diff --git a/rtengine/iimage.h b/rtengine/iimage.h
index f5f0d075e..2def35e06 100644
--- a/rtengine/iimage.h
+++ b/rtengine/iimage.h
@@ -82,22 +82,18 @@ public:
     // Read the raw dump of the data
     void readData  (FILE *fh) {}
     // Write a raw dump of the data
-    void writeData (FILE *fh) {}
+    void writeData (FILE *fh) const {}
 
     virtual void normalizeInt (int srcMinVal, int srcMaxVal) {};
     virtual void normalizeFloat (float srcMinVal, float srcMaxVal) {};
-    virtual void computeHistogramAutoWB (double &avg_r, double &avg_g, double &avg_b, int &n, LUTu &histogram, int compression) {}
+    virtual void computeHistogramAutoWB (double &avg_r, double &avg_g, double &avg_b, int &n, LUTu &histogram, int compression) const {}
     virtual void getSpotWBData (double &reds, double &greens, double &blues, int &rn, int &gn, int &bn,
                                 std::vector<Coord2D> &red, std::vector<Coord2D> &green, std::vector<Coord2D> &blue,
-                                int tran) {}
-    virtual void getAutoWBMultipliers (double &rm, double &gm, double &bm)
+                                int tran) const {}
+    virtual void getAutoWBMultipliers (double &rm, double &gm, double &bm) const
     {
         rm = gm = bm = 1.0;
     }
-    virtual const char* getType () const
-    {
-        return "unknown";
-    }
 
 };
 
@@ -232,7 +228,7 @@ public:
     }
 
     // Send back the row stride. WARNING: unit = byte, not element!
-    int getRowStride ()
+    int getRowStride () const
     {
         return rowstride;
     }
@@ -316,7 +312,7 @@ public:
     }
 
     /** Copy the data to another PlanarWhateverData */
-    void copyData(PlanarWhateverData<T> *dest)
+    void copyData(PlanarWhateverData<T> *dest) const
     {
         assert (dest != NULL);
         // Make sure that the size is the same, reallocate if necessary
@@ -389,7 +385,7 @@ public:
     }
 
     template <class IC>
-    void resizeImgTo (int nw, int nh, TypeInterpolation interp, PlanarWhateverData<IC> *imgPtr)
+    void resizeImgTo (int nw, int nh, TypeInterpolation interp, PlanarWhateverData<IC> *imgPtr) const
     {
         //printf("resizeImgTo: resizing %s image data (%d x %d) to %s (%d x %d)\n", getType(), width, height, imgPtr->getType(), imgPtr->width, imgPtr->height);
         if (width == nw && height == nh) {
@@ -499,17 +495,7 @@ public:
 #endif
     }
 
-    void calcHist(unsigned int *hist16)
-    {
-        for (int row = 0; row < height; row++)
-            for (int col = 0; col < width; col++) {
-                unsigned short idx;
-                convertTo(v(row, col), idx);
-                hist16[idx]++;
-            }
-    }
-
-    void transformPixel (int x, int y, int tran, int& tx, int& ty)
+    void transformPixel (int x, int y, int tran, int& tx, int& ty) const
     {
 
         if (!tran) {
@@ -552,7 +538,7 @@ public:
         }
     }
 
-    void getPipetteData (T &value, int posX, int posY, int squareSize, int tran)
+    void getPipetteData (T &value, int posX, int posY, int squareSize, int tran) const
     {
         int x;
         int y;
@@ -573,14 +559,14 @@ public:
         value = n ? T(accumulator / float(n)) : T(0);
     }
 
-    void readData   (FILE *f)
+    void readData (FILE *f)
     {
         for (int i = 0; i < height; i++) {
             fread (v(i), sizeof(T), width, f);
         }
     }
 
-    void writeData  (FILE *f)
+    void writeData (FILE *f) const
     {
         for (int i = 0; i < height; i++) {
             fwrite (v(i), sizeof(T), width, f);
@@ -622,12 +608,12 @@ public:
     }
 
     // Send back the row stride. WARNING: unit = byte, not element!
-    int getRowStride ()
+    int getRowStride () const
     {
         return rowstride;
     }
     // Send back the plane stride. WARNING: unit = byte, not element!
-    int getPlaneStride ()
+    int getPlaneStride () const
     {
         return planestride;
     }
@@ -732,7 +718,7 @@ public:
     }
 
