Render drop shadow for active window

Implement the twin_window_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.

Add the twin_stack_blur() function to implement Mario's Stack Blur
algorithm, which blurs the target pixel map. Additionally, create a blur
window in apps_blur() to show the effect of twin_stack_blur().

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

The function twin_window_drop_shadow() will first apply the
twin_shadow_border() and then apply twin_stack_blur() to blur the darker
border.

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 #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 +273,55 @@ static void apps_flower_start(twin_screen_t *screen, int x, int y, int w, int h)
     twin_window_show(window);
 }
 
+#if defined(CONFIG_LOADER_PNG)
+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);
+    if (!raw_background)
+        return;
+    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,
+                   scaled_background->width, scaled_background->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);
+    return;
+}
+#endif
+
 void apps_multi_start(twin_screen_t *screen,
                       const char *name,
                       int x,
@@ -286,4 +336,7 @@ 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);
+#if defined(CONFIG_LOADER_PNG)
+    apps_blur(screen, x += 20, y += 20, w / 2, h / 2);
+#endif
 }

configs/Kconfig

@@ -55,6 +55,28 @@ config CURSOR
     default n
     depends on !BACKEND_VNC
 
+config DROP_SHADOW
+    bool "Render drop shadow for active window"
+    default y
+
+config HORIZONTAL_OFFSET
+    int "Horizontal offset"
+    default 1
+    range 1 10
+    depends on DROP_SHADOW
+
+config VERTICAL_OFFSET
+    int "Vertical offset"
+    default 1
+    range 1 10
+    depends on DROP_SHADOW
+
+config SHADOW_BLUR
+    int "Shadow blur radius"
+    default 10
+    range 1 10
+    depends on DROP_SHADOW
+
 endmenu
 
 menu "Image Loaders"

include/twin.h

@@ -195,6 +195,15 @@ 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
@@ -423,6 +432,13 @@ 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_x;
+    twin_coord_t shadow_y;
+#endif
+
     twin_window_style_t style;
     twin_rect_t client;
     twin_rect_t damage;
@@ -652,8 +668,26 @@ void twin_fill(twin_pixmap_t *dst,
  * draw-common.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_premultiply_alpha(twin_pixmap_t *px);
 
+/*
+ * Overwrite the original pixel values for a specified number of pixels in
+ * width.
+ */
+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

@@ -181,13 +181,46 @@ typedef int64_t twin_xfixed_t;
     (((t) = twin_get_8(d, i) + twin_get_8(s, i)), (twin_argb32_t) twin_sat(t) \
                                                       << (i))
 
+#define _twin_add_ARGB(s, d, i, t) (((t) = (s) + twin_get_8(d, i)))
+#define _twin_add(s, d, t) (((t) = (s) + (d)))
+#define _twin_div(d, den, i, t)                     \
+    (((t) = (d) / (den)), (t) = twin_get_8((t), 0), \
+     (twin_argb32_t) twin_sat(t) << (i))
+#define _twin_sub_ARGB(s, d, i, t) (((t) = (s) - twin_get_8(d, i)))
+#define _twin_sub(s, d, t) (((t) = (s) - (d)))
+#define twin_put_8(d, i, t) (((t) = (d) << (i)))
+
 #define twin_argb32_to_rgb16(s) \
     ((((s) >> 3) & 0x001f) | (((s) >> 5) & 0x07e0) | (((s) >> 8) & 0xf800))
 #define twin_rgb16_to_argb32(s)                       \
     (((((s) << 3) & 0xf8) | (((s) >> 2) & 0x7)) |     \
      ((((s) << 5) & 0xfc00) | (((s) >> 1) & 0x300)) | \
      ((((s) << 8) & 0xf80000) | (((s) << 3) & 0x70000)) | 0xff000000)
 
+#if defined(__GNUC__) || defined(__clang__)
+#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
+#else /* generic implementation: potential side effects */
+#define min(x, y) ((x) < (y) ? (x) : (y))
+#define max(x, y) ((x) > (y) ? (x) : (y))
+#endif
+
 typedef union {
     twin_pointer_t p;
     twin_argb32_t c;
@@ -468,7 +501,7 @@ 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;
@@ -593,6 +626,21 @@ 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)
+/*
+ * Add a shadow with the specified color, horizontal offset, and vertical
+ * offset.
+ */
+void twin_shadow_border(twin_pixmap_t *shadow,
+                        twin_argb32_t color,
+                        twin_coord_t shift_x,
+                        twin_coord_t shift_y);
+#endif
+
 /* utility */
 
 #ifdef _MSC_VER