Build break: fix write order of colourspace chunks

cICP was written after PLTE, not before.  The other chunks were output
in an order which does not match the new PNG-v3 "priority" order.  This
change outputs all chunks in the "priority" order; highest precedence
first.  This means that the PNGs so written conform to PNG v3 (cICP) and
allow a streaming app to handle chunks in order without buffering data
which may later be overridden.  Note that PNV-v3 establishes the idea of
dropping ancillary chunks which are inconveniently ordered in the
definition of how APNG chunks are handled.

Signed-off-by: John Bowler <[email protected]>

pngwrite.c

@@ -127,29 +127,61 @@ png_write_info_before_PLTE(png_structrp png_ptr, png_const_inforp info_ptr)
        * the application continues writing the PNG.  So check the 'invalid'
        * flag here too.
        */
-#ifdef PNG_GAMMA_SUPPORTED
-#  ifdef PNG_WRITE_gAMA_SUPPORTED
-      if ((info_ptr->colorspace.flags & PNG_COLORSPACE_INVALID) == 0 &&
-          (info_ptr->colorspace.flags & PNG_COLORSPACE_FROM_gAMA) != 0 &&
-          (info_ptr->valid & PNG_INFO_gAMA) != 0)
-         png_write_gAMA_fixed(png_ptr, info_ptr->colorspace.gamma);
-#  endif
+#ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
+         /* Write unknown chunks first; PNG v3 establishes a precedence order
+          * for colourspace chunks.  It is certain therefore that new
+          * colourspace chunks will have a precedence and very likely it will be
+          * higher than all known so far.  Writing the unknown chunks here is
+          * most likely to present the chunks in the most convenient order.
+          *
+          * FUTURE: maybe write chunks in the order the app calls png_set_chnk
+          * to give the app control.
+          */
+         write_unknown_chunks(png_ptr, info_ptr, PNG_HAVE_IHDR);
+#endif
+
+#ifdef PNG_WRITE_sBIT_SUPPORTED
+         /* PNG v3: a streaming app will need to see this before cICP because
+          * the information is helpful in handling HL encoding (which is
+          * natively 10 bits but gets expanded to 16 in PNG.)
+          *
+          * The app shouldn't care about the order ideally but it might have
+          * no choice.  In PNG v3 apps are allowed to reject PNGs where the APNG
+          * chunks are out of order so it behooves libpng to be nice here.
+          */
+         if ((info_ptr->valid & PNG_INFO_sBIT) != 0)
+            png_write_sBIT(png_ptr, &(info_ptr->sig_bit), info_ptr->color_type);
 #endif
 
+   /* PNG v3: the July 2004 version of the TR introduced the concept of colour
+    * space priority.  As above it therefore behooves libpng to write the colour
+    * space chunks in the priority order so that a streaming app need not buffer
+    * them.
+    */
 #ifdef PNG_COLORSPACE_SUPPORTED
-      /* Write only one of sRGB or an ICC profile.  If a profile was supplied
-       * and it matches one of the known sRGB ones issue a warning.
+#  ifdef PNG_WRITE_cICP_SUPPORTED /* Priority 4 */
+   if ((info_ptr->valid & PNG_INFO_cICP) != 0)
+      {
+         png_write_cICP(png_ptr,
+                        info_ptr->cicp_colour_primaries,
+                        info_ptr->cicp_transfer_function,
+                        info_ptr->cicp_matrix_coefficients,
+                        info_ptr->cicp_video_full_range_flag);
+      }
+#  endif
+
+      /* PNG v3 change: it is now permitted to write both sRGB and ICC profiles,
+       * however because the libpng code auto-generates an sRGB for the
+       * corresponding ICC profiles and because PNG v2 disallowed this we need
+       * to only write one.
+       *
+       * Remove the PNG v2 warning about writing an sRGB ICC profile as well
+       * because it's invalid with PNG v3.
        */
-#  ifdef PNG_WRITE_iCCP_SUPPORTED
+#  ifdef PNG_WRITE_iCCP_SUPPORTED /* Priority 3 */
          if ((info_ptr->colorspace.flags & PNG_COLORSPACE_INVALID) == 0 &&
              (info_ptr->valid & PNG_INFO_iCCP) != 0)
          {
-#    ifdef PNG_WRITE_sRGB_SUPPORTED
-               if ((info_ptr->valid & PNG_INFO_sRGB) != 0)
-                  png_app_warning(png_ptr,
-                      "profile matches sRGB but writing iCCP instead");
-#     endif
-
             png_write_iCCP(png_ptr, info_ptr->iccp_name,
                 info_ptr->iccp_profile);
          }
@@ -158,31 +190,31 @@ png_write_info_before_PLTE(png_structrp png_ptr, png_const_inforp info_ptr)
 #     endif
 #  endif
 