     /** Copy the data to another PlanarRGBData */
-    void copyData(PlanarRGBData<T> *dest)
+    void copyData(PlanarRGBData<T> *dest) const
     {
         assert (dest != nullptr);
         // Make sure that the size is the same, reallocate if necessary
@@ -820,7 +806,7 @@ public:
     }
 
     template <class IC>
-    void resizeImgTo (int nw, int nh, TypeInterpolation interp, IC *imgPtr)
+    void resizeImgTo (int nw, int nh, TypeInterpolation interp, IC *imgPtr) const
     {
         //printf("resizeImgTo: resizing %s image data (%d x %d) to %s (%d x %d)\n", getType(), width, height, imgPtr->getType(), imgPtr->width, imgPtr->height);
         if (width == nw && height == nh) {
@@ -869,9 +855,9 @@ public:
             // This case should never occur!
             for (int i = 0; i < nh; i++) {
                 for (int j = 0; j < nw; j++) {
-                    r(i, j) = 0;
-                    g(i, j) = 0;
-                    b(i, j) = 0;
+                    imgPtr->r(i, j) = 0;
+                    imgPtr->g(i, j) = 0;
+                    imgPtr->b(i, j) = 0;
                 }
             }
         }
@@ -942,7 +928,7 @@ public:
 #endif
     }
 
-    void calcGrayscaleHist(unsigned int *hist16)
+    void calcGrayscaleHist(unsigned int *hist16) const
     {
         for (int row = 0; row < height; row++)
             for (int col = 0; col < width; col++) {
@@ -956,7 +942,7 @@ public:
             }
     }
 
-    void computeAutoHistogram (LUTu & histogram, int& histcompr)
+    void computeAutoHistogram (LUTu & histogram, int& histcompr) const
     {
         histcompr = 3;
 
@@ -975,7 +961,7 @@ public:
             }
     }
 
-    void computeHistogramAutoWB (double &avg_r, double &avg_g, double &avg_b, int &n, LUTu &histogram, const int compression)
+    void computeHistogramAutoWB (double &avg_r, double &avg_g, double &avg_b, int &n, LUTu &histogram, const int compression) const
     {
         histogram.clear();
         avg_r = avg_g = avg_b = 0.;
@@ -1007,7 +993,7 @@ public:
             }
     }
 
-    void getAutoWBMultipliers (double &rm, double &gm, double &bm)
+    void getAutoWBMultipliers (double &rm, double &gm, double &bm) const
     {
 
         double avg_r = 0.;
@@ -1038,7 +1024,7 @@ public:
         bm = avg_b / double(n);
     }
 
-    void transformPixel (int x, int y, int tran, int& tx, int& ty)
+    void transformPixel (int x, int y, int tran, int& tx, int& ty) const
     {
 
         if (!tran) {
@@ -1083,7 +1069,7 @@ public:
 
     virtual void getSpotWBData (double &reds, double &greens, double &blues, int &rn, int &gn, int &bn,
                                 std::vector<Coord2D> &red, std::vector<Coord2D> &green, std::vector<Coord2D> &blue,
-                                int tran)
+                                int tran) const
     {
         int x;
         int y;
@@ -1120,7 +1106,7 @@ public:
         }
     }
 
-    void getPipetteData (T &valueR, T &valueG, T &valueB, int posX, int posY, int squareSize, int tran)
+    void getPipetteData (T &valueR, T &valueG, T &valueB, int posX, int posY, int squareSize, int tran) const
     {
         int x;
         int y;
@@ -1147,7 +1133,7 @@ public:
         valueB = n ? T(accumulatorB / float(n)) : T(0);
     }
 
-    void readData   (FILE *f)
+    void readData (FILE *f)
     {
         for (int i = 0; i < height; i++) {
             fread (r(i), sizeof(T), width, f);
@@ -1162,7 +1148,7 @@ public:
         }
     }
 
-    void writeData  (FILE *f)
+    void writeData (FILE *f) const
     {
         for (int i = 0; i < height; i++) {
             fwrite (r(i), sizeof(T), width, f);
@@ -1345,7 +1331,7 @@ public:
     }
 
