Adopt CSS-style drop shadow mask

This commit generates shadow_gaussian lookup table during the build, so
the renderer stops depending on runtime math. It replaces the stack-blur
border writes with a lookup-table renderer that matches CSS behavior and
stays efficient.

For performance considerations, it caches the vertical and bottom falloff
tables and rely on memset() or pointer writes to trim per-frame work. It
exposes a small configurable fade tail and update defaults plus the API,
so it aligns with CSS box-shadow semantics.

Author: jserv

.gitignore

@@ -9,6 +9,7 @@ mado-perf
 font-edit
 .font-edit
 src/composite-decls.h
+src/shadow-gaussian-lut.h
 
 # Swap
 [._]*.s[a-v][a-z]

Makefile

@@ -83,6 +83,10 @@ src/composite-decls.h: scripts/gen-composite-decls.py
 	@echo "  GEN        $@"
 	@$< > $@
 
+src/shadow-gaussian-lut.h: scripts/gen-shadow-lut.py
+	@echo "  GEN        $@"
+	@./scripts/gen-shadow-lut.py $@
+
 # Optional features
 
 libtwin.a_files-$(CONFIG_LOGGING) += src/log.c
@@ -277,6 +281,9 @@ ifeq ($(filter config defconfig clean,$(MAKECMDGOALS)),)
     ifeq ($(wildcard src/composite-decls.h),)
         $(shell scripts/gen-composite-decls.py > src/composite-decls.h)
     endif
+    ifeq ($(wildcard src/shadow-gaussian-lut.h),)
+        $(shell ./scripts/gen-shadow-lut.py src/shadow-gaussian-lut.h)
+    endif
 endif
 
 # Only skip build rules when running ONLY config/defconfig (no other targets)

configs/Kconfig

@@ -191,32 +191,45 @@ config DROP_SHADOW
 
 config HORIZONTAL_OFFSET
     int "Horizontal offset"
-    default 1
+    default 6
     range 1 10
     depends on DROP_SHADOW
     help
       Horizontal shadow offset in pixels.
+      Default matches a CSS-style soft shadow (≈6 px offset).
       Larger values push shadow further right.
 
 config VERTICAL_OFFSET
     int "Vertical offset"
-    default 1
+    default 6
     range 1 10
     depends on DROP_SHADOW
     help
       Vertical shadow offset in pixels.
+      Default matches a CSS-style soft shadow (≈6 px offset).
       Larger values push shadow further down.
 
 config SHADOW_BLUR
     int "Shadow blur radius"
-    default 10
+    default 12
     range 1 15
     depends on DROP_SHADOW
     help
       Shadow blur kernel size (radius).
+      Default 12 approximates common CSS box-shadow softness.
       Larger values create softer, more diffuse shadows.
       Higher values increase rendering cost.
 
+config SHADOW_FADE_TAIL
+    int "Shadow fade tail"
+    default 2
+    range 0 3
+    depends on DROP_SHADOW
+    help
+      Additional zero-alpha pixels appended to the shadow mask.
+      Default adds ~2 px of clear space to match CSS feathering.
+      Higher values slightly enlarge the off-screen buffer usage.
+
 endmenu
 
 menu "Image Loaders"

scripts/gen-shadow-lut.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+"""
+Generate precomputed Gaussian-style weights for the drop shadow LUT.
+
+The curve approximates (1 - t^2)^2 * 0.92 + 0.08 sampled at 17 points
+between 0 and 1 (inclusive). Values are emitted in 16.16 fixed-point so
+the renderer can remain purely fixed-point.
+"""
+
+import math
+import pathlib
+import sys
+
+
+def compute_weights(samples: int = 16) -> list[int]:
+    weights = []
+    for idx in range(samples + 1):
+        t = idx / samples
+        value = ((1.0 - t * t) ** 2) * 0.92 + 0.08
+        fixed = int(round(value * 65536))  # 16.16 fixed-point
+        fixed = max(0, min(fixed, 0xFFFFFFFF))
+        weights.append(fixed)
+    return weights
+
+
+HEADER_TEMPLATE = """\
+/*
+ * This file is auto-generated by scripts/gen-shadow-lut.py.
+ * Do not edit manually.
+ */
+#ifndef SHADOW_GAUSSIAN_LUT_H
+#define SHADOW_GAUSSIAN_LUT_H
+
+#include "twin.h"
+
+static const twin_fixed_t shadow_gaussian_lut[{count}] = {{
+{body}
+}};
+
+#endif /* SHADOW_GAUSSIAN_LUT_H */
+"""
+
+
+def emit_header(path: pathlib.Path) -> None:
+    weights = compute_weights()
+    lines = []
+    for idx, value in enumerate(weights):
+        suffix = "," if idx < len(weights) - 1 else ""
+        lines.append(f"    0x{value:05x}{suffix}")
+    header = HEADER_TEMPLATE.format(count=len(weights), body="\n".join(lines))
+    path.write_text(header, encoding="utf-8")
+
+
+def main(argv: list[str]) -> int:
+    if len(argv) != 2:
+        sys.stderr.write("Usage: gen-shadow-lut.py <output>\n")
+        return 1
+    output_path = pathlib.Path(argv[1])
+    emit_header(output_path)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main(sys.argv))