diff --git a/src/backend/backend_common.c b/src/backend/backend_common.c
index c6a1690..632fe2a 100644
--- a/src/backend/backend_common.c
+++ b/src/backend/backend_common.c
@@ -71,7 +71,9 @@ xcb_image_t *make_shadow(xcb_connection_t *c, const conv *kernel,
 	 */
 	xcb_image_t *ximage;
 	const double *shadow_sum = kernel->rsum;
-	int d = kernel->size, r = d / 2;
+	// We only support square kernels for shadow
+	assert(kernel->w == kernel->h);
+	int d = kernel->w, r = d / 2;
 	int swidth = width + r * 2, sheight = height + r * 2;
 
 	assert(d % 2 == 1);
diff --git a/src/kernel.c b/src/kernel.c
index 86eb4af..07d810c 100644
--- a/src/kernel.c
+++ b/src/kernel.c
@@ -12,32 +12,30 @@
 /// top left corner is at (x, y)
 double sum_kernel(const conv *map, int x, int y, int width, int height) {
 	double ret = 0;
-	/*
-	 * Compute set of filter values which are "in range"
-	 */
 
-	int xstart = x;
-	if (xstart < 0)
+	// Compute sum of values which are "in range"
+	int xstart = x, xend = width + x;
+	if (xstart < 0) {
 		xstart = 0;
-	int xend = width + x;
-	if (xend > map->size)
-		xend = map->size;
-
-	int ystart = y;
-	if (ystart < 0)
+	}
+	if (xend > map->w) {
+		xend = map->w;
+	}
+	int ystart = y, yend = height + y;
+	if (ystart < 0) {
 		ystart = 0;
-	int yend = height + y;
-	if (yend > map->size)
-		yend = map->size;
+	}
+	if (yend > map->h) {
+		yend = map->h;
+	}
+	assert(yend >= ystart && xend >= xstart);
 
-	assert(yend > 0 && xend > 0);
-
-	int d = map->size;
+	int d = map->w;
 	if (map->rsum) {
+		// See sum_kernel_preprocess
 		double v1 = xstart ? map->rsum[(yend - 1) * d + xstart - 1] : 0;
 		double v2 = ystart ? map->rsum[(ystart - 1) * d + xend - 1] : 0;
-		double v3 =
-		    (xstart && ystart) ? map->rsum[(ystart - 1) * d + xstart - 1] : 0;
+		double v3 = (xstart && ystart) ? map->rsum[(ystart - 1) * d + xstart - 1] : 0;
 		return map->rsum[(yend - 1) * d + xend - 1] - v1 - v2 + v3;
 	}
 
@@ -77,7 +75,7 @@ conv *gaussian_kernel(double r) {
 	double t;
 
 	c = cvalloc(sizeof(conv) + size * size * sizeof(double));
-	c->size = size;
+	c->w = c->h = size;
 	c->rsum = NULL;
 	t = 0.0;
 
@@ -100,22 +98,22 @@ conv *gaussian_kernel(double r) {
 
 /// preprocess kernels to make shadow generation faster
 /// shadow_sum[x*d+y] is the sum of the kernel from (0, 0) to (x, y), inclusive
-void shadow_preprocess(conv *map) {
-	const int d = map->size;
-
-	if (map->rsum)
+void sum_kernel_preprocess(conv *map) {
+	if (map->rsum) {
 		free(map->rsum);
+	}
 
-	auto sum = map->rsum = ccalloc(d * d, double);
+	auto sum = map->rsum = ccalloc(map->w * map->h, double);
 	sum[0] = map->data[0];
 
-	for (int x = 1; x < d; x++) {
+	for (int x = 1; x < map->w; x++) {
 		sum[x] = sum[x - 1] + map->data[x];
 	}
 
-	for (int y = 1; y < d; y++) {
+	const int d = map->w;
+	for (int y = 1; y < map->h; y++) {
 		sum[y * d] = sum[(y - 1) * d] + map->data[y * d];
-		for (int x = 1; x < d; x++) {
+		for (int x = 1; x < map->w; x++) {
 			double tmp = sum[(y - 1) * d + x] + sum[y * d + x - 1] -
 			             sum[(y - 1) * d + x - 1];
 			sum[y * d + x] = tmp + map->data[y * d + x];
diff --git a/src/kernel.h b/src/kernel.h
index fdb6076..daf60d6 100644
--- a/src/kernel.h
+++ b/src/kernel.h
@@ -8,7 +8,7 @@
 /// Code for generating convolution kernels
 
 typedef struct conv {
-	int size;
+	int w, h;
 	double *rsum;
 	double data[];
 } conv;
@@ -23,7 +23,7 @@ conv *gaussian_kernel(double r);
 
 /// preprocess kernels to make shadow generation faster
 /// shadow_sum[x*d+y] is the sum of the kernel from (0, 0) to (x, y), inclusive
-void shadow_preprocess(conv *map);
+void sum_kernel_preprocess(conv *map);
 
 static inline void free_conv(conv *k) {
 	free(k->rsum);
diff --git a/src/render.c b/src/render.c
index 00c33e7..edf440b 100644
--- a/src/render.c
+++ b/src/render.c
@@ -1169,7 +1169,7 @@ bool init_render(session_t *ps) {
 	}
 
 	ps->gaussian_map = gaussian_kernel(ps->o.shadow_radius);
-	shadow_preprocess(ps->gaussian_map);
+	sum_kernel_preprocess(ps->gaussian_map);
 
 	ps->black_picture = solid_picture(ps, true, 1, 0, 0, 0);
 	ps->white_picture = solid_picture(ps, true, 1, 1, 1, 1);