]> git.saurik.com Git - wxWidgets.git/blobdiff - src/common/imagpng.cpp
partially working wxTaskBarIcon for X11
[wxWidgets.git] / src / common / imagpng.cpp
index 8272a0d383934b751860528b540b15e7a9e83091..1b8e2017189f27a9ed42cc7f0c1b2dd6e26baf46 100644 (file)
@@ -1,5 +1,5 @@
 /////////////////////////////////////////////////////////////////////////////
-// Name:        imagepng.cpp
+// Name:        src/common/imagepng.cpp
 // Purpose:     wxImage PNG handler
 // Author:      Robert Roebling
 // RCS-ID:      $Id$
@@ -7,10 +7,18 @@
 // Licence:     wxWindows licence
 /////////////////////////////////////////////////////////////////////////////
 
+// ============================================================================
+// declarations
+// ============================================================================
+
 #ifdef __GNUG__
 #pragma implementation "imagpng.h"
 #endif
 
+// ----------------------------------------------------------------------------
+// headers
+// ----------------------------------------------------------------------------
+
 // For compilers that support precompilation, includes "wx.h".
 #include "wx/wxprec.h"
 
@@ -22,7 +30,7 @@
   #include "wx/defs.h"
 #endif
 
-#if wxUSE_LIBPNG
+#if wxUSE_IMAGE && wxUSE_LIBPNG
 
 #include "wx/imagpng.h"
 #include "wx/bitmap.h"
 #endif
 #endif
 
-#ifdef __WXMSW__
-#include <windows.h>
-#endif
+// ----------------------------------------------------------------------------
+// constants
+// ----------------------------------------------------------------------------
 
-//-----------------------------------------------------------------------------
-// wxPNGHandler
-//-----------------------------------------------------------------------------
+// image can not 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
+};
 
-IMPLEMENT_DYNAMIC_CLASS(wxPNGHandler,wxImageHandler)
+// ----------------------------------------------------------------------------
+// local functions
+// ----------------------------------------------------------------------------
 
-#if wxUSE_LIBPNG
+// 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(const unsigned char *ptr,
+                  png_uint_32 x, png_uint_32 y, png_uint_32 w, png_uint_32 h);
 
-#if defined(__VISAGECPP__)
-#define LINKAGEMODE _Optlink
-#else
-#define LINKAGEMODE
+// 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);
+
+// ============================================================================
+// wxPNGHandler implementation
+// ============================================================================
+
+IMPLEMENT_DYNAMIC_CLASS(wxPNGHandler,wxImageHandler)
+
+#if wxUSE_STREAMS
+
+#ifndef PNGLINKAGEMODE
+    #ifdef __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
 
-static void LINKAGEMODE _PNG_stream_reader( png_structp png_ptr, png_bytep data, png_size_t length )
+
+// 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
+//     (this is for example the case with wxMGL port and SciTech MGL library
+//     that provides custom runtime-loadable libpng implementation with jmpbuf
+//     disabled altogether). 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
 {
-    ((wxInputStream*) png_get_io_ptr( png_ptr )) -> Read(data, length);
+    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"
+{
+
+void PNGLINKAGEMODE _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 LINKAGEMODE _PNG_stream_writer( png_structp png_ptr, png_bytep data, png_size_t length )
+void PNGLINKAGEMODE _PNG_stream_writer( png_structp png_ptr, png_bytep data, png_size_t length )
 {
-    ((wxOutputStream*) png_get_io_ptr( png_ptr )) -> Write(data, length);
+    WX_PNG_INFO(png_ptr)->stream.out->Write(data, length);
 }
 
