[devel] Use the old scaling method for background if png_set_chop_16() was

called.

ANNOUNCE

@@ -198,6 +198,8 @@ Version 1.5.4beta02 [June 14, 2011]
   Added png_set_chop_16() API, to match inaccurate results from previous
     libpng versions.
   Removed the ACCURATE and LEGACY options (they are no longer useable)
+  Use the old scaling method for background if png_set_chop_16() was
+    called.
 
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CHANGES

@@ -3461,6 +3461,8 @@ Version 1.5.4beta02 [June 14, 2011]
   Added png_set_chop_16() API, to match inaccurate results from previous
     libpng versions.
   Removed the ACCURATE and LEGACY options (they are no longer useable)
+  Use the old scaling method for background if png_set_chop_16() was
+    called.
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit

pngrtran.c

@@ -1453,12 +1453,25 @@ png_init_read_transformations(png_structp png_ptr)
        * The PNG_BACKGROUND_EXPAND code above does not expand to 16 bits at
        * present, so that case is ok (until do_expand_16 is moved.)
        */
+
+      if (png_ptr->transformations & PNG_CHOP_16_TO_8)
+      {
 #     define CHOP(x) (x)=((png_uint_16)(((png_uint_32)(x)*255+32895) >> 16))
       CHOP(png_ptr->background.red);
       CHOP(png_ptr->background.green);
       CHOP(png_ptr->background.blue);
       CHOP(png_ptr->background.gray);
 #     undef CHOP
+      }
+      else /* Use pre-libpng-1.5.4 inaccurate "ACCURATE" scaling */
+      {
+#     define CHOP(x) ((png_uint_16)((2*(png_uint_32)(x) + 257)/514))
+      CHOP(png_ptr->background.red);
+      CHOP(png_ptr->background.green);
+      CHOP(png_ptr->background.blue);
+      CHOP(png_ptr->background.gray);
+#     undef CHOP
+      }
    }
 #endif