diff --git a/floatimg/image.py b/floatimg/image.py
index addc10f..17ab2fe 100644
--- a/floatimg/image.py
+++ b/floatimg/image.py
@@ -6,8 +6,8 @@ from floatimg.settings import LIB
 # type constants
 GRAY = 1
 RGB = 3
-RGBA = 4
-RGBZ = 99
+RGBA = 4  # may not be used
+RGBZ = 99  # may not be used
 
 ############################################################################################################
 # Type mapping
@@ -32,7 +32,7 @@ C_FloatImg._fields_ = [
 ]
 
 ############################################################################################################
-# declaration of input / output types
+# declaration of input / output types for C binding of core functions
 c_fimgcreate = LIB.fimg_create
 c_fimgcreate.argtypes = (ct.POINTER(C_FloatImg), ct.c_int, ct.c_int, ct.c_int)
 c_fimgcreate.restype = ct.c_int
@@ -61,7 +61,6 @@ c_fimg_clear = LIB.fimg_clear
 c_fimg_clear.argtypes = (ct.POINTER(C_FloatImg),)
 c_fimg_clear.restype = ct.c_int
 
-
 c_fimg_plot_rgb = LIB.fimg_plot_rgb
 c_fimg_plot_rgb.argtypes = (
     ct.POINTER(C_FloatImg),
@@ -73,8 +72,6 @@ c_fimg_plot_rgb.argtypes = (
 )
 c_fimg_plot_rgb.restype = ct.c_int
 
-
-# int   fimg_get_rgb(FloatImg *head, int x, int y, float *rgb);
 c_fimg_get_rgb = LIB.fimg_get_rgb
 c_fimg_get_rgb.argtypes = (
     ct.POINTER(C_FloatImg),
@@ -84,7 +81,6 @@ c_fimg_get_rgb.argtypes = (
 )
 c_fimg_get_rgb.restype = ct.c_int
 
-# int   fimg_put_rgb(FloatImg *head, int x, int y, float *rgb);
 c_fimg_put_rgb = LIB.fimg_put_rgb
 c_fimg_put_rgb.argtypes = (
     ct.POINTER(C_FloatImg),
@@ -115,6 +111,17 @@ c_fimg_fileinfos = LIB.fimg_fileinfos
 c_fimg_fileinfos.argtypes = (ct.c_char_p, ct.POINTER(ct.c_int * 3))
 c_fimg_fileinfos.restype = ct.c_int
 
+c_fimg_add_rgb = LIB.fimg_add_rgb
+c_fimg_add_rgb.argtypes = (
+    ct.POINTER(C_FloatImg),
+    ct.c_int,
+    ct.c_int,
+    ct.c_float,
+    ct.c_float,
+    ct.c_float,
+)
+c_fimg_add_rgb.restype = ct.c_int
+
 ############################################################################################################
 class FloatImg:
     """
@@ -131,12 +138,6 @@ class FloatImg:
     fval = property(lambda self: self.c_img.fval)
     count = property(lambda self: self.c_img.count)
 
-    # oh yeah, really really sluggish access to pixels img.R[0].contents
-    # however, pixel data are not designed to be accessed this way
-    # R = property(
-    #     lambda self: pointer(self.c_img.contents.R)[: self.width * self.height]
-    # )
-
     #######################################################################################################
     def __init__(self, c_img):
         self.c_img = c_img
@@ -167,12 +168,8 @@ class FloatImg:
     #######################################################################################################
     def clone(self, copy_data=False):
         """return a clone of the current instance"""
-        if copy_data:
-            flags = 1
-        else:
-            flags = 0
         new_pic = C_FloatImg()
-        assert c_fimg_clone(self.c_img_p, ct.pointer(new_pic), flags) == 0
+        assert c_fimg_clone(self.c_img_p, ct.pointer(new_pic), int(copy_data)) == 0
         return FloatImg(new_pic)
 
     #######################################################################################################
@@ -192,19 +189,25 @@ class FloatImg:
     #######################################################################################################
     def get(self, x, y):
         """get r,g,b triplet from a pixel"""
-        rgb = (ct.c_float * 3)()
-        assert c_fimg_get_rgb(self.c_img_p, x, y, ct.pointer(rgb)) == 0
-        return rgb[:3]
+        color = (ct.c_float * 3)()
+        assert c_fimg_get_rgb(self.c_img_p, x, y, ct.pointer(color)) == 0
+        return color[:3]
 
     #######################################################################################################
-    def put(self, x, y, rgb):
-        """put r,g,b triplet to a pixel"""
-        # TODO may be a better way to create the array rather than iterating
+    def put(self, x, y, color):
+        """put color to a pixel"""
+        # TODO may be a better way to create the array rather than iterating that may cost too much in
+        # intensive usaage
         c_rgb = (ct.c_float * 3)()
-        for i, c in enumerate(rgb):
+        for i, c in enumerate(color):
             c_rgb[i] = c
         assert c_fimg_put_rgb(self.c_img_p, x, y, ct.pointer(c_rgb)) == 0
 
+    #######################################################################################################
+    def add(self, x, y, color):
+        """add color to a pixel """
+        assert c_fimg_add_rgb(self.c_img_p, x, y, *color) == 0
+
     #######################################################################################################
     def dump(self, fname):
         """save data to a dump file"""
diff --git a/tests/basics.py b/tests/basics.py
index 46e1357..c98869b 100644
--- a/tests/basics.py
+++ b/tests/basics.py
@@ -54,7 +54,6 @@ def test_rgb_constant():
             assert img.get(x, y) == color
 
 
-
 def test_copy_data():
     width = 5
     height = 5
@@ -104,3 +103,12 @@ def test_put_rgb():
     assert img.get(0, 0) == [0.0, 0.0, 0.0]
     img.put(0, 0, (127.0, 127.0, 127.0))
     assert img.get(0, 0) == [127.0, 127.0, 127.0]
+
+
+def test_add():
+    width, height = 5, 5
+    img = floatimg.create_rgb(width, height)
+    color = (32.0, 32.0, 32.0)
+    img.put(0, 0, color)
+    img.add(0, 0, color)
+    assert img.get(0, 0) == [64.0, 64.0, 64.0]