 // from pngerror.c
 // so that the libpng doesn't send anything on stderr
 void
-LINKAGEMODE png_silent_error(png_structp png_ptr, png_const_charp WXUNUSED(message))
+PNGLINKAGEMODE wx_png_error(png_structp png_ptr, png_const_charp message)
 {
+    wxPNGInfoStruct *info = WX_PNG_INFO(png_ptr);
+    if (info->verbose)
+        wxLogError( wxString::FromAscii(message) );
+
 #ifdef USE_FAR_KEYWORD
-   {
-      jmp_buf jmpbuf;
-      png_memcpy(jmpbuf,png_ptr->jmpbuf,sizeof(jmp_buf));
-      longjmp(jmpbuf, 1);
-   }
+    {
+       jmp_buf jmpbuf;
+       png_memcpy(jmpbuf,info->jmpbuf,sizeof(jmp_buf));
+       longjmp(jmpbuf, 1);
+    }
 #else
-   longjmp(png_ptr->jmpbuf, 1);
+    longjmp(info->jmpbuf, 1);
 #endif
 }
 
 void
-LINKAGEMODE png_silent_warning(png_structp WXUNUSED(png_ptr), png_const_charp WXUNUSED(message))
+PNGLINKAGEMODE wx_png_warning(png_structp png_ptr, png_const_charp message)
 {
+    wxPNGInfoStruct *info = WX_PNG_INFO(png_ptr);
+    if (info->verbose)
+        wxLogWarning( wxString::FromAscii(message) );
+}
+
+} // extern "C"
+
+// ----------------------------------------------------------------------------
+// LoadFile() helpers
+// ----------------------------------------------------------------------------
+
+Transparency
+CheckTransparency(const unsigned char *ptr,
+                  png_uint_32 x, png_uint_32 y, png_uint_32 w, png_uint_32 h)
+{
+    // suppose that a mask will suffice
+    Transparency transparency = Transparency_Mask;
+
+    // and check all the remaining alpha values to see if it does
+    unsigned char a2;
+    unsigned const char *ptr2 = ptr;
+    for ( png_uint_32 y2 = y; y2 < h; y2++ )
+    {
+        for ( png_uint_32 x2 = x + 1; x2 < w; x2++ )
+        {
+            // skip the grey byte
+            a2 = *++ptr2;
+
+            if ( a2 && a2 != 0xff )
+            {
+                // not fully opeaque nor fully transparent, hence need alpha
+                transparency = Transparency_Alpha;
+                break;
+            }
+
+            ++ptr2;
+        }
+
+        if ( transparency == Transparency_Alpha )
+        {
+            // no need to continue
+            break;
+        }
+    }
+
+    return transparency;
+}
+
+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
+    for ( png_uint_32 y2 = 0; y2 <= y; y2++ )
+    {
+        for ( png_uint_32 x2 = 0; x2 < x; x2++ )
+        {
+            // 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++;
+
+            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)) )
+        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 ( a != 0xff && 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(ptrSrc, x, y,
+                                                     width, height);
+
+                    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_Alpha:
+                        *alpha++ = a;
+                        // fall through
+
+                    case Transparency_None:
+                        *ptrDst++ = g;
+                        *ptrDst++ = g;
+                        *ptrDst++ = g;
+                        break;
+
+                    case Transparency_Mask:
+                        *ptrDst++ = rMask;
+                        *ptrDst++ = bMask;
+                        *ptrDst++ = gMask;
+                }
+            }
+        }
+    }
+    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 ( a != 0xff && transparency == Transparency_None )
+                {
+                    transparency = CheckTransparency(ptrSrc, x, y,
+                                                     width, height);
+
+                    if ( transparency == Transparency_Mask )
+                    {
+                        FindMaskColour(lines, width, height,
+                                       rMask, gMask, bMask);
+                    }
+                    else // transparency == Transparency_Alpha
+                    {
+                        alpha = InitAlpha(image, x, y);
+                    }
+
+                }
+
+                switch ( transparency )
+                {
+                    case Transparency_Alpha:
+                        *alpha++ = a;
+                        // fall through
+
+                    case Transparency_None:
+                        *ptrDst++ = r;
+                        *ptrDst++ = g;
+                        *ptrDst++ = b;
+                        break;
+
+                    case Transparency_Mask:
+                        // 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++;
+                        }
+
+                        *ptrDst++ = rMask;
+                        *ptrDst++ = bMask;
+                        *ptrDst++ = gMask;
+                }
+            }
+        }
+    }
+
+    if ( transparency == Transparency_Mask )
+    {
+        image->SetMaskColour(rMask, gMask, bMask);
+    }
 }
 
