note PNG background colour in metadata

see https://github.com/libvips/libvips/issues/2111

ChangeLog

@@ -18,6 +18,7 @@
 - fix ref leaks in mosaicing package
 - run libvips leak test in CI 
 - add vips_fitsload_source()m vips_niftiload_source()
+- png load notes background colour as metadata [781545872]
 
 22/12/20 start 8.10.6
 - don't seek on bad file descriptors [kleisauke]

libvips/foreign/spngload.c

@@ -2,6 +2,8 @@
  *
  * 1/5/20
  * 	- from pngload.c
+ * 19/2/21 781545872
+ * 	- read out background, if we can
  */
 
 /*
@@ -199,6 +201,7 @@ vips_foreign_load_png_set_header( VipsForeignLoadPng *png, VipsImage *image )
 	struct spng_iccp iccp;
 	struct spng_exif exif;
 	struct spng_phys phys;
+	struct spng_bkgd bkgd;
 	guint32 n_text;
 
 	/* Get resolution. Default to 72 pixels per inch.
@@ -267,6 +270,47 @@ vips_foreign_load_png_set_header( VipsForeignLoadPng *png, VipsImage *image )
 	 */
 	if( png->ihdr.interlace_method != SPNG_INTERLACE_NONE ) 
 		vips_image_set_int( image, "interlaced", 1 ); 
+
+	if( !spng_get_bkgd( png->ctx, &bkgd ) ) {
+		const int scale = image->BandFmt == 
+			VIPS_FORMAT_UCHAR ? 1 : 256;
+
+		double array[3];
+		int n;
+
+		switch( png->ihdr.color_type ) { 
+		case SPNG_COLOR_TYPE_GRAYSCALE:
+		case SPNG_COLOR_TYPE_GRAYSCALE_ALPHA:
+			array[0] = bkgd.gray / scale;
+			n = 1;
+			break;
+
+		case SPNG_COLOR_TYPE_TRUECOLOR:
+		case SPNG_COLOR_TYPE_TRUECOLOR_ALPHA:
+			array[0] = bkgd.red / scale;
+			array[1] = bkgd.green / scale;
+			array[2] = bkgd.blue / scale;
+			n = 3;
+			break;
+
+		case SPNG_COLOR_TYPE_INDEXED:
+		default:
+			/* Not sure what to do here. I suppose we should read
+			 * the palette.
+			 */
+			n = 0;
+			break;
+		}
+
+		if( n > 0 ) {
+			GValue value = { 0 };
+
+			g_value_init( &value, VIPS_TYPE_ARRAY_DOUBLE );
+			vips_value_set_array_double( &value, array, n );
+			vips_image_set( image, "background", &value );
+			g_value_unset( &value );
+		}
+	}
 }
 
 static int
@@ -308,7 +352,6 @@ vips_foreign_load_png_header( VipsForeignLoad *load )
 		return( -1 );
 	}
 
-
 #ifdef DEBUG
 	printf( "width: %d\nheight: %d\nbit depth: %d\ncolor type: %d\n",
 		png->ihdr.width, png->ihdr.height,

libvips/foreign/vipspng.c

@@ -79,6 +79,8 @@
  * 	- revise for connection IO
  * 11/5/20
  * 	- only warn for saving bad profiles, don't fail
+ * 19/2/21 781545872
+ * 	- read out background, if we can
  */
 
 /*
@@ -559,6 +561,54 @@ png2vips_header( Read *read, VipsImage *out )
 	if( color_type == PNG_COLOR_TYPE_PALETTE )
 		vips_image_set_int( out, "palette-bit-depth", bitdepth );
 
+	/* Note the PNG background colour, if any.
+	 */
+#ifdef PNG_bKGD_SUPPORTED
+{
+	png_color_16 *background;
+
+	if( png_get_bKGD( read->pPng, read->pInfo, &background ) ) {
+		const int scale = out->BandFmt == VIPS_FORMAT_UCHAR ? 1 : 256;
+
+		double array[3];
+		int n;
+
+		switch( color_type ) {
+		case PNG_COLOR_TYPE_GRAY:
+		case PNG_COLOR_TYPE_GRAY_ALPHA:
+			array[0] = background->gray / scale;
+			n = 1;
+			break;
+
+		case PNG_COLOR_TYPE_RGB:
+		case PNG_COLOR_TYPE_RGB_ALPHA:
+			array[0] = background->red / scale;
+			array[1] = background->green / scale;
+			array[2] = background->blue / scale;
+			n = 3;
+			break;
+
+		case PNG_COLOR_TYPE_PALETTE:
+		default:
+			/* Not sure what to do here. I suppose we should read
+			 * the palette.
+			 */
+			n = 0;
+			break;
+		}
+
+		if( n > 0 ) {
+			GValue value = { 0 };
+
+			g_value_init( &value, VIPS_TYPE_ARRAY_DOUBLE );
+			vips_value_set_array_double( &value, array, n );
+			vips_image_set( out, "background", &value );
+			g_value_unset( &value );
+		}
+	}
+}
+#endif
+
 	return( 0 );
 }