Incorporated Changjiang Yang's patch that makes load_image() use the file

headers to decide how to load images instead of using their file extensions.
This makes load_image() work even when the files have the incorrect file
extensions.

dlib/image_loader/load_image.h

-// Copyright (C) 2011  Davis E. King (davis@dlib.net), Nils Labugt
+// Copyright (C) 2011  Davis E. King (davis@dlib.net), Nils Labugt, Changjiang Yang (yangcha@leidos.com)
 // License: Boost Software License   See LICENSE.txt for the full license.
 #ifndef DLIB_LOAd_IMAGE_Hh_
 #define DLIB_LOAd_IMAGE_Hh_
@@ -8,37 +8,72 @@
 #include "png_loader.h"
 #include "jpeg_loader.h"
 #include "image_loader.h"
+#include <fstream>
 
 namespace dlib
 {
+    namespace image_file_type
+    {
+        enum type
+        {
+            BMP,
+            JPG,
+            PNG,
+            DNG,
+            UNKNOWN
+        };
+
+        type read_type(const std::string& fileName) 
+        {
+            std::ifstream file(fileName, std::ios::in|std::ios::binary);
+            if (!file)
+                throw image_load_error("Unable to open file: " + fileName);
+
+            char buffer[9];
+            file.read((char*)buffer, 8);
+            buffer[8] = 0;
+
+            // Determine the true image type using link:
+            // http://en.wikipedia.org/wiki/List_of_file_signatures
+
+            if (strcmp(buffer, "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A") == 0) 
+                return PNG;
+            else if(buffer[0]=='\xff' && buffer[1]=='\xd8' && buffer[2]=='\xff') 
+                return JPG;
+            else if(buffer[0]=='B' && buffer[1]=='M') 
+                return BMP;
+            else if(buffer[0]=='D' && buffer[1]=='N' && buffer[2] == 'G') 
+                return DNG;
+
+            return UNKNOWN;
+        }
+    };
+
     template <typename image_type>
     void load_image (
         image_type& image,
         const std::string& file_name
     )
     {
-        const std::string extension = tolower(right_substr(file_name,"."));
-        if (extension == "bmp")
-            load_bmp(image, file_name);
+        const image_file_type::type im_type = image_file_type::read_type(file_name);
+        switch (im_type)
+        {
+            case image_file_type::BMP: load_bmp(image, file_name); return;
+            case image_file_type::DNG: load_dng(image, file_name); return;
 #ifdef DLIB_PNG_SUPPORT
-        else if (extension == "png")
-            load_png(image, file_name);
+            case image_file_type::PNG: load_png(image, file_name); return;
 #endif
 #ifdef DLIB_JPEG_SUPPORT
-        else if (extension == "jpeg" || extension == "jpg")
-            load_jpeg(image, file_name);
+            case image_file_type::JPG: load_jpeg(image, file_name); return;
 #endif
-        else if (extension == "dng")
-            load_dng(image, file_name);
-        else
-        {
-            if (extension == "jpeg" || extension == "jpg")
+        }
+
+        if (im_type == image_file_type::JPG)
             throw image_load_error("DLIB_JPEG_SUPPORT not #defined: Unable to load image in file " + file_name);
-            else if (extension == "png")
+        else if (im_type == image_file_type::PNG)
             throw image_load_error("DLIB_PNG_SUPPORT not #defined: Unable to load image in file " + file_name);
         else
-                throw image_load_error("Unknown file extension: Unable to load image in file " + file_name);
-        }
+            throw image_load_error("Unknown image file format: Unable to load image in file " + file_name);
     }
 
 }

dlib/image_loader/load_image_abstract.h

@@ -19,18 +19,10 @@ namespace dlib
             - image_type == an image object that implements the interface defined in
               dlib/image_processing/generic_image.h 
         ensures
-            - let EXT == the extension of the file given by file_name converted
-              to lower case (i.e.  the part of the file after the '.')
-            - if (EXT == "png") then
-                - performs: load_png(image, file_name);
-            - else if (EXT == "jpg" || EXT == "jpeg") then
-                - performs: load_jpeg(image, file_name);
-            - else if (EXT == "bmp") then
-                - performs: load_bmp(image, file_name);
-            - else if (EXT == "dng") then
-                - performs: load_dng(image, file_name);
-            - else
-                - throws image_load_error
+            - This function looks at the file extensions and file headers to try and figure
+              out what kind of image format is inside the given file.  It then calls one of
+              load_png(), load_jpeg(), load_bmp(), or load_dng() as appropriate and stores
+              the resulting image into #image.
         throws
             - image_load_error
                 This exception is thrown if there is some error that prevents