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diff --git a/src/kernel.h b/src/kernel.h
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+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <[email protected]>
+
+#pragma once
+#include <stdlib.h>
+#include "compiler.h"
+
+/// Code for generating convolution kernels
+
+typedef struct conv {
+	int w, h;
+	double *rsum;
+	double data[];
+} conv;
+
+/// Calculate the sum of a rectangle part of the convolution kernel
+/// the rectangle is defined by top left (x, y), and a size (width x height)
+double attr_pure sum_kernel(const conv *map, int x, int y, int width, int height);
+double attr_pure sum_kernel_normalized(const conv *map, int x, int y, int width, int height);
+
+/// Create a kernel with gaussian distribution with standard deviation `r`, and size
+/// `size`.
+conv *gaussian_kernel(double r, int size);
+
+/// Create a gaussian kernel with auto detected standard deviation. The choosen standard
+/// deviation tries to make sure the outer most pixels of the shadow are completely
+/// transparent.
+///
+/// @param[in] shadow_radius the radius of the shadow
+conv *gaussian_kernel_autodetect_deviation(int shadow_radius);
+
+/// 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 sum_kernel_preprocess(conv *map);
+
+static inline void free_conv(conv *k) {
+	free(k->rsum);
+	free(k);
+}