better png import

better handling of 1-bit and palette png images, though png 1.2.9 and
later only now argh

TODO

@@ -5,6 +5,12 @@
 
   a 1-bit PNG image ... we get 0/1 in the vips file, argh, not 0/255
 
+- try:
+
+	http://upload.wikimedia.org/wikipedia/commons/3/31/A_large_blank_world_map_with_oceans_marked_in_blue-edited.png
+
+  pallette png, we just get RGBA all zero atm
+
 
 - vips_attach_save() in image.c has problems if save fails ... there's no way
   for the result of the "written" callback to propogate the error

configure.in

@@ -495,7 +495,7 @@ AC_ARG_WITH([png],
   AS_HELP_STRING([--without-png], [build without libpng (default: test)]))
 
 if test x"$with_png" != "xno"; then
-  PKG_CHECK_MODULES(PNG, libpng, 
+  PKG_CHECK_MODULES(PNG, libpng >= 1.2.9, 
     [AC_DEFINE(HAVE_PNG,1,[define if you have libpng installed.])
      with_png=yes
      PACKAGES_USED="$PACKAGES_USED libpng"],
@@ -683,6 +683,7 @@ file import with matio: 		$with_matio
 file import with cfitsio: 		$with_cfitsio
 text rendering with pangoft2: 		$with_pangoft2
 file import/export with libpng: 	$with_png
+  (requires libpng-1.2.9 or later)
 file import/export with libtiff:	$with_tiff
 file import/export with libjpeg:	$with_jpeg
 use libexif to load/save JPEG metadata: $with_libexif

libvips/format/im_png2vips.c

@@ -20,6 +20,8 @@
  * 	- get png resolution (thanks Zhiyu Wu)
  * 17/3/11
  * 	- update for libpng-1.5 API changes
+ * 	- better handling of palette and 1-bit images
+ * 	- ... but we are now png 1.2.9 and later only :-( argh
  */
 
 /*
@@ -64,8 +66,7 @@
 int
 im_png2vips( const char *name, IMAGE *out )
 {
-	im_error( "im_png2vips", "%s",
-		_( "PNG support disabled" ) );
+	im_error( "im_png2vips", "%s", _( "PNG support disabled" ) );
 	return( -1 );
 }
 
@@ -236,8 +237,6 @@ png2vips( Read *read, int header_only )
 {
 	png_uint_32 width, height;
 	int bit_depth, color_type, interlace_type;
-	int num_trans;
-	png_color_8p sig_bit;
 
 	png_uint_32 res_x, res_y;
 	int unit_type;
@@ -253,8 +252,6 @@ png2vips( Read *read, int header_only )
 	png_get_IHDR( read->pPng, read->pInfo, 
 		&width, &height, &bit_depth, &color_type,
 		&interlace_type, NULL, NULL );
-	png_get_tRNS( read->pPng, read->pInfo, NULL, &num_trans, NULL );
-	png_get_sBIT( read->pPng, read->pInfo, &sig_bit );
 
 	/* png_get_channels() gives us 1 band for palette images ... so look
 	 * at colour_type for output bands.
@@ -263,10 +260,9 @@ png2vips( Read *read, int header_only )
 	case PNG_COLOR_TYPE_PALETTE: 
 		bands = 3; 
 
-		/* Don't know if this is really correct. If there are
-		 * transparent pixels, assume we're going to output RGBA.
+		/* If there's transparency in the palette, we make an alpha.
 		 */
-		if( num_trans )
+		if( png_get_valid( read->pPng, read->pInfo, PNG_INFO_tRNS ) ) 
 			bands = 4; 
 
 		break;
@@ -309,17 +305,18 @@ png2vips( Read *read, int header_only )
 			type = IM_TYPE_sRGB;
 	}
 
-	/* Expand palette images.
+	/* Expand palette images, expand transparency too.
 	 */
 	if( color_type == PNG_COLOR_TYPE_PALETTE )
-	        png_set_expand( read->pPng );
+		png_set_palette_to_rgb( read->pPng );
+	if( png_get_valid( read->pPng, read->pInfo, PNG_INFO_tRNS ) ) 
+		png_set_tRNS_to_alpha( read->pPng );
 
 	/* Expand <8 bit images to full bytes.
 	 */
-	if( bit_depth < 8 ) {
-		png_set_packing( read->pPng );
-	        png_set_shift( read->pPng, sig_bit );
-	}
+	if( color_type == PNG_COLOR_TYPE_GRAY &&
+		bit_depth < 8 ) 
+		png_set_expand_gray_1_2_4_to_8( read->pPng );
 
 	/* If we're an INTEL byte order machine and this is 16bits, we need
 	 * to swap bytes.