add support for loading of RGBA JPEG images (#409)

dlib/image_loader/jpeg_loader.cpp

@@ -60,6 +60,13 @@ namespace dlib
         return (output_components_ == 3);
     }
 
+// ----------------------------------------------------------------------------------------
+
+    bool jpeg_loader::is_rgba() const
+    {
+        return (output_components_ == 4);
+    }
+
 // ----------------------------------------------------------------------------------------
 
     struct jpeg_loader_error_mgr 
@@ -123,7 +130,8 @@ namespace dlib
         output_components_ = cinfo.output_components;
 
         if (output_components_ != 1 && 
-            output_components_ != 3)
+            output_components_ != 3 &&
+            output_components_ != 4)
         {
             fclose( fp );
             jpeg_destroy_decompress(&cinfo);

dlib/image_loader/jpeg_loader.h

@@ -23,6 +23,7 @@ namespace dlib
 
         bool is_gray() const;
         bool is_rgb() const;
+        bool is_rgba() const;
 
         template<typename T>
         void get_image( T& t_) const
@@ -37,7 +38,6 @@ namespace dlib
             COMPILE_TIME_ASSERT(sizeof(T) == 0);
 #endif
             image_view<T> t(t_);
-
             t.set_size( height_, width_ );
             for ( unsigned n = 0; n < height_;n++ )
             {
@@ -49,6 +49,14 @@ namespace dlib
                         unsigned char p = v[m];
                         assign_pixel( t[n][m], p );
                     }
+                    else if ( is_rgba() ) {
+                        rgb_alpha_pixel p;
+                        p.red = v[m*4];
+                        p.green = v[m*4+1];
+                        p.blue = v[m*4+2];
+                        p.alpha = v[m*4+3];
+                        assign_pixel( t[n][m], p );
+                    }
                     else // if ( is_rgb() )
                     {
                         rgb_pixel p;