 // temporarily disable the warning C4611 (interaction between '_setjmp' and
@@ -99,14 +453,21 @@ LINKAGEMODE png_silent_warning(png_structp WXUNUSED(png_ptr), png_const_charp WX
     #pragma warning(disable:4611)
 #endif /* VC++ */
 
-bool wxPNGHandler::LoadFile( wxImage *image, wxInputStream& stream, bool verbose, int WXUNUSED(index) )
+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;
-    unsigned int i;
+    unsigned char **lines = NULL;
     png_infop info_ptr = (png_infop) NULL;
+    wxPNGInfoStruct wxinfo;
+
+    wxinfo.verbose = verbose;
+    wxinfo.stream.in = &stream;
 
     image->Destroy();
 
@@ -115,25 +476,23 @@ bool wxPNGHandler::LoadFile( wxImage *image, wxInputStream& stream, bool verbose
         (png_error_ptr) NULL,
         (png_error_ptr) NULL );
     if (!png_ptr)
-        goto error_nolines;
+        goto error;
+
+    png_set_error_fn(png_ptr, (png_voidp)NULL, wx_png_error, wx_png_warning);
 
-    // the file example.c explain how to guess if the stream is a png image
-    if (!verbose) png_set_error_fn(png_ptr, (png_voidp)NULL, png_silent_error, png_silent_warning);
+    // NB: please see the comment near wxPNGInfoStruct declaration for
+    //     explanation why this line is mandatory
+    png_set_read_fn( png_ptr, &wxinfo, _PNG_stream_reader);
 
     info_ptr = png_create_info_struct( png_ptr );
     if (!info_ptr)
-        goto error_nolines;
+        goto error;
 
-    if (setjmp(png_ptr->jmpbuf))
-        goto error_nolines;
+    if (setjmp(wxinfo.jmpbuf))
+        goto error;
 
-    if (info_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-        goto error_nolines;
-
-    png_set_read_fn( png_ptr, &stream, _PNG_stream_reader);
-
-    png_uint_32 width,height;
-    int bit_depth,color_type,interlace_type;
+    png_uint_32 i, width, height;
+    int bit_depth, color_type, interlace_type;
 
     png_read_info( png_ptr, info_ptr );
     png_get_IHDR( png_ptr, info_ptr, &width, &height, &bit_depth, &color_type, &interlace_type, (int*) NULL, (int*) NULL );
@@ -141,6 +500,10 @@ bool wxPNGHandler::LoadFile( wxImage *image, wxInputStream& stream, bool verbose
     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))
@@ -150,11 +513,11 @@ bool wxPNGHandler::LoadFile( wxImage *image, wxInputStream& stream, bool verbose
     image->Create( (int)width, (int)height );
 
     if (!image->Ok())
-        goto error_nolines;
+        goto error;
 
     lines = (unsigned char **)malloc( (size_t)(height * sizeof(unsigned char *)) );
-    if (lines == NULL)
-        goto error_nolines;
+    if ( !lines )
+        goto error;
 
     for (i = 0; i < height; i++)
     {
@@ -166,87 +529,20 @@ bool wxPNGHandler::LoadFile( wxImage *image, wxInputStream& stream, bool verbose
         }
     }
 
