Clarify licenses

- add license headers to everything
- convert a few files from public domain or LGPL2.1+ to GPL2+
- some archaeology to clarify which files are from which library

72 files changed, 189 insertions, 117 deletions

diff --git a/src/extension/internal/bitmap/adaptiveThreshold.cpp b/src/extension/internal/bitmap/adaptiveThreshold.cpp
index 3d7475cb3..a632ba226 100644
--- a/src/extension/internal/bitmap/adaptiveThreshold.cpp
+++ b/src/extension/internal/bitmap/adaptiveThreshold.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/adaptiveThreshold.h b/src/extension/internal/bitmap/adaptiveThreshold.h
index 75501720b..631c8e2e0 100644
--- a/src/extension/internal/bitmap/adaptiveThreshold.h
+++ b/src/extension/internal/bitmap/adaptiveThreshold.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/addNoise.cpp b/src/extension/internal/bitmap/addNoise.cpp
index 02bb877b7..606099638 100644
--- a/src/extension/internal/bitmap/addNoise.cpp
+++ b/src/extension/internal/bitmap/addNoise.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/addNoise.h b/src/extension/internal/bitmap/addNoise.h
index b1ac6a9f5..ab7936432 100644
--- a/src/extension/internal/bitmap/addNoise.h
+++ b/src/extension/internal/bitmap/addNoise.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/blur.cpp b/src/extension/internal/bitmap/blur.cpp
index 37a56b8a9..712d135a0 100644
--- a/src/extension/internal/bitmap/blur.cpp
+++ b/src/extension/internal/bitmap/blur.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/blur.h b/src/extension/internal/bitmap/blur.h
index be58ba5ba..269d57567 100644
--- a/src/extension/internal/bitmap/blur.h
+++ b/src/extension/internal/bitmap/blur.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/channel.cpp b/src/extension/internal/bitmap/channel.cpp
index 57ef0421b..4ca666814 100644
--- a/src/extension/internal/bitmap/channel.cpp
+++ b/src/extension/internal/bitmap/channel.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/channel.h b/src/extension/internal/bitmap/channel.h
index 898d3af46..067a5fd3b 100644
--- a/src/extension/internal/bitmap/channel.h
+++ b/src/extension/internal/bitmap/channel.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/charcoal.cpp b/src/extension/internal/bitmap/charcoal.cpp
index 2cfd741d1..69b76614b 100644
--- a/src/extension/internal/bitmap/charcoal.cpp
+++ b/src/extension/internal/bitmap/charcoal.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/charcoal.h b/src/extension/internal/bitmap/charcoal.h
index 2bedba76e..5c2a8d513 100644
--- a/src/extension/internal/bitmap/charcoal.h
+++ b/src/extension/internal/bitmap/charcoal.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/colorize.cpp b/src/extension/internal/bitmap/colorize.cpp
index 1c4b8a696..b7ce01a2a 100644
--- a/src/extension/internal/bitmap/colorize.cpp
+++ b/src/extension/internal/bitmap/colorize.cpp
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Authors:
  *   Christopher Brown <audiere@gmail.com>
@@ -5,7 +6,7 @@
  *
  * Copyright (C) 2007 Authors
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/colorize.h b/src/extension/internal/bitmap/colorize.h
index 5b55a1814..5faccf5a9 100644
--- a/src/extension/internal/bitmap/colorize.h
+++ b/src/extension/internal/bitmap/colorize.h
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Authors:
  *   Christopher Brown <audiere@gmail.com>
@@ -5,7 +6,7 @@
  *
  * Copyright (C) 2007 Authors
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/contrast.cpp b/src/extension/internal/bitmap/contrast.cpp
index a3be77808..35d04f50d 100644
--- a/src/extension/internal/bitmap/contrast.cpp
+++ b/src/extension/internal/bitmap/contrast.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/contrast.h b/src/extension/internal/bitmap/contrast.h
index 53a636647..c8099176f 100644
