diff --git a/PIL/ImageOps.py b/PIL/ImageOps.py
index fec77620a..30377dc62 100644
--- a/PIL/ImageOps.py
+++ b/PIL/ImageOps.py
@@ -446,14 +446,16 @@ usm = unsharp_mask
 
 def box_blur(image, radius):
     """
-    Apply box blur to given image. Box blur is operation where
-    each pixel becomes the average value of pixels in given radius.
-    Supports float radius and very large ones. Fast implementation,
-    works in linear time relative to the radius.
+    Blur the image by setting each pixel to the average value of the pixels
+    in a square box extending radius pixels in each direction.
+    Supports float radius of arbitrary size. Uses an optimized implementation
+    which runs in linear time relative to the size of the image
+    for any radius value.
 
     :param image: The image to blur.
     :param radius: Size of the box in one direction. Radius 0 does not blur,
-    radius 1 takes 1 pixel in all directions, i.e. 9 pixels in total.
+                   returns an identical image. Radius 1 takes 1 pixel
+                   in each direction, i.e. 9 pixels in total.
     :return: An image.
     """
     image.load()
diff --git a/_imaging.c b/_imaging.c
index 75a72465d..dfb0fd46e 100644
--- a/_imaging.c
+++ b/_imaging.c
@@ -872,7 +872,7 @@ _gaussian_blur(ImagingObject* self, PyObject* args)
     if (!imOut)
         return NULL;
 
-    if ( ! ImagingGaussianBlur(imOut, imIn, radius, passes))
+    if (!ImagingGaussianBlur(imOut, imIn, radius, passes))
         return NULL;
 
     return PyImagingNew(imOut);
@@ -1797,7 +1797,7 @@ _unsharp_mask(ImagingObject* self, PyObject* args)
     if (!imOut)
         return NULL;
 
-    if ( ! ImagingUnsharpMask(imOut, imIn, radius, percent, threshold))
+    if (!ImagingUnsharpMask(imOut, imIn, radius, percent, threshold))
         return NULL;
 
     return PyImagingNew(imOut);
diff --git a/libImaging/BoxBlur.c b/libImaging/BoxBlur.c
index 64d4fd430..a85c5eadc 100644
--- a/libImaging/BoxBlur.c
+++ b/libImaging/BoxBlur.c
@@ -16,11 +16,11 @@ LineBoxBlur32(pixel *line, UINT32 *lineOut, int lastx, int radius, int edgeA,
     UINT32 acc[4];
     UINT32 bulk[4];
 
-    #define MOVE_ACC(acc, substract, add) \
-        acc[0] += line[add][0] - line[substract][0]; \
-        acc[1] += line[add][1] - line[substract][1]; \
-        acc[2] += line[add][2] - line[substract][2]; \
-        acc[3] += line[add][3] - line[substract][3];
+    #define MOVE_ACC(acc, subtract, add) \
+        acc[0] += line[add][0] - line[subtract][0]; \
+        acc[1] += line[add][1] - line[subtract][1]; \
+        acc[2] += line[add][2] - line[subtract][2]; \
+        acc[3] += line[add][3] - line[subtract][3];
 
     #define ADD_FAR(bulk, acc, left, right) \
         bulk[0] = (acc[0] * ww) + (line[left][0] + line[right][0]) * fw; \
@@ -28,9 +28,9 @@ LineBoxBlur32(pixel *line, UINT32 *lineOut, int lastx, int radius, int edgeA,
         bulk[2] = (acc[2] * ww) + (line[left][2] + line[right][2]) * fw; \
         bulk[3] = (acc[3] * ww) + (line[left][3] + line[right][3]) * fw;
 
