diff --git a/rtengine/camconst.cc b/rtengine/camconst.cc
index d136d6a21..c5cfc26fa 100644
--- a/rtengine/camconst.cc
+++ b/rtengine/camconst.cc
@@ -638,7 +638,7 @@ CameraConst::update_globalGreenEquilibration(bool other)
 }
 
 bool
-CameraConstantsStore::parse_camera_constants_file(Glib::ustring filename_)
+CameraConstantsStore::parse_camera_constants_file(const Glib::ustring& filename_)
 {
     // read the file into a single long string
     const char *filename = filename_.c_str();
@@ -809,7 +809,7 @@ CameraConstantsStore::~CameraConstantsStore()
     }
 }
 
-void CameraConstantsStore::init(Glib::ustring baseDir, Glib::ustring userSettingsDir)
+void CameraConstantsStore::init(const Glib::ustring& baseDir, const Glib::ustring& userSettingsDir)
 {
     parse_camera_constants_file(Glib::build_filename(baseDir, "camconst.json"));
 
diff --git a/rtengine/camconst.h b/rtengine/camconst.h
index 1096e1767..fbc16cb13 100644
--- a/rtengine/camconst.h
+++ b/rtengine/camconst.h
@@ -3,10 +3,16 @@
  */
 #pragma once
 
-#include <glibmm/ustring.h>
 #include <map>
 #include <vector>
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtengine
 {
 
@@ -62,11 +68,11 @@ private:
     std::map<std::string, CameraConst *> mCameraConstants;
 
     CameraConstantsStore();
-    bool parse_camera_constants_file(Glib::ustring filename);
+    bool parse_camera_constants_file(const Glib::ustring& filename);
 
 public:
     ~CameraConstantsStore();
-    void init(Glib::ustring baseDir, Glib::ustring userSettingsDir);
+    void init(const Glib::ustring& baseDir, const Glib::ustring& userSettingsDir);
     static CameraConstantsStore *getInstance(void);
     CameraConst *get(const char make[], const char model[]);
 };
diff --git a/rtengine/color.h b/rtengine/color.h
index 211615de1..8e0015c42 100644
--- a/rtengine/color.h
+++ b/rtengine/color.h
@@ -20,7 +20,6 @@
 #pragma once
 
 #include <array>
-#include <glibmm/ustring.h>
 
 #include "rt_math.h"
 #include "LUT.h"
@@ -30,6 +29,13 @@
 
 #define SAT(a,b,c) ((float)max(a,b,c)-(float)min(a,b,c))/(float)max(a,b,c)
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtengine
 {
 
diff --git a/rtengine/curves.cc b/rtengine/curves.cc
index b2105a091..35a38d3b8 100644
--- a/rtengine/curves.cc
+++ b/rtengine/curves.cc
@@ -21,8 +21,7 @@
 #include <memory>
 #include <cmath>
 #include <cstring>
-#include <glib.h>
-#include <glib/gstdio.h>
+#include <glibmm/ustring.h>
 
 #include "rt_math.h"
 
@@ -2186,7 +2185,7 @@ void PerceptualToneCurve::init()
     }
 }
 
-void PerceptualToneCurve::initApplyState(PerceptualToneCurveState & state, Glib::ustring workingSpace) const
+void PerceptualToneCurve::initApplyState(PerceptualToneCurveState & state, const Glib::ustring &workingSpace) const
 {
 
     // Get the curve's contrast value, and convert to a chroma scaling
diff --git a/rtengine/curves.h b/rtengine/curves.h
index bc8193b76..27f5a8adc 100644
--- a/rtengine/curves.h
+++ b/rtengine/curves.h
@@ -22,12 +22,9 @@
 #include <string>
 #include <vector>
 
-#include <glibmm/ustring.h>
-
 #include "rt_math.h"
 #include "flatcurvetypes.h"
 #include "diagonalcurvetypes.h"
-#include "pipettebuffer.h"
 #include "noncopyable.h"
 #include "LUT.h"
 #include "sleef.h"
@@ -37,6 +34,13 @@
 
