Defined the jpeg quality measure.

dlib/image_saver/save_jpeg.cpp

@@ -48,6 +48,11 @@ namespace dlib
             "\t save_jpeg()"
             << "\n\t You can't save an empty image as a JPEG."
             );
+        DLIB_CASSERT(0 <= quality && quality <= 100,
+            "\t save_jpeg()"
+            << "\n\t Invalid quality value."
+            << "\n\t quality: " << quality
+            );
 
         FILE* outfile = fopen(filename.c_str(), "wb");
         if (!outfile)
@@ -104,6 +109,12 @@ namespace dlib
             "\t save_jpeg()"
             << "\n\t You can't save an empty image as a JPEG."
             );
+        DLIB_CASSERT(0 <= quality && quality <= 100,
+            "\t save_jpeg()"
+            << "\n\t Invalid quality value."
+            << "\n\t quality: " << quality
+            );
+
 
         FILE* outfile = fopen(filename.c_str(), "wb");
         if (!outfile)

dlib/image_saver/save_jpeg_abstract.h

@@ -25,6 +25,7 @@ namespace dlib
             - image_type == an image object that implements the interface defined in
               dlib/image_processing/generic_image.h or a matrix expression
             - image.size() != 0
+            - 0 <= quality <= 100
         ensures
             - writes the image to the file indicated by file_name in the JPEG format.
             - image[0][0] will be in the upper left corner of the image.
@@ -34,6 +35,8 @@ namespace dlib
               save_jpeg() can only natively store rgb_pixel and uint8 pixel types.  All
               other pixel types will be converted into one of these types as appropriate
               before being saved to disk.
+            - The quality value determines how lossy the compression is.  Larger quality 
+              values result in larger output images but the images will look better.
         throws
             - image_save_error
                 This exception is thrown if there is an error that prevents us from saving