Render drop shadow for active window

Add the twin_stack_blur() function to implement Mario's Stack Blur
algorithm, which blurs the target pixel map. Additionally, create a blur
window to show the effect of twin_stack_blur(). Implement the
twin_drop_shadow() function to handle the pixels within the drop shadow
area of the active window's pixel map. The drop shadow effect of the
window is only visible when the window is on the top layer, ensuring the
active window stands out visually.

Implement the twin_shadow_border() function to create a darker border of
the active window that gives a more dimensional appearance.

In the twin_drop_shadow() function, twin_stack_blur() will apply a blur
effect to the pixels beneath the active window's pixel map, which aims
to create a frosted glass appearance.

Furthermore, implement the twin_cover() function to cover the target
pixels with the desired color.

Ref: https://melatonin.dev/blog/implementing-marios-stack-blur-15-times-in-cpp/
Close sysprog21#34

Signed-off-by: Wei-Hsin Yeh <weihsinyeh168@gmail.com>

Author: weihsinyeh

apps/multi.c

@@ -7,6 +7,7 @@
 #include "apps_multi.h"
 
 #define D(x) twin_double_to_fixed(x)
+#define ASSET_PATH "assets/"
 
 static void apps_line_start(twin_screen_t *screen, int x, int y, int w, int h)
 {
@@ -272,6 +273,50 @@ static void apps_flower_start(twin_screen_t *screen, int x, int y, int w, int h)
     twin_window_show(window);
 }
 
+static void apps_blur(twin_screen_t *screen, int x, int y, int w, int h)
+{
+    twin_pixmap_t *raw_background =
+        twin_pixmap_from_file(ASSET_PATH "tux.png", TWIN_ARGB32);
+    twin_window_t *window = twin_window_create(
+        screen, TWIN_ARGB32, TwinWindowApplication, x, y, w, h);
+    twin_window_set_name(window, "Blur");
+    twin_pixmap_t *scaled_background = twin_pixmap_create(
+        TWIN_ARGB32, window->pixmap->width, window->pixmap->height);
+    twin_fixed_t sx, sy;
+    sx = twin_fixed_div(
+        twin_int_to_fixed(raw_background->width),
+        twin_int_to_fixed(window->client.right - window->client.left));
+    sy = twin_fixed_div(
+        twin_int_to_fixed(raw_background->height),
+        twin_int_to_fixed(window->client.bottom - window->client.top));
+
+    twin_matrix_scale(&raw_background->transform, sx, sy);
+    twin_operand_t srcop = {
+        .source_kind = TWIN_PIXMAP,
+        .u.pixmap = raw_background,
+    };
+
+    twin_composite(scaled_background, 0, 0, &srcop, 0, 0, 0, 0, 0, TWIN_SOURCE,
+                   screen->width, screen->height);
+
+    twin_pointer_t src, dst;
+    for (int y = window->client.top; y < window->client.bottom; y++)
+        for (int x = window->client.left; x < window->client.right; x++) {
+            src =
+                twin_pixmap_pointer(scaled_background, x - window->client.left,
+                                    y - window->client.top);
+            dst = twin_pixmap_pointer(window->pixmap, x, y);
+            *dst.argb32 = *src.argb32 | 0xff000000;
+        }
+    twin_stack_blur(window->pixmap, 5, window->client.left,
+                    window->client.right, window->client.top,
+                    window->client.bottom);
+
+    twin_pixmap_destroy(scaled_background);
+    twin_pixmap_destroy(raw_background);
+    twin_window_show(window);
+}
+
 void apps_multi_start(twin_screen_t *screen,
                       const char *name,
                       int x,
@@ -286,4 +331,5 @@ void apps_multi_start(twin_screen_t *screen,
     apps_ascii_start(screen, x += 20, y += 20, w, h);
     apps_jelly_start(screen, x += 20, y += 20, w / 2, h);
     apps_flower_start(screen, x += 20, y += 20, w, h);
+    apps_blur(screen, x += 20, y += 20, w / 2, h / 2);
 }