-    // loaded successfully!
-    {
-        int transp = 0;
-        png_read_image( png_ptr, lines );
-        png_read_end( png_ptr, info_ptr );
-        png_destroy_read_struct( &png_ptr, &info_ptr, (png_infopp) NULL );
-        unsigned char *ptr = image->GetData();
-        if ((color_type == PNG_COLOR_TYPE_GRAY) ||
-            (color_type == PNG_COLOR_TYPE_GRAY_ALPHA))
-        {
-            for (unsigned int y = 0; y < height; y++)
-            {
-                unsigned char *ptr2 = lines[y];
-                for (unsigned int x = 0; x < width; x++)
-                {
-                    unsigned char r = *ptr2++;
-                    unsigned char a = *ptr2++;
-                    if (a < 128)
-                    {
-                        *ptr++ = 255;
-                        *ptr++ = 0;
-                        *ptr++ = 255;
-                        transp = 1;
-                    }
-                    else
-                    {
-                        *ptr++ = r;
-                        *ptr++ = r;
-                        *ptr++ = r;
-                    }
-                }
-            }
-        }
-        else
-        {
-            for (unsigned int y = 0; y < height; y++)
-            {
-                unsigned char *ptr2 = lines[y];
-                for (unsigned int x = 0; x < width; x++)
-                {
-                    unsigned char r = *ptr2++;
-                    unsigned char g = *ptr2++;
-                    unsigned char b = *ptr2++;
-                    unsigned char a = *ptr2++;
-                    if (a < 128)
-                    {
-                        *ptr++ = 255;
-                        *ptr++ = 0;
-                        *ptr++ = 255;
-                        transp = 1;
-                    }
-                    else
-                    {
-                        if ((r == 255) && (g == 0) && (b == 255)) r = 254;
-                        *ptr++ = r;
-                        *ptr++ = g;
-                        *ptr++ = b;
-                    }
-                }
-            }
-        }
+    png_read_image( png_ptr, lines );
+    png_read_end( png_ptr, info_ptr );
+    png_destroy_read_struct( &png_ptr, &info_ptr, (png_infopp) NULL );
 
-        for ( unsigned int j = 0; j < height; j++ )
-            free( lines[j] );
-        free( lines );
+    // loaded successfully, now init wxImage with this data
+    CopyDataFromPNG(image, lines, width, height, color_type);
 
-        if (transp)
-        {
-            image->SetMaskColour( 255, 0, 255 );
-        }
-        else
-        {
-            image->SetMask( FALSE );
-        }
-    }
+    for ( i = 0; i < height; i++ )
+        free( lines[i] );
+    free( lines );
 
     return TRUE;
 
- error_nolines:
-    lines = NULL; // called from before it was set
- error:
+error:
     if (verbose)
        wxLogError(_("Couldn't load a PNG image - file is corrupted or not enough memory."));
 
@@ -273,81 +569,97 @@ bool wxPNGHandler::LoadFile( wxImage *image, wxInputStream& stream, bool verbose
     return FALSE;
 }
 
+// ----------------------------------------------------------------------------
+// writing PNGs
+// ----------------------------------------------------------------------------
+
 bool wxPNGHandler::SaveFile( wxImage *image, wxOutputStream& stream, bool verbose )
 {
-    {
-        png_structp png_ptr = png_create_write_struct( PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
-        if (!png_ptr)
-        {
-            return FALSE;
-        }
+    wxPNGInfoStruct wxinfo;
 
-        if (!verbose) png_set_error_fn(png_ptr, (png_voidp)NULL, png_silent_error, png_silent_warning);
+    wxinfo.verbose = verbose;
+    wxinfo.stream.out = &stream;
 
-        png_infop info_ptr = png_create_info_struct(png_ptr);
-        if (info_ptr == NULL)
-        {
-            png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
-            return FALSE;
-        }
+    png_structp png_ptr = png_create_write_struct( PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
+    if (!png_ptr)
+    {
+        if (verbose)
+           wxLogError(_("Couldn't save PNG image."));
+        return FALSE;
+    }
 
-        if (setjmp(png_ptr->jmpbuf))
-        {
-            png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
-            return FALSE;
-        }
+    png_set_error_fn(png_ptr, (png_voidp)NULL, wx_png_error, wx_png_warning);
 
