From b420a7738ce5d309eaddda4d54bc6b7f7f643fbe Mon Sep 17 00:00:00 2001
From: Alx Sa <cmyk.student@gmail.com>
Date: Thu, 30 Oct 2025 01:16:25 +0000
Subject: [PATCH] app, pdb: fix gimp_drawable_get_sub_thumbnail() on high-bit
 depth drawable.

Resolves #13598
Applies the same logic designed by Jehan
in f78a0629 to the gimp_drawable_get_sub_thumbnail
function.
This allows GimpZoomPreview and
GimpDrawablePreview in older
plug-ins to show correct previews for
images that are larger than 8-bit.
---
 app/pdb/drawable-cmds.c | 41 ++++++++++++++++++++++++++++++-----
 pdb/groups/drawable.pdb | 47 ++++++++++++++++++++++++++++++++++-------
 2 files changed, 75 insertions(+), 13 deletions(-)

diff --git a/app/pdb/drawable-cmds.c b/app/pdb/drawable-cmds.c
index a702feba1a..ded88a68c7 100644
--- a/app/pdb/drawable-cmds.c
+++ b/app/pdb/drawable-cmds.c
@@ -1094,11 +1094,42 @@ drawable_sub_thumbnail_invoker (GimpProcedure         *procedure,
 
           if (buf)
             {
-              width                = gimp_temp_buf_get_width  (buf);
-              height               = gimp_temp_buf_get_height (buf);
-              bpp                  = babl_format_get_bytes_per_pixel (gimp_temp_buf_get_format (buf));
-              thumbnail_data       = g_bytes_new (gimp_temp_buf_get_data (buf),
-                                                  gimp_temp_buf_get_data_size (buf));
+              const Babl *format      = gimp_temp_buf_get_format (buf);
+              const Babl *rgb_format  = babl_format ("R'G'B' u8");
+              const Babl *rgba_format = babl_format ("R'G'B'A u8");
+              const Babl *fish        = NULL;
+
+              if (babl_format_has_alpha (format) && format != rgba_format)
+                {
+                  fish   = babl_fish (format, rgba_format);
+                  format = rgba_format;
+                }
+              else if (! babl_format_has_alpha (format) && format != rgb_format)
+                {
+                  fish   = babl_fish (format, rgb_format);
+                  format = rgb_format;
+                }
+
+              width  = gimp_temp_buf_get_width  (buf);
+              height = gimp_temp_buf_get_height (buf);
+              bpp    = babl_format_get_bytes_per_pixel (format);
+              if (fish)
+                {
+                  guchar *data;
+                  gint    data_size = bpp * width * height;
+
+                  data = g_malloc (data_size);
+                  babl_process (fish, gimp_temp_buf_get_data (buf), data,
+                                width * height);
+
+                  thumbnail_data = g_bytes_new (data, data_size);
+                  g_free (data);
+                }
+              else
+                {
+                  thumbnail_data = g_bytes_new (gimp_temp_buf_get_data (buf),
+                                                gimp_temp_buf_get_data_size (buf));
+                }
 
               gimp_temp_buf_unref (buf);
             }
diff --git a/pdb/groups/drawable.pdb b/pdb/groups/drawable.pdb
index f71921d6fd..8464fb4767 100644
--- a/pdb/groups/drawable.pdb
+++ b/pdb/groups/drawable.pdb
@@ -1074,9 +1074,9 @@ HELP
 	  desc => 'The previews height' },
 	{ name => 'bpp', type => 'int32',
 	  desc => 'The previews bpp' },
-  { name => 'thumbnail_data', type => 'bytes',
-	  desc => 'The thumbnail data' }
-    );
+    { name => 'thumbnail_data', type => 'bytes',
+	    desc => 'The thumbnail data' }
+      );
 
     %invoke = (
 	headers => [ qw("core/gimpdrawable-preview.h") ],
@@ -1100,11 +1100,42 @@ HELP
 
       if (buf)
         {
-          width                = gimp_temp_buf_get_width  (buf);
-          height               = gimp_temp_buf_get_height (buf);
-          bpp                  = babl_format_get_bytes_per_pixel (gimp_temp_buf_get_format (buf));
-          thumbnail_data       = g_bytes_new (gimp_temp_buf_get_data (buf),
-                                              gimp_temp_buf_get_data_size (buf));
+          const Babl *format      = gimp_temp_buf_get_format (buf);
+          const Babl *rgb_format  = babl_format ("R'G'B' u8");
+          const Babl *rgba_format = babl_format ("R'G'B'A u8");
+          const Babl *fish        = NULL;
+
+          if (babl_format_has_alpha (format) && format != rgba_format)
+            {
+              fish   = babl_fish (format, rgba_format);
+              format = rgba_format;
+            }
+          else if (! babl_format_has_alpha (format) && format != rgb_format)
+            {
+              fish   = babl_fish (format, rgb_format);
+              format = rgb_format;
+            }
+
+          width  = gimp_temp_buf_get_width  (buf);
+          height = gimp_temp_buf_get_height (buf);
+          bpp    = babl_format_get_bytes_per_pixel (format);
+          if (fish)
+            {
+              guchar *data;
+              gint    data_size = bpp * width * height;
+
+              data = g_malloc (data_size);
+              babl_process (fish, gimp_temp_buf_get_data (buf), data,
+                            width * height);
+
+              thumbnail_data = g_bytes_new (data, data_size);
+              g_free (data);
+            }
+          else
+            {
+              thumbnail_data = g_bytes_new (gimp_temp_buf_get_data (buf),
+                                            gimp_temp_buf_get_data_size (buf));
+            }
 
           gimp_temp_buf_unref (buf);
         }