Made the PNG loader able to load in grayscale images with an alpha channel.

dlib/image_loader/png_loader.cpp

@@ -73,6 +73,13 @@ namespace dlib
         return ( color_type_ == PNG_COLOR_TYPE_GRAY );
     }
 
+// ----------------------------------------------------------------------------------------
+
+    bool png_loader::is_graya() const
+    {
+        return ( color_type_ == PNG_COLOR_TYPE_GRAY_ALPHA );
+    }
+
 // ----------------------------------------------------------------------------------------
 
     bool png_loader::is_rgb() const
@@ -169,7 +176,8 @@ namespace dlib
 
         if (color_type_ != PNG_COLOR_TYPE_GRAY && 
             color_type_ != PNG_COLOR_TYPE_RGB && 
-            color_type_ != PNG_COLOR_TYPE_RGB_ALPHA )
+            color_type_ != PNG_COLOR_TYPE_RGB_ALPHA &&
+            color_type_ != PNG_COLOR_TYPE_GRAY_ALPHA)
         {
             fclose( fp );
             png_destroy_read_struct( &( ld_->png_ptr_ ), &( ld_->info_ptr_ ), &( ld_->end_info_ ) );

dlib/image_loader/png_loader.h

@@ -23,6 +23,7 @@ namespace dlib
         ~png_loader();
 
         bool is_gray() const;
+        bool is_graya() const;
         bool is_rgb() const;
         bool is_rgba() const;
 
@@ -41,6 +42,7 @@ namespace dlib
             COMPILE_TIME_ASSERT(sizeof(T) == 0);
 #endif
 
+            typedef typename T::type pixel_type;
             t.set_size( height_, width_ );
 
 
@@ -68,6 +70,52 @@ namespace dlib
                     }
                 }
             }
+            else if (is_graya() && bit_depth_ == 8)
+            {
+                for ( unsigned n = 0; n < height_;n++ )
+                {
+                    const unsigned char* v = get_row( n );
+                    for ( unsigned m = 0; m < width_; m++ )
+                    {
+                        unsigned char p = v[m*2];
+                        if (!pixel_traits<pixel_type>::has_alpha)
+                        {
+                            assign_pixel( t[n][m], p );
+                        }
+                        else
+                        {
+                            unsigned char pa = v[m*2+1];
+                            rgb_alpha_pixel pix;
+                            assign_pixel(pix, p);
+                            assign_pixel(pix.alpha, pa);
+                            assign_pixel(t[n][m], pix);
+                        }
+                    }
+                }
+            }
+            else if (is_graya() && bit_depth_ == 16)
+            {
+                for ( unsigned n = 0; n < height_;n++ )
+                {
+                    const uint16* v = (uint16*)get_row( n );
+                    for ( unsigned m = 0; m < width_; m++ )
+                    {
+                        dlib::uint16 p = v[m*2];
+                        if (!pixel_traits<pixel_type>::has_alpha)
+                        {
+                            assign_pixel( t[n][m], p );
+                        }
+                        else
+                        {
+                            dlib::uint16 pa = v[m*2+1];
+                            rgb_alpha_pixel pix;
+                            assign_pixel(pix, p);
+                            assign_pixel(pix.alpha, pa);
+                            assign_pixel(t[n][m], pix);
+                        }
+                    }
+                }
+            }
             else if (is_rgb() && bit_depth_ == 8)
             {
                 for ( unsigned n = 0; n < height_;n++ )

dlib/image_loader/png_loader_abstract.h

@@ -75,7 +75,17 @@ namespace dlib
         ) const;
         /*!
             ensures
-                - if (this object contains a grayscale image) then
+                - if (this object contains a grayscale image without an alpha channel) then
+                    - returns true
+                - else
+                    - returns false
+        !*/
+        
+        bool is_graya(
+        ) const;
+        /*!
+            ensures
+                - if (this object contains a grayscale image with an alpha channel) then
                     - returns true
                 - else
                     - returns false