--- a/src/extension/internal/bitmap/contrast.h
+++ b/src/extension/internal/bitmap/contrast.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/crop.cpp b/src/extension/internal/bitmap/crop.cpp
index 99faef9d3..7b36bd340 100644
--- a/src/extension/internal/bitmap/crop.cpp
+++ b/src/extension/internal/bitmap/crop.cpp
@@ -1,8 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2011 Authors:
  *   Nicolas Dufour <nicoduf@yahoo.fr>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "2geom/transforms.h"
diff --git a/src/extension/internal/bitmap/crop.h b/src/extension/internal/bitmap/crop.h
index ce9b92797..168c093c5 100644
--- a/src/extension/internal/bitmap/crop.h
+++ b/src/extension/internal/bitmap/crop.h
@@ -1,10 +1,11 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2010 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *   Nicolas Dufour <nicoduf@yahoo.fr>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/cycleColormap.cpp b/src/extension/internal/bitmap/cycleColormap.cpp
index 8d55087a6..f7198eba5 100644
--- a/src/extension/internal/bitmap/cycleColormap.cpp
+++ b/src/extension/internal/bitmap/cycleColormap.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/cycleColormap.h b/src/extension/internal/bitmap/cycleColormap.h
index 7d8f49504..eacbb8c85 100644
--- a/src/extension/internal/bitmap/cycleColormap.h
+++ b/src/extension/internal/bitmap/cycleColormap.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/despeckle.cpp b/src/extension/internal/bitmap/despeckle.cpp
index c40e6e227..e0b2ed7e6 100644
--- a/src/extension/internal/bitmap/despeckle.cpp
+++ b/src/extension/internal/bitmap/despeckle.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/despeckle.h b/src/extension/internal/bitmap/despeckle.h
index 7a9be90a3..0c846b651 100644
--- a/src/extension/internal/bitmap/despeckle.h
+++ b/src/extension/internal/bitmap/despeckle.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/edge.cpp b/src/extension/internal/bitmap/edge.cpp
index 486ce0e4e..4212dee0d 100644
--- a/src/extension/internal/bitmap/edge.cpp
+++ b/src/extension/internal/bitmap/edge.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/edge.h b/src/extension/internal/bitmap/edge.h
index 40277a89e..8c7b8f9d1 100644
--- a/src/extension/internal/bitmap/edge.h
+++ b/src/extension/internal/bitmap/edge.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/emboss.cpp b/src/extension/internal/bitmap/emboss.cpp
index 9518cf320..44f65b049 100644
--- a/src/extension/internal/bitmap/emboss.cpp
+++ b/src/extension/internal/bitmap/emboss.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/emboss.h b/src/extension/internal/bitmap/emboss.h
index 4dcfc89d4..ff897089b 100644
--- a/src/extension/internal/bitmap/emboss.h
+++ b/src/extension/internal/bitmap/emboss.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/enhance.cpp b/src/extension/internal/bitmap/enhance.cpp
index 5b06a2d27..8c7dc52fd 100644
--- a/src/extension/internal/bitmap/enhance.cpp
+++ b/src/extension/internal/bitmap/enhance.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/enhance.h b/src/extension/internal/bitmap/enhance.h
index 3f5df47f6..574781fbe 100644
--- a/src/extension/internal/bitmap/enhance.h
+++ b/src/extension/internal/bitmap/enhance.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/equalize.cpp b/src/extension/internal/bitmap/equalize.cpp
index 6098f74b1..25d5e3c45 100644
--- a/src/extension/internal/bitmap/equalize.cpp
+++ b/src/extension/internal/bitmap/equalize.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/equalize.h b/src/extension/internal/bitmap/equalize.h
index 22cadf554..ea897dafb 100644
--- a/src/extension/internal/bitmap/equalize.h
+++ b/src/extension/internal/bitmap/equalize.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/gaussianBlur.cpp b/src/extension/internal/bitmap/gaussianBlur.cpp