-        png_set_write_fn( png_ptr, &stream, _PNG_stream_writer, NULL);
+    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;
+    }
 
-        png_set_IHDR( png_ptr, info_ptr, image->GetWidth(), image->GetHeight(), 8,
-            PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE,
-            PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
+    if (setjmp(wxinfo.jmpbuf))
+    {
+        png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
+        if (verbose)
+           wxLogError(_("Couldn't save PNG image."));
+        return FALSE;
+    }
 
-        png_color_8 sig_bit;
-        sig_bit.red = 8;
-        sig_bit.green = 8;
-        sig_bit.blue = 8;
-        sig_bit.alpha = 8;
-        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 );
+    // NB: please see the comment near wxPNGInfoStruct declaration for
+    //     explanation why this line is mandatory
+    png_set_write_fn( png_ptr, &wxinfo, _PNG_stream_writer, NULL);
+
+    png_set_IHDR( png_ptr, info_ptr, image->GetWidth(), image->GetHeight(), 8,
+        PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE,
+        PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
+
+    png_color_8 sig_bit;
+    sig_bit.red = 8;
+    sig_bit.green = 8;
+    sig_bit.blue = 8;
+    sig_bit.alpha = 8;
+    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()*4 );
-        if (!data)
-        {
-            png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
-            return FALSE;
-        }
+    unsigned char *data = (unsigned char *)malloc( image->GetWidth()*4 );
+    if (!data)
+    {
+        png_destroy_write_struct( &png_ptr, (png_infopp)NULL );
+        return FALSE;
+    }
 
-        for (int y = 0; y < image->GetHeight(); y++)
+    for (int y = 0; y < image->GetHeight(); y++)
+    {
+        unsigned char *ptr = image->GetData() + (y * image->GetWidth() * 3);
+        for (int x = 0; x < image->GetWidth(); x++)
         {
-            unsigned char *ptr = image->GetData() + (y * image->GetWidth() * 3);
-            for (int x = 0; x < image->GetWidth(); x++)
+            data[(x << 2) + 0] = *ptr++;
+            data[(x << 2) + 1] = *ptr++;
+            data[(x << 2) + 2] = *ptr++;
+            if (( !image->HasMask() ) || \
+                (data[(x << 2) + 0] != image->GetMaskRed()) || \
+                (data[(x << 2) + 1] != image->GetMaskGreen()) || \
+                (data[(x << 2) + 2] != image->GetMaskBlue()))
             {
-                data[(x << 2) + 0] = *ptr++;
-                data[(x << 2) + 1] = *ptr++;
-                data[(x << 2) + 2] = *ptr++;
-                if (( !image->HasMask() ) || \
-                    (data[(x << 2) + 0] != image->GetMaskRed()) || \
-                    (data[(x << 2) + 1] != image->GetMaskGreen()) || \
-                    (data[(x << 2) + 2] != image->GetMaskBlue()))
-                {
-                    data[(x << 2) + 3] = 255;
-                }
-                else
-                {
-                    data[(x << 2) + 3] = 0;
-                }
+                data[(x << 2) + 3] = 255;
+            }
+            else
+            {
+                data[(x << 2) + 3] = 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 );
+        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;
 }
 
@@ -355,15 +667,6 @@ bool wxPNGHandler::SaveFile( wxImage *image, wxOutputStream& stream, bool verbos
     #pragma warning(default:4611)
 #endif /* VC++ */
 
-bool wxPNGHandler::DoCanRead( wxInputStream& stream )
-{
-    unsigned char hdr[4];
-
-    stream.Read(&hdr, 4);
-    stream.SeekI(-4, wxFromCurrent);
-    return (hdr[0] == 0x89 && hdr[1] == 'P' && hdr[2] == 'N' && hdr[3] == 'G');
-}
-
 #endif  // wxUSE_STREAMS
 
 #endif  // wxUSE_LIBPNG