     /** Copy the data to another ChunkyRGBData */
-    void copyData(ChunkyRGBData<T> *dest)
+    void copyData(ChunkyRGBData<T> *dest) const
     {
         assert (dest != nullptr);
         // Make sure that the size is the same, reallocate if necessary
@@ -1421,7 +1407,7 @@ public:
     }
 
     template <class IC>
-    void resizeImgTo (int nw, int nh, TypeInterpolation interp, IC *imgPtr)
+    void resizeImgTo (int nw, int nh, TypeInterpolation interp, IC *imgPtr) const
     {
         //printf("resizeImgTo: resizing %s image data (%d x %d) to %s (%d x %d)\n", getType(), width, height, imgPtr->getType(), imgPtr->width, imgPtr->height);
         if (width == nw && height == nh) {
@@ -1485,9 +1471,9 @@ public:
             // This case should never occur!
             for (int i = 0; i < nh; i++) {
                 for (int j = 0; j < nw; j++) {
-                    r(i, j) = 0;
-                    g(i, j) = 0;
-                    b(i, j) = 0;
+                    imgPtr->r(i, j) = 0;
+                    imgPtr->g(i, j) = 0;
+                    imgPtr->b(i, j) = 0;
                 }
             }
         }
@@ -1545,7 +1531,7 @@ public:
         }
     }
 
-    void calcGrayscaleHist(unsigned int *hist16)
+    void calcGrayscaleHist(unsigned int *hist16) const
     {
         for (int row = 0; row < height; row++)
             for (int col = 0; col < width; col++) {
@@ -1559,7 +1545,7 @@ public:
             }
     }
 
-    void computeAutoHistogram (LUTu & histogram, int& histcompr)
+    void computeAutoHistogram (LUTu & histogram, int& histcompr) const
     {
         histcompr = 3;
 
@@ -1578,7 +1564,7 @@ public:
             }
     }
 
-    void computeHistogramAutoWB (double &avg_r, double &avg_g, double &avg_b, int &n, LUTu &histogram, const int compression)
+    void computeHistogramAutoWB (double &avg_r, double &avg_g, double &avg_b, int &n, LUTu &histogram, const int compression) const
     {
         histogram.clear();
         avg_r = avg_g = avg_b = 0.;
@@ -1610,7 +1596,7 @@ public:
             }
     }
 
-    void getAutoWBMultipliers (double &rm, double &gm, double &bm)
+    void getAutoWBMultipliers (double &rm, double &gm, double &bm) const
     {
 
         double avg_r = 0.;
@@ -1641,7 +1627,7 @@ public:
         bm = avg_b / double(n);
     }
 
-    void transformPixel (int x, int y, int tran, int& tx, int& ty)
+    void transformPixel (int x, int y, int tran, int& tx, int& ty) const
     {
 
         if (!tran) {
@@ -1686,7 +1672,7 @@ public:
 
     virtual void getSpotWBData (double &reds, double &greens, double &blues, int &rn, int &gn, int &bn,
                                 std::vector<Coord2D> &red, std::vector<Coord2D> &green, std::vector<Coord2D> &blue,
-                                int tran)
+                                int tran) const
     {
         int x;
         int y;
@@ -1723,14 +1709,14 @@ public:
         }
     }
 
-    void readData   (FILE *f)
+    void readData (FILE *f)
     {
         for (int i = 0; i < height; i++) {
             fread (r(i), sizeof(T), 3 * width, f);
         }
     }
 
-    void writeData  (FILE *f)
+    void writeData (FILE *f) const
     {
         for (int i = 0; i < height; i++) {
             fwrite (r(i), sizeof(T), 3 * width, f);
@@ -1758,24 +1744,24 @@ public:
     /** @brief Saves the image to file. It autodetects the format (jpg, tif, png are supported).
       * @param fname is the name of the file
         @return the error code, 0 if none */
-    virtual int saveToFile (Glib::ustring fname) = 0;
+    virtual int saveToFile (const Glib::ustring &fname) const = 0;
     /** @brief Saves the image to file in a png format.
       * @param fname is the name of the file
       * @param compression is the amount of compression (0-6), -1 corresponds to the default
       * @param bps can be 8 or 16 depending on the bits per pixels the output file will have
         @return the error code, 0 if none */
-    virtual int saveAsPNG  (Glib::ustring fname, int bps = -1) = 0;
+    virtual int saveAsPNG (const Glib::ustring &fname, int bps = -1) const = 0;
     /** @brief Saves the image to file in a jpg format.
       * @param fname is the name of the file
       * @param quality is the quality of the jpeg (0...100), set it to -1 to use default
         @return the error code, 0 if none */
-    virtual int saveAsJPEG (Glib::ustring fname, int quality = 100, int subSamp = 3 ) = 0;
+    virtual int saveAsJPEG (const Glib::ustring &fname, int quality = 100, int subSamp = 3 ) const = 0;
     /** @brief Saves the image to file in a tif format.
       * @param fname is the name of the file
       * @param bps can be 8 or 16 depending on the bits per pixels the output file will have
       * @param isFloat is true for saving float images. Will be ignored by file format not supporting float data
         @return the error code, 0 if none */
-    virtual int saveAsTIFF (Glib::ustring fname, int bps = -1, bool isFloat = false, bool uncompressed = false) = 0;
+    virtual int saveAsTIFF (const Glib::ustring &fname, int bps = -1, bool isFloat = false, bool uncompressed = false) const = 0;
     /** @brief Sets the progress listener if you want to follow the progress of the image saving operations (optional).
       * @param pl is the pointer to the class implementing the ProgressListener interface */
     virtual void setSaveProgressListener (ProgressListener* pl) = 0;
diff --git a/rtengine/image16.cc b/rtengine/image16.cc
index 42a2df028..618d31641 100644
--- a/rtengine/image16.cc
+++ b/rtengine/image16.cc
@@ -124,7 +124,7 @@ void Image16::setScanline(int row, unsigned char* buffer, int bps, unsigned int
      */
 }
 