index d68cd59bc..944d84838 100644
--- a/src/extension/internal/bitmap/gaussianBlur.cpp
+++ b/src/extension/internal/bitmap/gaussianBlur.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/gaussianBlur.h b/src/extension/internal/bitmap/gaussianBlur.h
index 3b6ca2137..2227fde5b 100644
--- a/src/extension/internal/bitmap/gaussianBlur.h
+++ b/src/extension/internal/bitmap/gaussianBlur.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/imagemagick.cpp b/src/extension/internal/bitmap/imagemagick.cpp
index 270fb2f35..e476a7370 100644
--- a/src/extension/internal/bitmap/imagemagick.cpp
+++ b/src/extension/internal/bitmap/imagemagick.cpp
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Authors:
  *   Christopher Brown <audiere@gmail.com>
@@ -5,7 +6,7 @@
  *
  * Copyright (C) 2007 Authors
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include <libintl.h>
diff --git a/src/extension/internal/bitmap/imagemagick.h b/src/extension/internal/bitmap/imagemagick.h
index 86c828d29..2793e457e 100644
--- a/src/extension/internal/bitmap/imagemagick.h
+++ b/src/extension/internal/bitmap/imagemagick.h
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 #ifndef INKSCAPE_EXTENSION_INTERNAL_BITMAP_IMAGEMAGICK_H
 #define INKSCAPE_EXTENSION_INTERNAL_BITMAP_IMAGEMAGICK_H
 
@@ -6,7 +7,7 @@
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
  *
- * Released under GNU GPL, read the file 'COPYING' for more information
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/implementation/implementation.h"
diff --git a/src/extension/internal/bitmap/implode.cpp b/src/extension/internal/bitmap/implode.cpp
index c24fb7bf5..b48fb47cc 100644
--- a/src/extension/internal/bitmap/implode.cpp
+++ b/src/extension/internal/bitmap/implode.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/implode.h b/src/extension/internal/bitmap/implode.h
index b25b35412..24218dc9f 100644
--- a/src/extension/internal/bitmap/implode.h
+++ b/src/extension/internal/bitmap/implode.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/level.cpp b/src/extension/internal/bitmap/level.cpp
index 168328e8f..cc4ade583 100644
--- a/src/extension/internal/bitmap/level.cpp
+++ b/src/extension/internal/bitmap/level.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/level.h b/src/extension/internal/bitmap/level.h
index 111e83af7..600a82a79 100644
--- a/src/extension/internal/bitmap/level.h
+++ b/src/extension/internal/bitmap/level.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/levelChannel.cpp b/src/extension/internal/bitmap/levelChannel.cpp
index d5c636059..ddaf2e958 100644
--- a/src/extension/internal/bitmap/levelChannel.cpp
+++ b/src/extension/internal/bitmap/levelChannel.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/levelChannel.h b/src/extension/internal/bitmap/levelChannel.h
index 832619d5f..5e2b20ca7 100644
--- a/src/extension/internal/bitmap/levelChannel.h
+++ b/src/extension/internal/bitmap/levelChannel.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/medianFilter.cpp b/src/extension/internal/bitmap/medianFilter.cpp
index 64837850d..eb7c374a9 100644
--- a/src/extension/internal/bitmap/medianFilter.cpp
+++ b/src/extension/internal/bitmap/medianFilter.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/medianFilter.h b/src/extension/internal/bitmap/medianFilter.h
index befc57001..343a11d3c 100644
--- a/src/extension/internal/bitmap/medianFilter.h
+++ b/src/extension/internal/bitmap/medianFilter.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/modulate.cpp b/src/extension/internal/bitmap/modulate.cpp
index 3b3a2495c..086b0aafa 100644
--- a/src/extension/internal/bitmap/modulate.cpp
+++ b/src/extension/internal/bitmap/modulate.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/modulate.h b/src/extension/internal/bitmap/modulate.h