-#  ifdef PNG_WRITE_sRGB_SUPPORTED
+#  ifdef PNG_WRITE_sRGB_SUPPORTED /* Priority 2 */
          if ((info_ptr->colorspace.flags & PNG_COLORSPACE_INVALID) == 0 &&
              (info_ptr->valid & PNG_INFO_sRGB) != 0)
             png_write_sRGB(png_ptr, info_ptr->colorspace.rendering_intent);
 #  endif /* WRITE_sRGB */
 #endif /* COLORSPACE */
 
-#ifdef PNG_WRITE_sBIT_SUPPORTED
-         if ((info_ptr->valid & PNG_INFO_sBIT) != 0)
-            png_write_sBIT(png_ptr, &(info_ptr->sig_bit), info_ptr->color_type);
+#ifdef PNG_GAMMA_SUPPORTED
+#  ifdef PNG_WRITE_gAMA_SUPPORTED /* Priority 1 */
+      if ((info_ptr->colorspace.flags & PNG_COLORSPACE_INVALID) == 0 &&
+          (info_ptr->colorspace.flags & PNG_COLORSPACE_FROM_gAMA) != 0 &&
+          (info_ptr->valid & PNG_INFO_gAMA) != 0)
+         png_write_gAMA_fixed(png_ptr, info_ptr->colorspace.gamma);
+#  endif
 #endif
 
 #ifdef PNG_COLORSPACE_SUPPORTED
-#  ifdef PNG_WRITE_cHRM_SUPPORTED
+#  ifdef PNG_WRITE_cHRM_SUPPORTED /* Also priority 1 */
          if ((info_ptr->colorspace.flags & PNG_COLORSPACE_INVALID) == 0 &&
              (info_ptr->colorspace.flags & PNG_COLORSPACE_FROM_cHRM) != 0 &&
              (info_ptr->valid & PNG_INFO_cHRM) != 0)
             png_write_cHRM_fixed(png_ptr, &info_ptr->colorspace.end_points_xy);
 #  endif
 #endif
 
-#ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
-         write_unknown_chunks(png_ptr, info_ptr, PNG_HAVE_IHDR);
-#endif
-
       png_ptr->mode |= PNG_WROTE_INFO_BEFORE_PLTE;
    }
 }
@@ -236,17 +268,6 @@ png_write_info(png_structrp png_ptr, png_const_inforp info_ptr)
       png_write_bKGD(png_ptr, &(info_ptr->background), info_ptr->color_type);
 #endif
 
-#ifdef PNG_WRITE_cICP_SUPPORTED
-   if ((info_ptr->valid & PNG_INFO_cICP) != 0)
-   {
-      png_write_cICP(png_ptr,
-                     info_ptr->cicp_colour_primaries,
-                     info_ptr->cicp_transfer_function,
-                     info_ptr->cicp_matrix_coefficients,
-                     info_ptr->cicp_video_full_range_flag);
-   }
-#endif
-
 #ifdef PNG_WRITE_eXIf_SUPPORTED
    if ((info_ptr->valid & PNG_INFO_eXIf) != 0)
    {