plug-ins: follow the assigned profile TRC in PNG export.

Similar to JPEG export (commit c5f7bac2ba)
as discussed with Ell. GIMP should follow and save as-is any *assigned*
profile. We only make a decision about whether to convert from storage
precision to another format when the profile is the default GIMP one.

Picked from commit 72c5d24fe9.
Some changes may have to happen later on the master code as the space
invasion changes a bit how default profiles are handled.

plug-ins/common/file-png.c

@@ -1471,7 +1471,7 @@ save_image (const gchar  *filename,
   gint              num;              /* Number of rows to load */
   FILE             *fp;               /* File pointer */
   GimpColorProfile *profile = NULL;   /* Color profile */
-  gboolean          linear;           /* Save linear RGB */
+  gboolean          out_linear;       /* Save linear RGB */
   GeglBuffer       *buffer;           /* GEGL buffer for layer */
   const Babl       *file_format;      /* BABL format of file */
   png_structp       pp;               /* PNG read pointer */
@@ -1486,44 +1486,58 @@ save_image (const gchar  *filename,
   time_t            cutime;           /* Time since epoch */
   struct tm        *gmt;              /* GMT broken down */
   gint              color_type;       /* PNG color type */
-  gint              bit_depth = 16;   /* Default to bit bepth 16 */
+  gint              bit_depth;        /* Default to bit depth 16 */
 
   guchar            remap[256];       /* Re-mapping for the palette */
 
   png_textp         text = NULL;
 
+  out_linear = FALSE;
 #if defined(PNG_iCCP_SUPPORTED)
+  profile = gimp_image_get_color_profile (orig_image_ID);
   if (pngvals.save_profile)
-    profile = gimp_image_get_effective_color_profile (orig_image_ID);
+    {
+      if (profile && gimp_color_profile_is_linear (profile) &&
+          pngvals.export_format == PNG_FORMAT_AUTO)
+        out_linear = TRUE;
+
+      if (! profile)
+        profile = gimp_image_get_effective_color_profile (orig_image_ID);
+
+      if (! out_linear && gimp_color_profile_is_linear (profile))
+        {
+          /* This should happen only when using default linear profile
+           * (no profile explicitly set) or when not exporting to
+           * PNG_FORMAT_AUTO.
+           */
+          GimpColorProfile *saved_profile;
+
+          saved_profile = gimp_color_profile_new_srgb_trc_from_color_profile (profile);
+          g_object_unref (profile);
+          profile = saved_profile;
+        }
+    }
 #endif
 
+  /* We save as 8-bit PNG only if:
+   * (1) Work image is 8-bit linear with linear profile to be saved.
+   * (2) Work image is 8-bit non-linear or perceptual with or without
+   * profile.
+   */
+  bit_depth = 16;
   switch (gimp_image_get_precision (image_ID))
     {
     case GIMP_PRECISION_U8_LINEAR:
-      /* only keep 8 bit linear RGB if we also save a profile */
+      if (out_linear)
-      if (profile)
         bit_depth = 8;
-
-    case GIMP_PRECISION_U16_LINEAR:
-    case GIMP_PRECISION_U32_LINEAR:
-    case GIMP_PRECISION_HALF_LINEAR:
-    case GIMP_PRECISION_FLOAT_LINEAR:
-    case GIMP_PRECISION_DOUBLE_LINEAR:
-      /* save linear RGB only if we save a profile, or a loader won't
-       * do the right thing
-       */
-      if (profile)
-        linear = TRUE;
-      else
-        linear = FALSE;
       break;
 
     case GIMP_PRECISION_U8_NON_LINEAR:
     case GIMP_PRECISION_U8_PERCEPTUAL:
       bit_depth = 8;
+      break;
 
     default:
-      linear = FALSE;
       break;
     }
 
@@ -1609,14 +1623,14 @@ save_image (const gchar  *filename,
         color_type = PNG_COLOR_TYPE_RGB;
         if (bit_depth == 8)
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("RGB u8");
             else
               file_format = babl_format ("R'G'B' u8");
           }
         else
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("RGB u16");
             else
               file_format = babl_format ("R'G'B' u16");
@@ -1627,14 +1641,14 @@ save_image (const gchar  *filename,
         color_type = PNG_COLOR_TYPE_RGB_ALPHA;
         if (bit_depth == 8)
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("RGBA u8");
             else
               file_format = babl_format ("R'G'B'A u8");
           }
         else
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("RGBA u16");
             else
               file_format = babl_format ("R'G'B'A u16");
@@ -1645,14 +1659,14 @@ save_image (const gchar  *filename,
         color_type = PNG_COLOR_TYPE_GRAY;
         if (bit_depth == 8)
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("Y u8");
             else
               file_format = babl_format ("Y' u8");
           }
         else
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("Y u16");
             else
               file_format = babl_format ("Y' u16");
@@ -1663,14 +1677,14 @@ save_image (const gchar  *filename,
         color_type = PNG_COLOR_TYPE_GRAY_ALPHA;
         if (bit_depth == 8)
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("YA u8");
             else
               file_format = babl_format ("Y'A u8");
           }
         else
           {
-            if (linear)
+            if (out_linear)
               file_format = babl_format ("YA u16");
             else
               file_format = babl_format ("Y'A u16");
@@ -1744,14 +1758,6 @@ save_image (const gchar  *filename,
           bit_depth = 16;
           break;
       }
-      if (linear && profile)
-        {
-          GimpColorProfile *saved_profile;
-
-          saved_profile = gimp_color_profile_new_srgb_trc_from_color_profile (profile);
-          g_object_unref (profile);
-          profile = saved_profile;
-        }
     }
 
   bpp = babl_format_get_bytes_per_pixel (file_format);
@@ -1836,7 +1842,7 @@ save_image (const gchar  *filename,
     }
 
 #if defined(PNG_iCCP_SUPPORTED)
-  if (profile)
+  if (pngvals.save_profile)
     {
       GimpParasite *parasite;
       gchar        *profile_name = NULL;
@@ -1861,10 +1867,10 @@ save_image (const gchar  *filename,
 
       g_free (profile_name);
 
-      g_object_unref (profile);
-
       *profile_saved = TRUE;
     }
+  if (profile)
+    g_object_unref (profile);
 #endif
 
 #ifdef PNG_zTXt_SUPPORTED