index f7c6fe9a5..1eb69cbdb 100644
--- a/src/extension/internal/bitmap/modulate.h
+++ b/src/extension/internal/bitmap/modulate.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/negate.cpp b/src/extension/internal/bitmap/negate.cpp
index 4737e6443..0465c7f2c 100644
--- a/src/extension/internal/bitmap/negate.cpp
+++ b/src/extension/internal/bitmap/negate.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/negate.h b/src/extension/internal/bitmap/negate.h
index 7ebe3b79e..c8464664a 100644
--- a/src/extension/internal/bitmap/negate.h
+++ b/src/extension/internal/bitmap/negate.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/normalize.cpp b/src/extension/internal/bitmap/normalize.cpp
index 1444aa654..91029aa2c 100644
--- a/src/extension/internal/bitmap/normalize.cpp
+++ b/src/extension/internal/bitmap/normalize.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/normalize.h b/src/extension/internal/bitmap/normalize.h
index 0637dbf44..5da81ed38 100644
--- a/src/extension/internal/bitmap/normalize.h
+++ b/src/extension/internal/bitmap/normalize.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/oilPaint.cpp b/src/extension/internal/bitmap/oilPaint.cpp
index 88c85a590..66c8941c9 100644
--- a/src/extension/internal/bitmap/oilPaint.cpp
+++ b/src/extension/internal/bitmap/oilPaint.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/oilPaint.h b/src/extension/internal/bitmap/oilPaint.h
index 822119d00..77d4ad10e 100644
--- a/src/extension/internal/bitmap/oilPaint.h
+++ b/src/extension/internal/bitmap/oilPaint.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/opacity.cpp b/src/extension/internal/bitmap/opacity.cpp
index a25e99543..bcffbc497 100644
--- a/src/extension/internal/bitmap/opacity.cpp
+++ b/src/extension/internal/bitmap/opacity.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/opacity.h b/src/extension/internal/bitmap/opacity.h
index 2c3a4ecb3..287b10f8c 100644
--- a/src/extension/internal/bitmap/opacity.h
+++ b/src/extension/internal/bitmap/opacity.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/raise.cpp b/src/extension/internal/bitmap/raise.cpp
index e85cc747a..55d2d6e7d 100644
--- a/src/extension/internal/bitmap/raise.cpp
+++ b/src/extension/internal/bitmap/raise.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/raise.h b/src/extension/internal/bitmap/raise.h
index 81a6c65c3..fca23ccb1 100644
--- a/src/extension/internal/bitmap/raise.h
+++ b/src/extension/internal/bitmap/raise.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/reduceNoise.cpp b/src/extension/internal/bitmap/reduceNoise.cpp
index b0d8b9c2d..43846b816 100644
--- a/src/extension/internal/bitmap/reduceNoise.cpp
+++ b/src/extension/internal/bitmap/reduceNoise.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/reduceNoise.h b/src/extension/internal/bitmap/reduceNoise.h
index 0a4847d63..f7270db25 100644
--- a/src/extension/internal/bitmap/reduceNoise.h
+++ b/src/extension/internal/bitmap/reduceNoise.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/sample.cpp b/src/extension/internal/bitmap/sample.cpp
index 081303b73..f75dd966f 100644
--- a/src/extension/internal/bitmap/sample.cpp
+++ b/src/extension/internal/bitmap/sample.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/sample.h b/src/extension/internal/bitmap/sample.h
index a378229d9..a87929ea8 100644
--- a/src/extension/internal/bitmap/sample.h
+++ b/src/extension/internal/bitmap/sample.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/shade.cpp b/src/extension/internal/bitmap/shade.cpp
index 927a3a578..1db1ce90f 100644
--- a/src/extension/internal/bitmap/shade.cpp
+++ b/src/extension/internal/bitmap/shade.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/shade.h b/src/extension/internal/bitmap/shade.h
index 2f8349265..a7580a23c 100644
