diff --git a/src/imageformats/xcf.cpp b/src/imageformats/xcf.cpp
index d92f5e9..d727867 100644
--- a/src/imageformats/xcf.cpp
+++ b/src/imageformats/xcf.cpp
@@ -167,11 +167,11 @@ private:
 
     //! The bottom-most layer is copied into the final QImage by this
     //! routine.
-    typedef void (*PixelCopyOperation)(Layer &layer, uint i, uint j, int k, int l,
+    typedef void (*PixelCopyOperation)(const Layer &layer, uint i, uint j, int k, int l,
                                        QImage &image, int m, int n);
 
     //! Higher layers are merged into the final QImage by this routine.
-    typedef void (*PixelMergeOperation)(Layer &layer, uint i, uint j, int k, int l,
+    typedef void (*PixelMergeOperation)(const Layer &layer, uint i, uint j, int k, int l,
                                         QImage &image, int m, int n);
 
     //! Layer mode static data.
@@ -201,37 +201,37 @@ private:
                      int data_length, qint32 bpp);
 
     static void copyLayerToImage(XCFImage &xcf_image);
-    static void copyRGBToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void copyRGBToRGB(const Layer &layer, uint i, uint j, int k, int l,
                              QImage &image, int m, int n);
-    static void copyGrayToGray(Layer &layer, uint i, uint j, int k, int l,
+    static void copyGrayToGray(const Layer &layer, uint i, uint j, int k, int l,
                                QImage &image, int m, int n);
-    static void copyGrayToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void copyGrayToRGB(const Layer &layer, uint i, uint j, int k, int l,
                               QImage &image, int m, int n);
-    static void copyGrayAToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void copyGrayAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                QImage &image, int m, int n);
-    static void copyIndexedToIndexed(Layer &layer, uint i, uint j, int k, int l,
+    static void copyIndexedToIndexed(const Layer &layer, uint i, uint j, int k, int l,
                                      QImage &image, int m, int n);
-    static void copyIndexedAToIndexed(Layer &layer, uint i, uint j, int k, int l,
+    static void copyIndexedAToIndexed(const Layer &layer, uint i, uint j, int k, int l,
                                       QImage &image, int m, int n);
-    static void copyIndexedAToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void copyIndexedAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                   QImage &image, int m, int n);
 
     static void mergeLayerIntoImage(XCFImage &xcf_image);
-    static void mergeRGBToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeRGBToRGB(const Layer &layer, uint i, uint j, int k, int l,
                               QImage &image, int m, int n);
-    static void mergeGrayToGray(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeGrayToGray(const Layer &layer, uint i, uint j, int k, int l,
                                 QImage &image, int m, int n);
-    static void mergeGrayAToGray(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeGrayAToGray(const Layer &layer, uint i, uint j, int k, int l,
                                  QImage &image, int m, int n);
-    static void mergeGrayToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeGrayToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                QImage &image, int m, int n);
-    static void mergeGrayAToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeGrayAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                 QImage &image, int m, int n);
-    static void mergeIndexedToIndexed(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeIndexedToIndexed(const Layer &layer, uint i, uint j, int k, int l,
                                       QImage &image, int m, int n);
-    static void mergeIndexedAToIndexed(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeIndexedAToIndexed(const Layer &layer, uint i, uint j, int k, int l,
                                        QImage &image, int m, int n);
-    static void mergeIndexedAToRGB(Layer &layer, uint i, uint j, int k, int l,
+    static void mergeIndexedAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                    QImage &image, int m, int n);
 
