, info_ptr, &sig_bit );
    png_write_info( png_ptr, info_ptr );
    png_set_shift( png_ptr, &sig_bit );
    png_set_packing( png_ptr );

    unsigned char *
        data = (unsigned char *)malloc( image->GetWidth() * iElements );
    if ( !data )
    {
        png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
        return false;
    }

    const unsigned char *
        pAlpha = (const unsigned char *)(bHasAlpha ? image->GetAlpha() : NULL);

    const unsigned char *pColors = image->GetData();

    for (int y = 0; y != iHeight; ++y)
    {
        unsigned char *pData = data;
        for (int x = 0; x != iWidth; x++)
        {
            png_color_8 clr;
            clr.red   = *pColors++;
            clr.green = *pColors++;
            clr.blue  = *pColors++;
            clr.gray  = 0;
            clr.alpha = (bUsePalette && pAlpha) ? *pAlpha++ : 0; // use with wxPNG_TYPE_PALETTE only

            switch ( iColorType )
            {
                default:
                    wxFAIL_MSG( wxT("unknown wxPNG_TYPE_XXX") );
                    // fall through

                case wxPNG_TYPE_COLOUR:
                    *pData++ = clr.red;
                    if ( iBitDepth == 16 )
                        *pData++ = 0;
                    *pData++ = clr.green;
                    if ( iBitDepth == 16 )
                        *pData++ = 0;
                    *pData++ = clr.blue;
                    if ( iBitDepth == 16 )
                        *pData++ = 0;
                    break;

                case wxPNG_TYPE_GREY:
                    {
                        // where do these coefficients come from? maybe we
                        // should have image options for them as well?
                        unsigned uiColor =
                            (unsigned) (76.544*(unsigned)clr.red +
                                        150.272*(unsigned)clr.green +
                                        36.864*(unsigned)clr.blue);

                        *pData++ = (unsigned char)((uiColor >> 8) & 0xFF);
                        if ( iBitDepth == 16 )
                            *pData++ = (unsigned char)(uiColor & 0xFF);
                    }
                    break;

                case wxPNG_TYPE_GREY_RED:
                    *pData++ = clr.red;
                    if ( iBitDepth == 16 )
                        *pData++ = 0;
                    break;

                case wxPNG_TYPE_PALETTE:
                    *pData++ = (unsigned char) PaletteFind(palette, clr);
                    break;
            }

            if ( bUseAlpha )
            {
                unsigned char uchAlpha = 255;
                if ( bHasAlpha )
                    uchAlpha = *pAlpha++;

                if ( bHasMask )
                {
                    if ( (clr.red == mask.red)
                            && (clr.green == mask.green)
                                && (clr.blue == mask.blue) )
                        uchAlpha = 0;
                }

                *pData++ = uchAlpha;
                if ( iBitDepth == 16 )
                    *pData++ = 0;
            }
        }

        png_bytep row_ptr = data;
        png_write_rows( png_ptr, &row_ptr, 1 );
    }

    free(data);
    png_write_end( png_ptr, info_ptr );
    png_destroy_write_struct( &png_ptr, (png_infopp)&info_ptr );

    return true;
}

#ifdef __VISUALC__
    #pragma warning(default:4611)
#endif /* VC++ */

#endif  // wxUSE_STREAMS

/*static*/ wxVersionInfo wxPNGHandler::GetLibraryVersionInfo()
{
    // The version string seems to always have a leading space and a trailing
    // new line, get rid of them both.
    wxString str = png_get_header_version(NULL) + 1;
    str.Replace("\n", "");

    return wxVersionInfo("libpng",
                         PNG_LIBPNG_VER_MAJOR,
                         PNG_LIBPNG_VER_MINOR,
                         PNG_LIBPNG_VER_RELEASE,
                         str);
}

#endif  // wxUSE_LIBPNG