-    #define SAVE(acc) \
-        (UINT8)((acc[0] + (1 << 23)) >> 24) << 0  | (UINT8)((acc[1] + (1 << 23)) >> 24) << 8 | \
-        (UINT8)((acc[2] + (1 << 23)) >> 24) << 16 | (UINT8)((acc[3] + (1 << 23)) >> 24) << 24
+    #define SAVE(bulk) \
+        (UINT8)((bulk[0] + (1 << 23)) >> 24) << 0  | (UINT8)((bulk[1] + (1 << 23)) >> 24) << 8 | \
+        (UINT8)((bulk[2] + (1 << 23)) >> 24) << 16 | (UINT8)((bulk[3] + (1 << 23)) >> 24) << 24
 
     /* Compute acc for -1 pixel (outside of image):
        From "-radius-1" to "-1" get first pixel,
@@ -54,21 +54,21 @@ LineBoxBlur32(pixel *line, UINT32 *lineOut, int lastx, int radius, int edgeA,
 
     if (edgeA <= edgeB)
     {
-        /* Substract pixel from left ("0").
+        /* Subtract pixel from left ("0").
            Add pixels from radius. */
         for (x = 0; x < edgeA; x++) {
             MOVE_ACC(acc, 0, x + radius);
             ADD_FAR(bulk, acc, 0, x + radius + 1);
             lineOut[x] = SAVE(bulk);
         }
-        /* Substract previous pixel from "-radius".
+        /* Subtract previous pixel from "-radius".
            Add pixels from radius. */
         for (x = edgeA; x < edgeB; x++) {
             MOVE_ACC(acc, x - radius - 1, x + radius);
             ADD_FAR(bulk, acc, x - radius - 1, x + radius + 1);
             lineOut[x] = SAVE(bulk);
         }
-        /* Substract previous pixel from "-radius".
+        /* Subtract previous pixel from "-radius".
            Add last pixel. */
         for (x = edgeB; x <= lastx; x++) {
             MOVE_ACC(acc, x - radius - 1, lastx);
@@ -109,14 +109,14 @@ LineBoxBlur8(UINT8 *line, UINT8 *lineOut, int lastx, int radius, int edgeA,
     UINT32 acc;
     UINT32 bulk;
 
-    #define MOVE_ACC(acc, substract, add) \
-        acc += line[add] - line[substract];
+    #define MOVE_ACC(acc, subtract, add) \
+        acc += line[add] - line[subtract];
 
     #define ADD_FAR(bulk, acc, left, right) \
         bulk = (acc * ww) + (line[left] + line[right]) * fw;
 
-    #define SAVE(acc) \
-        (UINT8)((acc + (1 << 23)) >> 24)
+    #define SAVE(bulk) \
+        (UINT8)((bulk + (1 << 23)) >> 24)
 
     acc = line[0] * (radius + 1);
     for (x = 0; x < edgeA - 1; x++) {
@@ -252,17 +252,17 @@ ImagingBoxBlur(Imaging imOut, Imaging imIn, float radius, int n)
     if (imIn->type != IMAGING_TYPE_UINT8)
         return ImagingError_ModeError();
 
-    if ( ! (strcmp(imIn->mode, "RGB") == 0 ||
-            strcmp(imIn->mode, "RGBA") == 0 ||
-            strcmp(imIn->mode, "RGBX") == 0 ||
-            strcmp(imIn->mode, "CMYK") == 0 ||
-            strcmp(imIn->mode, "L") == 0 ||
-            strcmp(imIn->mode, "LA") == 0))
+    if (!(strcmp(imIn->mode, "RGB") == 0 ||
+          strcmp(imIn->mode, "RGBA") == 0 ||
+          strcmp(imIn->mode, "RGBX") == 0 ||
+          strcmp(imIn->mode, "CMYK") == 0 ||
+          strcmp(imIn->mode, "L") == 0 ||
+          strcmp(imIn->mode, "LA") == 0))
         return ImagingError_ModeError();
 
     /* Create transposed temp image (imIn->ysize x imIn->xsize). */
     Imaging temp = ImagingNew(imIn->mode, imIn->ysize, imIn->xsize);
-    if ( ! temp)
+    if (!temp)
         return NULL;
 
     /* Apply one-dimensional blur.