     static void initializeRandomTable();
@@ -1640,7 +1640,7 @@ void XCFImageFormat::copyLayerToImage(XCFImage &xcf_image)
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::copyRGBToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::copyRGBToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                   QImage &image, int m, int n)
 {
     QRgb src = layer.image_tiles[j][i].pixel(k, l);
@@ -1671,7 +1671,7 @@ void XCFImageFormat::copyRGBToRGB(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::copyGrayToGray(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::copyGrayToGray(const Layer &layer, uint i, uint j, int k, int l,
                                     QImage &image, int m, int n)
 {
     int src = layer.image_tiles[j][i].pixelIndex(k, l);
@@ -1691,7 +1691,7 @@ void XCFImageFormat::copyGrayToGray(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::copyGrayToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::copyGrayToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                    QImage &image, int m, int n)
 {
     QRgb src = layer.image_tiles[j][i].pixel(k, l);
@@ -1712,7 +1712,7 @@ void XCFImageFormat::copyGrayToRGB(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::copyGrayAToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::copyGrayAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                     QImage &image, int m, int n)
 {
     QRgb src = layer.image_tiles[j][i].pixel(k, l);
@@ -1740,7 +1740,7 @@ void XCFImageFormat::copyGrayAToRGB(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::copyIndexedToIndexed(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::copyIndexedToIndexed(const Layer &layer, uint i, uint j, int k, int l,
         QImage &image, int m, int n)
 {
     int src = layer.image_tiles[j][i].pixelIndex(k, l);
@@ -1758,7 +1758,7 @@ void XCFImageFormat::copyIndexedToIndexed(Layer &layer, uint i, uint j, int k, i
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::copyIndexedAToIndexed(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::copyIndexedAToIndexed(const Layer &layer, uint i, uint j, int k, int l,
         QImage &image, int m, int n)
 {
     uchar src = layer.image_tiles[j][i].pixelIndex(k, l);
@@ -1793,7 +1793,7 @@ void XCFImageFormat::copyIndexedAToIndexed(Layer &layer, uint i, uint j, int k,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::copyIndexedAToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::copyIndexedAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                        QImage &image, int m, int n)
 {
     QRgb src = layer.image_tiles[j][i].pixel(k, l);
@@ -1930,7 +1930,7 @@ void XCFImageFormat::mergeLayerIntoImage(XCFImage &xcf_image)
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeRGBToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeRGBToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                    QImage &image, int m, int n)
 {
     QRgb src = layer.image_tiles[j][i].pixel(k, l);
@@ -2256,7 +2256,7 @@ void XCFImageFormat::mergeRGBToRGB(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeGrayToGray(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeGrayToGray(const Layer &layer, uint i, uint j, int k, int l,
                                      QImage &image, int m, int n)
 {
     int src = layer.image_tiles[j][i].pixelIndex(k, l);
@@ -2274,7 +2274,7 @@ void XCFImageFormat::mergeGrayToGray(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeGrayAToGray(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeGrayAToGray(const Layer &layer, uint i, uint j, int k, int l,
                                       QImage &image, int m, int n)
 {
     int src = qGray(layer.image_tiles[j][i].pixel(k, l));
@@ -2411,7 +2411,7 @@ void XCFImageFormat::mergeGrayAToGray(Layer &layer, uint i, uint j, int k, int l
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeGrayToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeGrayToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                     QImage &image, int m, int n)
 {
     QRgb src = layer.image_tiles[j][i].pixel(k, l);
@@ -2432,7 +2432,7 @@ void XCFImageFormat::mergeGrayToRGB(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeGrayAToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeGrayAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                      QImage &image, int m, int n)
 {
     int src = qGray(layer.image_tiles[j][i].pixel(k, l));
@@ -2586,7 +2586,7 @@ void XCFImageFormat::mergeGrayAToRGB(Layer &layer, uint i, uint j, int k, int l,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeIndexedToIndexed(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeIndexedToIndexed(const Layer &layer, uint i, uint j, int k, int l,
         QImage &image, int m, int n)
 {
     int src = layer.image_tiles[j][i].pixelIndex(k, l);
@@ -2604,7 +2604,7 @@ void XCFImageFormat::mergeIndexedToIndexed(Layer &layer, uint i, uint j, int k,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeIndexedAToIndexed(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeIndexedAToIndexed(const Layer &layer, uint i, uint j, int k, int l,
         QImage &image, int m, int n)
 {
     uchar src = layer.image_tiles[j][i].pixelIndex(k, l);
@@ -2636,7 +2636,7 @@ void XCFImageFormat::mergeIndexedAToIndexed(Layer &layer, uint i, uint j, int k,
  * \param m x pixel of destination image.
  * \param n y pixel of destination image.
  */
-void XCFImageFormat::mergeIndexedAToRGB(Layer &layer, uint i, uint j, int k, int l,
+void XCFImageFormat::mergeIndexedAToRGB(const Layer &layer, uint i, uint j, int k, int l,
                                         QImage &image, int m, int n)
 {
     QRgb src = layer.image_tiles[j][i].pixel(k, l);