 #define CLIPI(a) ((a)>0?((a)<65534?(a):65534):0)
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 using namespace std;
 
 namespace rtengine
@@ -940,7 +944,7 @@ private:
     float calculateToneCurveContrastValue() const;
 public:
     static void init();
-    void initApplyState(PerceptualToneCurveState & state, Glib::ustring workingSpace) const;
+    void initApplyState(PerceptualToneCurveState & state, const Glib::ustring& workingSpace) const;
     void BatchApply(const size_t start, const size_t end, float *r, float *g, float *b, const PerceptualToneCurveState &state) const;
 };
 
diff --git a/rtengine/diagonalcurves.cc b/rtengine/diagonalcurves.cc
index bb20b7cc1..e81f2fe92 100644
--- a/rtengine/diagonalcurves.cc
+++ b/rtengine/diagonalcurves.cc
@@ -16,8 +16,6 @@
  *  You should have received a copy of the GNU General Public License
  *  along with RawTherapee.  If not, see <https://www.gnu.org/licenses/>.
  */
-#include <glib.h>
-#include <glib/gstdio.h>
 #include "curves.h"
 #include <cmath>
 #include <vector>
diff --git a/rtengine/iimage.h b/rtengine/iimage.h
index 7309dd91f..c2e9dfd3b 100644
--- a/rtengine/iimage.h
+++ b/rtengine/iimage.h
@@ -20,7 +20,6 @@
 
 #include <vector>
 
-#include <glibmm/ustring.h>
 #include <lcms2.h>
 
 #include "alignedbuffer.h"
@@ -41,6 +40,13 @@
 
 #define CHECK_BOUNDS 0
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtengine
 {
 
diff --git a/rtengine/imagedata.h b/rtengine/imagedata.h
index ff8ed4b86..5765d9aec 100644
--- a/rtengine/imagedata.h
+++ b/rtengine/imagedata.h
@@ -23,12 +23,18 @@
 #include <string>
 #include <vector>
 
-#include <glibmm/ustring.h>
 
 #include <libiptcdata/iptc-data.h>
 
 #include "imageio.h"
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtexif
 {
 
diff --git a/rtengine/improcfun.h b/rtengine/improcfun.h
index 8f4ae7771..6538bd6b1 100644
--- a/rtengine/improcfun.h
+++ b/rtengine/improcfun.h
@@ -19,11 +19,17 @@
 #pragma once
 
 #include <memory>
+#include <vector>
 
 #include "coord2d.h"
 #include "gamutwarning.h"
-#include "pipettebuffer.h"
 
+namespace Glib
+{
+
+class ustring;
+
+}
 template<typename T>
 class LUT;
 
@@ -44,6 +50,7 @@ class FramesMetaData;
 class LensCorrection;
 class NoiseCurve;
 class OpacityCurve;
+class PipetteBuffer;
 class ToneCurve;
 class WavCurve;
 class WavOpacityCurveBY;
diff --git a/rtengine/previewimage.h b/rtengine/previewimage.h
index e6c3ea070..2143509a3 100644
--- a/rtengine/previewimage.h
+++ b/rtengine/previewimage.h
@@ -18,10 +18,16 @@
  */
 #pragma once
 
-#include <glibmm/ustring.h>
 
 #include <cairomm/cairomm.h>
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtengine
 {
 
diff --git a/rtengine/rtthumbnail.h b/rtengine/rtthumbnail.h
index dcc9596f6..c8d657a62 100644
--- a/rtengine/rtthumbnail.h
+++ b/rtengine/rtthumbnail.h
@@ -18,8 +18,6 @@
  */
 #pragma once
 
-#include <glibmm/ustring.h>
-
 #include <lcms2.h>
 
 #include "image16.h"
@@ -30,6 +28,13 @@
 
 #include "../rtgui/threadutils.h"
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtengine
 {
 
diff --git a/rtgui/previewloader.h b/rtgui/previewloader.h
index 9a74ee2eb..a8032fcaf 100644
--- a/rtgui/previewloader.h
+++ b/rtgui/previewloader.h
@@ -20,10 +20,15 @@
 
 #include <set>
 
-#include <glibmm/ustring.h>
-
 #include "../rtengine/noncopyable.h"
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 class FileBrowserEntry;
 
 class PreviewLoaderListener