-Image16* Image16::copy()
+Image16* Image16::copy() const
 {
 
     Image16* cp = new Image16(width, height);
@@ -295,8 +295,7 @@ void Image16::getStdImage(const ColorTemp &ctemp, int tran, Imagefloat* image, P
 #undef GCLIP
 }
 
-Image8*
-Image16::to8()
+Image8* Image16::to8() const
 {
     Image8* img8 = new Image8(width, height);
 
@@ -311,8 +310,7 @@ Image16::to8()
     return img8;
 }
 
-Imagefloat*
-Image16::tofloat()
+Imagefloat* Image16::tofloat() const
 {
     Imagefloat* imgfloat = new Imagefloat(width, height);
 
diff --git a/rtengine/image16.h b/rtengine/image16.h
index 8e9f686c9..973b09504 100644
--- a/rtengine/image16.h
+++ b/rtengine/image16.h
@@ -40,10 +40,10 @@ public:
     Image16(int width, int height);
     ~Image16();
 
-    Image16* copy();
+    Image16* copy() const;
 
-    Image8*  to8();
-    Imagefloat* tofloat();
+    Image8* to8() const;
+    Imagefloat* tofloat() const;
 
     virtual void getStdImage(const ColorTemp &ctemp, int tran, Imagefloat* image, PreviewProps pp) const;
 
@@ -74,22 +74,22 @@ public:
         return 8 * sizeof(unsigned short);
     }
 
-    virtual int saveToFile(Glib::ustring fname)
+    virtual int saveToFile(const Glib::ustring &fname) const
     {
         return save(fname);
     }
 
-    virtual int saveAsPNG(Glib::ustring fname, int bps = -1)
+    virtual int saveAsPNG(const Glib::ustring &fname, int bps = -1) const
     {
         return savePNG(fname, bps);
     }
 
-    virtual int saveAsJPEG(Glib::ustring fname, int quality = 100, int subSamp = 3)
+    virtual int saveAsJPEG(const Glib::ustring &fname, int quality = 100, int subSamp = 3) const
     {
         return saveJPEG(fname, quality, subSamp);
     }
 
-    virtual int saveAsTIFF(Glib::ustring fname, int bps = -1, bool isFloat = false, bool uncompressed = false)
+    virtual int saveAsTIFF(const Glib::ustring &fname, int bps = -1, bool isFloat = false, bool uncompressed = false) const
     {
         return saveTIFF(fname, bps, isFloat, uncompressed);
     }
diff --git a/rtengine/image8.cc b/rtengine/image8.cc
index 64fe59ecd..edced2fe5 100644
--- a/rtengine/image8.cc
+++ b/rtengine/image8.cc
@@ -89,7 +89,7 @@ void Image8::setScanline (int row, unsigned char* buffer, int bps, unsigned int
     }
 }
 
-Image8* Image8::copy ()
+Image8* Image8::copy () const
 {
 
     Image8* cp = new Image8 (width, height);
diff --git a/rtengine/image8.h b/rtengine/image8.h
index 521605009..9460a4fbb 100644
--- a/rtengine/image8.h
+++ b/rtengine/image8.h
@@ -38,7 +38,7 @@ public:
     Image8 (int width, int height);
     ~Image8 ();
 
-    Image8*              copy ();
+    Image8* copy () const;
 
     virtual void getStdImage (const ColorTemp &ctemp, int tran, Imagefloat* image, PreviewProps pp) const;
 
@@ -69,22 +69,22 @@ public:
         return 8 * sizeof(unsigned char);
     }
 
-    virtual int saveToFile (Glib::ustring fname)
+    virtual int saveToFile (const Glib::ustring &fname) const
     {
         return save (fname);
     }
 
