Gave the spatially_filter_image routines the ability to add their

results to an output image rather than overwrite the contents
of the output image.

dlib/image_transforms/spatial_filtering.h

@@ -27,7 +27,8 @@ namespace dlib
         out_image_type& out_img,
         const matrix_exp<EXP>& filter,
         T scale,
-        bool use_abs = false
+        bool use_abs = false,
+        bool add_to = false
     )
     {
         COMPILE_TIME_ASSERT( pixel_traits<typename in_image_type::type>::has_alpha == false );
@@ -92,8 +93,15 @@ namespace dlib
                 }
 
                 // save this pixel to the output image
-                assign_pixel(out_img[r][c], in_img[r][c]);
-                assign_pixel_intensity(out_img[r][c], temp);
+                if (add_to == false)
+                {
+                    assign_pixel(out_img[r][c], in_img[r][c]);
+                    assign_pixel_intensity(out_img[r][c], temp);
+                }
+                else
+                {
+                    assign_pixel(out_img[r][c], temp + get_pixel_intensity(out_img[r][c]));
+                }
             }
         }
     }
@@ -127,7 +135,8 @@ namespace dlib
         const matrix_exp<EXP1>& row_filter,
         const matrix_exp<EXP2>& col_filter,
         T scale,
-        bool use_abs = false
+        bool use_abs = false,
+        bool add_to = false
     )
     {
         COMPILE_TIME_ASSERT( pixel_traits<typename in_image_type::type>::has_alpha == false );
@@ -213,8 +222,15 @@ namespace dlib
                 }
 
                 // save this pixel to the output image
-                assign_pixel(out_img[r][c], in_img[r][c]);
-                assign_pixel_intensity(out_img[r][c], temp);
+                if (add_to == false)
+                {
+                    assign_pixel(out_img[r][c], in_img[r][c]);
+                    assign_pixel_intensity(out_img[r][c], temp);
+                }
+                else
+                {
+                    assign_pixel(out_img[r][c], temp + get_pixel_intensity(out_img[r][c]));
+                }
             }
         }
     }

dlib/image_transforms/spatial_filtering_abstract.h

@@ -22,7 +22,8 @@ namespace dlib
         out_image_type& out_img,
         const matrix_exp<EXP>& filter,
         T scale = 1,
-        bool use_abs = false
+        bool use_abs = false,
+        bool add_to = false
     );
     /*!
         requires
@@ -41,14 +42,16 @@ namespace dlib
             - The intermediate filter computations will be carried out using variables of type EXP::type.
               This is whatever scalar type is used inside the filter matrix. 
             - Pixel values are stored into out_img using the assign_pixel() function and therefore
-              any applicable color space conversion or value saturation is performed.
+              any applicable color space conversion or value saturation is performed.  Note that if 
+              add_to is true then the filtered output value will be added to out_img rather than 
+              overwriting the original value.
             - if (pixel_traits<typename in_image_type::type>::grayscale == false) then
                 - the pixel values are converted to the HSI color space and the filtering
                   is done on the intensity channel only.
             - if (use_abs == true) then
                 - pixel values after filtering that are < 0 are converted to their absolute values.
             - Pixels close enough to the edge of in_img to not have the filter still fit 
-              inside the image are set to zero.
+              inside the image are always set to zero.
             - #out_img.nc() == in_img.nc()
             - #out_img.nr() == in_img.nr()
     !*/
@@ -68,7 +71,8 @@ namespace dlib
         const matrix_exp<EXP1>& row_filter,
         const matrix_exp<EXP2>& col_filter,
         T scale = 1,
-        bool use_abs = false
+        bool use_abs = false,
+        bool add_to = false
     );
     /*!
         requires
@@ -92,14 +96,16 @@ namespace dlib
             - The intermediate filter computations will be carried out using variables of type EXP1::type.
               This is whatever scalar type is used inside the row_filter matrix. 
             - Pixel values are stored into out_img using the assign_pixel() function and therefore
-              any applicable color space conversion or value saturation is performed.
+              any applicable color space conversion or value saturation is performed.  Note that if 
+              add_to is true then the filtered output value will be added to out_img rather than 
+              overwriting the original value.
             - if (pixel_traits<typename in_image_type::type>::grayscale == false) then
                 - the pixel values are converted to the HSI color space and the filtering
                   is done on the intensity channel only.
             - if (use_abs == true) then
                 - pixel values after filtering that are < 0 are converted to their absolute values
             - Pixels close enough to the edge of in_img to not have the filter still fit 
-              inside the image are set to zero.
+              inside the image are always set to zero.
             - #out_img.nc() == in_img.nc()
             - #out_img.nr() == in_img.nr()
     !*/

dlib/test/image.cpp

@@ -924,6 +924,23 @@ namespace
             DLIB_TEST(img2[2][1] == 0);
             DLIB_TEST(img2[2][2] == 0);
             DLIB_TEST(img2[2][3] == 0);
+
+            assign_all_pixels(img2, 3);
+            spatially_filter_image_separable(img,img2,rowf,colf,1,true, true);
+            DLIB_TEST(img2[0][0] == 0);
+            DLIB_TEST(img2[0][1] == 0);
+            DLIB_TEST(img2[0][2] == 0);
+            DLIB_TEST(img2[0][3] == 0);
+
+            DLIB_TEST(img2[1][0] == 0);
+            DLIB_TEST_MSG(img2[1][1] == 9+3, img2[1][1] );
+            DLIB_TEST(img2[1][2] == 9+3);
+            DLIB_TEST(img2[1][3] == 0);
+
+            DLIB_TEST(img2[2][0] == 0);
+            DLIB_TEST(img2[2][1] == 0);
+            DLIB_TEST(img2[2][2] == 0);
+            DLIB_TEST(img2[2][3] == 0);
         }
         {
             array2d<double> img, img2;
@@ -941,8 +958,36 @@ namespace
             DLIB_TEST(img2[0][3] == 0);
 
             DLIB_TEST(img2[1][0] == 0);
-            DLIB_TEST((img2[1][1] -  -4.5) < 1e-14);
-            DLIB_TEST((img2[1][2] -  -4.5) < 1e-14);
+            DLIB_TEST(std::abs(img2[1][1] -  -4.5) < 1e-14);
+            DLIB_TEST(std::abs(img2[1][2] -  -4.5) < 1e-14);
+            DLIB_TEST(img2[1][3] == 0);
+
+            DLIB_TEST(img2[2][0] == 0);
+            DLIB_TEST(img2[2][1] == 0);
+            DLIB_TEST(img2[2][2] == 0);
+            DLIB_TEST(img2[2][3] == 0);
+
+        }
+        {
+            array2d<double> img, img2;
+            img.set_size(3,4);
+            img2.set_size(3,4);
+            assign_all_pixels(img2, 8);
+
+            matrix<double> filter(3,3);
+            filter = 1;
+            assign_all_pixels(img,-1);
+
+            spatially_filter_image(img,img2,filter,2, false, true);
+
+            DLIB_TEST(img2[0][0] == 0);
+            DLIB_TEST(img2[0][1] == 0);
+            DLIB_TEST(img2[0][2] == 0);
+            DLIB_TEST(img2[0][3] == 0);
+
+            DLIB_TEST(img2[1][0] == 0);
+            DLIB_TEST(std::abs(img2[1][1] -  -4.5 - 8) < 1e-14);
+            DLIB_TEST(std::abs(img2[1][2] -  -4.5 - 8) < 1e-14);
             DLIB_TEST(img2[1][3] == 0);
 
             DLIB_TEST(img2[2][0] == 0);