[gimp] plug-ins - compress the PNG comment when over a given size.

[Jehan Pagès <jehanp src gnome org>]

commit d38f209e2ff5294764ec4882ecab1d66a00e3008
Author: Jehan <jehan girinstud io>
Date:   Thu Sep 12 12:52:09 2013 +1200

    plug-ins - compress the PNG comment when over a given size.

 plug-ins/common/file-png.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)
---
diff --git a/plug-ins/common/file-png.c b/plug-ins/common/file-png.c
index 223f69d..13d7ccb 100644
--- a/plug-ins/common/file-png.c
+++ b/plug-ins/common/file-png.c
@@ -1558,6 +1558,12 @@ save_image (const gchar  *filename,
   }
 #endif
 
+#ifdef PNG_zTXt_SUPPORTED
+/* Small texts are not worth compressing and will be even bigger if compressed.
+   Empirical length limit of a text being worth compressing. */
+#define COMPRESSION_WORTHY_LENGTH 200
+#endif
+
   if (pngvals.comment)
     {
       GimpParasite *parasite;
@@ -1595,7 +1601,12 @@ save_image (const gchar  *filename,
                   text[0].text        = g_strdup (comment);
                   text[0].itxt_length = strlen (text[0].text);
 
+#ifdef PNG_zTXt_SUPPORTED
+                  text[0].compression = strlen (text[0].text) > COMPRESSION_WORTHY_LENGTH ?
+                                        PNG_ITXT_COMPRESSION_zTXt : PNG_ITXT_COMPRESSION_NONE;
+#else
                   text[0].compression = PNG_ITXT_COMPRESSION_NONE;
+#endif /* PNG_zTXt_SUPPORTED */
                 }
               else
                   /* The comment is ISO-8859-1 compatible, so we use tEXt
@@ -1617,7 +1628,13 @@ save_image (const gchar  *filename,
                                                           NULL);
 #endif
 
+#ifdef PNG_zTXt_SUPPORTED
+                  text[0].compression = strlen (text[0].text) > COMPRESSION_WORTHY_LENGTH ?
+                                        PNG_TEXT_COMPRESSION_zTXt : PNG_TEXT_COMPRESSION_NONE;
+#else
                   text[0].compression = PNG_TEXT_COMPRESSION_NONE;
+#endif /* PNG_zTXt_SUPPORTED */
+
                   text[0].text_length = text_length;
                  }
 
@@ -1633,6 +1650,10 @@ save_image (const gchar  *filename,
         }
     }
 
+#ifdef PNG_zTXt_SUPPORTED
+#undef COMPRESSION_WORTHY_LENGTH
+#endif
+
   if (text)
     png_set_text (pp, info, text, 1);