backend: gl: use gaussian blur for shadow

dual kawase is fast but its blur size is coarse grained, which sometimes
makes the shadow ugly.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/gl_common.c

@@ -1174,11 +1174,11 @@ struct backend_shadow_context *gl_create_shadow_context(backend_t *base, double
 	auto ctx = ccalloc(1, struct gl_shadow_context);
 	ctx->radius = radius;
 
-	struct dual_kawase_blur_args args = {
+	struct gaussian_blur_args args = {
 	    .size = (int)radius,
-	    .strength = 0,
+	    .deviation = gaussian_kernel_std_for_size(radius, 0.5 / 256.0),
 	};
-	ctx->blur_context = gl_create_blur_context(base, BLUR_METHOD_DUAL_KAWASE, &args);
+	ctx->blur_context = gl_create_blur_context(base, BLUR_METHOD_GAUSSIAN, &args);
 	return (struct backend_shadow_context *)ctx;
 }
 

src/kernel.c

@@ -103,7 +103,7 @@ static inline double estimate_first_row_sum(double size, double r) {
 /// Pick a suitable gaussian kernel standard deviation for a given kernel size. The
 /// returned radius is the maximum possible radius (<= size*2) that satisfies no sum of
 /// the rows in the kernel are less than `row_limit` (up to certain precision).
-static inline double gaussian_kernel_std_for_size(double size, double row_limit) {
+double gaussian_kernel_std_for_size(double size, double row_limit) {
 	assert(size > 0);
 	if (row_limit >= 1.0 / 2.0 / size) {
 		return size * 2;

src/kernel.h

@@ -22,6 +22,9 @@ double attr_pure sum_kernel_normalized(const conv *map, int x, int y, int width,
 /// `size`.
 conv *gaussian_kernel(double r, int size);
 
+/// Estimate the best standard deviation for a give kernel size.
+double gaussian_kernel_std_for_size(double size, double row_limit);
+
 /// 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.