configs/Kconfig

@@ -55,6 +55,10 @@ config CURSOR
     default n
     depends on !BACKEND_VNC
 
+config DROP_SHADOW
+    bool "Render drop shadow for active window"
+    default y
+
 endmenu
 
 menu "Image Loaders"

include/twin.h

@@ -194,6 +194,14 @@ typedef struct _twin_pixmap {
      * Pixels
      */
     twin_animation_t *animation;
+#if defined(CONFIG_DROP_SHADOW)
+    /*
+     * When the pixel map is within the active window, it will have a drop
+     * shadow to enhance its visual distinction.
+     */
+    bool shadow;
+#endif
+
     twin_pointer_t p;
     /*
      * When representing a window, this point
@@ -422,6 +430,13 @@ typedef void (*twin_destroy_func_t)(twin_window_t *window);
 struct _twin_window {
     twin_screen_t *screen;
     twin_pixmap_t *pixmap;
+
+#if defined(CONFIG_DROP_SHADOW)
+    /* Set the shadow range for horizontal and vertical directions. */
+    twin_coord_t shadow_offset_x;
+    twin_coord_t shadow_offset_y;
+#endif
+
     twin_window_style_t style;
     twin_rect_t client;
     twin_rect_t damage;
@@ -626,6 +641,14 @@ void twin_dispatch(twin_context_t *ctx);
  * draw.c
  */
 
+/* Blur the specified area in the pixel map. */
+void twin_stack_blur(twin_pixmap_t *px,
+                     int radius,
+                     twin_coord_t left,
+                     twin_coord_t right,
+                     twin_coord_t top,
+                     twin_coord_t bottom);
+
 void twin_composite(twin_pixmap_t *dst,
                     twin_coord_t dst_x,
                     twin_coord_t dst_y,
@@ -649,6 +672,15 @@ void twin_fill(twin_pixmap_t *dst,
 
 void twin_premultiply_alpha(twin_pixmap_t *px);
 
+/*
+ * Overwrite the original pixel values of the specified area in the pixel map.
+ */
+void twin_cover(twin_pixmap_t *dst,
+                twin_argb32_t color,
+                twin_coord_t x,
+                twin_coord_t y,
+                twin_coord_t width);
+
 /*
  * event.c
  */

include/twin_private.h

@@ -187,6 +187,25 @@ typedef int64_t twin_xfixed_t;
      ((((s) << 5) & 0xfc00) | (((s) >> 1) & 0x300)) | \
      ((((s) << 8) & 0xf80000) | (((s) << 3) & 0x70000)) | 0xff000000)
 
+#ifndef min
+#define min(x, y)            \
+    ({                       \
+        typeof(x) _x = (x);  \
+        typeof(y) _y = (y);  \
+        (void) (&_x == &_y); \
+        _x < _y ? _x : _y;   \
+    })
+#endif
+#ifndef max
+#define max(x, y)            \
+    ({                       \
+        typeof(x) _x = (x);  \
+        typeof(y) _y = (y);  \
+        (void) (&_x == &_y); \
+        _x > _y ? _x : _y;   \
+    })
+#endif
+
 typedef union {
     twin_pointer_t p;
     twin_argb32_t c;
@@ -467,7 +486,7 @@ void _twin_path_sfinish(twin_path_t *path);
 #define twin_glyph_snap_y(g) (twin_glyph_snap_x(g) + twin_glyph_n_snap_x(g))
 
 /*
- * dispatch stuff
+ * Dispatch stuff
  */
 typedef struct _twin_queue {
     struct _twin_queue *next;
@@ -592,6 +611,14 @@ void _twin_button_init(twin_button_t *button,
                        twin_style_t font_style,
                        twin_dispatch_proc_t dispatch);
 
+/*
+ * Visual effect stuff
+ */
+
+#if defined(CONFIG_DROP_SHADOW)
+void twin_shadow_border(twin_pixmap_t *shadow);
+#endif
+
 /* utility */
 
 #ifdef _MSC_VER