--- a/src/extension/internal/bitmap/shade.h
+++ b/src/extension/internal/bitmap/shade.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/sharpen.cpp b/src/extension/internal/bitmap/sharpen.cpp
index d397d46c8..4c9c38d9b 100644
--- a/src/extension/internal/bitmap/sharpen.cpp
+++ b/src/extension/internal/bitmap/sharpen.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/sharpen.h b/src/extension/internal/bitmap/sharpen.h
index b8f92ddb3..56baff42b 100644
--- a/src/extension/internal/bitmap/sharpen.h
+++ b/src/extension/internal/bitmap/sharpen.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/solarize.cpp b/src/extension/internal/bitmap/solarize.cpp
index 5f8cdd17c..994f4b48b 100644
--- a/src/extension/internal/bitmap/solarize.cpp
+++ b/src/extension/internal/bitmap/solarize.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/solarize.h b/src/extension/internal/bitmap/solarize.h
index 4acdf3c5c..8e161c532 100644
--- a/src/extension/internal/bitmap/solarize.h
+++ b/src/extension/internal/bitmap/solarize.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/spread.cpp b/src/extension/internal/bitmap/spread.cpp
index e38e2a927..87ce35796 100644
--- a/src/extension/internal/bitmap/spread.cpp
+++ b/src/extension/internal/bitmap/spread.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/spread.h b/src/extension/internal/bitmap/spread.h
index 82d955890..dd6591336 100644
--- a/src/extension/internal/bitmap/spread.h
+++ b/src/extension/internal/bitmap/spread.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/swirl.cpp b/src/extension/internal/bitmap/swirl.cpp
index 98efa6238..199359f9e 100644
--- a/src/extension/internal/bitmap/swirl.cpp
+++ b/src/extension/internal/bitmap/swirl.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/swirl.h b/src/extension/internal/bitmap/swirl.h
index b54b83581..320c31214 100644
--- a/src/extension/internal/bitmap/swirl.h
+++ b/src/extension/internal/bitmap/swirl.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/threshold.cpp b/src/extension/internal/bitmap/threshold.cpp
index 9f10b2c42..0f06c7c79 100644
--- a/src/extension/internal/bitmap/threshold.cpp
+++ b/src/extension/internal/bitmap/threshold.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/threshold.h b/src/extension/internal/bitmap/threshold.h
index 2e65a7a6f..13221ed33 100644
--- a/src/extension/internal/bitmap/threshold.h
+++ b/src/extension/internal/bitmap/threshold.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/unsharpmask.cpp b/src/extension/internal/bitmap/unsharpmask.cpp
index c5a7e9291..c54008342 100644
--- a/src/extension/internal/bitmap/unsharpmask.cpp
+++ b/src/extension/internal/bitmap/unsharpmask.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/unsharpmask.h b/src/extension/internal/bitmap/unsharpmask.h
index 5d3a47ecf..29c0c4253 100644
--- a/src/extension/internal/bitmap/unsharpmask.h
+++ b/src/extension/internal/bitmap/unsharpmask.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"
diff --git a/src/extension/internal/bitmap/wave.cpp b/src/extension/internal/bitmap/wave.cpp
index 1ccaa1d5e..c3a81a4e0 100644
--- a/src/extension/internal/bitmap/wave.cpp
+++ b/src/extension/internal/bitmap/wave.cpp
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "extension/effect.h"
diff --git a/src/extension/internal/bitmap/wave.h b/src/extension/internal/bitmap/wave.h
index 5bbde9e48..d88ce68d6 100644
--- a/src/extension/internal/bitmap/wave.h
+++ b/src/extension/internal/bitmap/wave.h
@@ -1,9 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2007 Authors:
  *   Christopher Brown <audiere@gmail.com>
  *   Ted Gould <ted@gould.cx>
- *   
- * Released under GNU GPL, read the file 'COPYING' for more information
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
  */
 
 #include "imagemagick.h"