Fixed a bug in save_dng() that happened sometimes when saving

images with unsigned char pixels.

dlib/image_saver/image_saver.h

@@ -325,7 +325,8 @@ namespace dlib
                     {
                         unsigned char cur;
                         assign_pixel(cur, image[r][c]);
-                        eem.encode(cur - predictor_grayscale(image,r,c));
+                        cur -= predictor_grayscale(image,r,c);
+                        eem.encode(cur);
                     }
                 }
                 // write out the magic byte to mark the end of the data

dlib/test/image.cpp

@@ -488,7 +488,7 @@ namespace
             {
                 for (long c = 0; c < 15; ++c)
                 {
-                    img[r][c] = static_cast<unsigned char>(r*14 + c);
+                    img[r][c] = static_cast<unsigned char>(r*14 + c*111);
                 }
             }
 
@@ -514,7 +514,7 @@ namespace
                 {
                     for (long c = 0; c < 15; ++c)
                     {
-                        DLIB_TEST(img[r][c] == r*14 + c);
+                        DLIB_TEST(img[r][c] == static_cast<unsigned char>(r*14 + c*111));
                     }
                 }
 
@@ -532,7 +532,7 @@ namespace
                 {
                     for (long c = 0; c < 15; ++c)
                     {
-                        DLIB_TEST(img[r][c] == r*14 + c);
+                        DLIB_TEST(img[r][c] == static_cast<unsigned char>(r*14 + c*111));
                     }
                 }
             }