-    virtual int saveAsPNG  (Glib::ustring fname, int bps = -1)
+    virtual int saveAsPNG (const Glib::ustring &fname, int bps = -1) const
     {
         return savePNG (fname, bps);
     }
 
-    virtual int saveAsJPEG (Glib::ustring fname, int quality = 100, int subSamp = 3)
+    virtual int saveAsJPEG (const Glib::ustring &fname, int quality = 100, int subSamp = 3) const
     {
         return saveJPEG (fname, quality, subSamp);
     }
 
-    virtual int saveAsTIFF (Glib::ustring fname, int bps = -1, bool isFloat = false, bool uncompressed = false)
+    virtual int saveAsTIFF (const Glib::ustring &fname, int bps = -1, bool isFloat = false, bool uncompressed = false) const
     {
         return saveTIFF (fname, bps, isFloat, uncompressed);
     }
diff --git a/rtengine/imagefloat.cc b/rtengine/imagefloat.cc
index baf872e44..63c521040 100644
--- a/rtengine/imagefloat.cc
+++ b/rtengine/imagefloat.cc
@@ -159,7 +159,7 @@ void Imagefloat::getScanline (int row, unsigned char* buffer, int bps, bool isFl
     }
 }
 
-Imagefloat* Imagefloat::copy ()
+Imagefloat* Imagefloat::copy () const
 {
 
     Imagefloat* cp = new Imagefloat (width, height);
@@ -330,7 +330,7 @@ void Imagefloat::getStdImage (const ColorTemp &ctemp, int tran, Imagefloat* imag
 }
 
 Image8*
-Imagefloat::to8()
+Imagefloat::to8() const
 {
     Image8* img8 = new Image8(width, height);
 #ifdef _OPENMP
@@ -349,7 +349,7 @@ Imagefloat::to8()
 }
 
 Image16*
-Imagefloat::to16()
+Imagefloat::to16() const
 {
     Image16* img16 = new Image16(width, height);
 #ifdef _OPENMP
diff --git a/rtengine/imagefloat.h b/rtengine/imagefloat.h
index e163b6d4f..49dce8284 100644
--- a/rtengine/imagefloat.h
+++ b/rtengine/imagefloat.h
@@ -44,10 +44,10 @@ public:
     Imagefloat (int width, int height);
     ~Imagefloat ();
 
-    Imagefloat*          copy ();
+    Imagefloat* copy () const;
 
-    Image8*              to8();
-    Image16*             to16();
+    Image8* to8() const;
+    Image16* to16() const;
 
     virtual void         getStdImage (const ColorTemp &ctemp, int tran, Imagefloat* image, PreviewProps pp) const;
 
@@ -75,23 +75,23 @@ public:
     {
         return 8 * sizeof(float);
     }
-    virtual int          saveToFile (Glib::ustring fname)
+    virtual int saveToFile (const Glib::ustring &fname) const
     {
         return save (fname);
     }
-    virtual int          saveAsPNG  (Glib::ustring fname, int bps = -1)
+    virtual int saveAsPNG  (const Glib::ustring &fname, int bps = -1) const
     {
         return savePNG (fname, bps);
     }
-    virtual int          saveAsJPEG (Glib::ustring fname, int quality = 100, int subSamp = 3)
+    virtual int saveAsJPEG (const Glib::ustring &fname, int quality = 100, int subSamp = 3) const
     {
         return saveJPEG (fname, quality, subSamp);
     }
-    virtual int          saveAsTIFF (Glib::ustring fname, int bps = -1, bool isFloat = false, bool uncompressed = false)
+    virtual int saveAsTIFF (const Glib::ustring &fname, int bps = -1, bool isFloat = false, bool uncompressed = false) const
     {
         return saveTIFF (fname, bps, isFloat, uncompressed);
     }
-    virtual void         setSaveProgressListener (ProgressListener* pl)
+    virtual void setSaveProgressListener (ProgressListener* pl)
     {
         setProgressListener (pl);
     }
diff --git a/rtengine/imageio.h b/rtengine/imageio.h
index 09fcbab81..5970866f0 100644
--- a/rtengine/imageio.h
+++ b/rtengine/imageio.h
@@ -84,6 +84,7 @@ public:
     virtual int getBPS () const = 0;
     virtual void getScanline (int row, unsigned char* buffer, int bps, bool isFloat = false) const = 0;
     virtual void setScanline (int row, unsigned char* buffer, int bps, unsigned int numSamples = 3) = 0;
+    virtual const char* getType () const = 0;
 
     int load (const Glib::ustring &fname);
     int save (const Glib::ustring &fname) const;