Adopt CSS-style drop shadow mask #146

jserv · 2025-10-26T16:04:20Z

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.

Summary by cubic

Switches drop shadow rendering to a CSS-style box-shadow mask using a precomputed Gaussian LUT. Improves visual match and speeds up rendering while simplifying the shadow API.

New Features
- Generate a fixed-point Gaussian weight LUT at build time (scripts/gen-shadow-lut.py → src/shadow-gaussian-lut.h), removing runtime math.
- Replace stack-blur edge writes with a LUT-based renderer for right, bottom, and corner falloffs to match CSS behavior.
- Cache vertical and bottom falloff tables and use fast clears to cut per-frame work.
- Add SHADOW_FADE_TAIL (default 2) and update defaults: HORIZONTAL_OFFSET 6, VERTICAL_OFFSET 6, SHADOW_BLUR 12; set default shadow color to 50% black (0x80000000).
Migration
- Update calls to twin_shadow_border(shadow, color); remove shift_x and shift_y as offsets now come from Kconfig.

cubic-dev-ai

No issues found across 7 files

src/draw-common.c

weihsinyeh · 2025-10-28T08:54:58Z

src/draw-common.c

+
+    /* Clear stale shadow pixels before repainting the mask. */
+    if (shadow->window->shadow_x > 0) {
+        for (twin_coord_t y = 0; y < shadow->height; y++) {


This line can be changed to for (twin_coord_t y = 0; y < win_height; y++) {
This is because the next part, if (shadow->window->shadow_y > 0), handles the area between win_height and shadow->height.

weihsinyeh · 2025-10-28T10:04:53Z

src/draw-common.c

+            twin_pointer_t clear = twin_pixmap_pointer(shadow, 0, y);
+            if (is_argb32) {
+                memset(clear.argb32, 0,
+                       (size_t) win_width * sizeof(*clear.argb32));


(size_t) shadow->width * sizeof(*clear.argb32));
This change allows it to handle the bottom-right corner. However, I don’t observe any difference after applying it.

src/draw-common.c

weihsinyeh · 2025-10-28T10:08:53Z

src/draw-common.c

+    const twin_coord_t fade_tail = CONFIG_SHADOW_FADE_TAIL;
+    const twin_coord_t lut_x_len = x_offset + fade_tail;
+    const twin_coord_t lut_y_len = y_offset + fade_tail;
+    twin_coord_t right_extent = shadow->width - win_width;


twin_coord_t win_width = shadow->width - shadow->window->shadow_x;
Therefore, right_extent is equal to shadow->window->shadow_x.

twin_coord_t right_extent = shadow->window->shadow_x;

weihsinyeh · 2025-10-28T10:09:53Z

src/draw-common.c

+    const twin_coord_t lut_x_len = x_offset + fade_tail;
+    const twin_coord_t lut_y_len = y_offset + fade_tail;
+    twin_coord_t right_extent = shadow->width - win_width;
+    twin_coord_t bottom_extent = shadow->height - win_height;


twin_coord_t win_height = shadow->height - shadow->window->shadow_y;
Therefore, bottom_extent is equal to shadow->window->shadow_y.

weihsinyeh · 2025-10-28T10:30:10Z

src/draw-common.c

+#define SHADOW_LUT_Y_LEN (CONFIG_VERTICAL_OFFSET + CONFIG_SHADOW_FADE_TAIL)
+
+/* Fast lookup: 17-entry approximation of (1 - t^2)^2 * 0.92 + 0.08. */
+static inline twin_fixed_t shadow_gaussian_weight(twin_fixed_t t)


It seems that the shadow_gaussian_weight() function has the same effect as twin_stack_blur(). Removing the call to twin_stack_blur() still produces a gradient color effect.

weihsinyeh · 2025-10-28T12:39:25Z

src/draw-common.c

+
+                if (alpha_bottom) {
+                    for (twin_coord_t x = 0; x < bottom_width; x++) {
+                        twin_a8_t alpha = (alpha_y * alpha_bottom[x]) >> 8;


I think this should be twin_a8_t alpha = (alpha_y * alpha_bottom[0]) >> 8;.

I’m not sure why it’s written as twin_a8_t alpha = (alpha_y * alpha_bottom[x]) >> 8;. Was this done on purpose?

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.

weihsinyeh · 2025-10-29T06:18:13Z

src/draw-common.c

+    const twin_coord_t fade_tail = CONFIG_SHADOW_FADE_TAIL;
+    const twin_coord_t lut_x_len = x_offset + fade_tail;
+    const twin_coord_t lut_y_len = y_offset + fade_tail;
+    twin_coord_t right_extent = shadow->width - win_width;


twin_coord_t right_extent = shadow->window->shadow_x;

weihsinyeh · 2025-10-29T06:28:04Z

src/draw-common.c

+        twin_coord_t bottom_start = win_height;
+        if (bottom_start < 0)
+            bottom_start = 0;
+        twin_coord_t bottom_end = win_height + bottom_extent;


twin_coord_t win_height = shadow->height - shadow->window->shadow_y;
twin_coord_t bottom_extent = shadow->height - win_height;
Therefore, bottom_end == shadow->height

weihsinyeh · 2025-10-29T06:28:42Z

src/draw-common.c

+        if (bottom_start < 0)
+            bottom_start = 0;
+        twin_coord_t bottom_end = win_height + bottom_extent;
+        if (bottom_end > shadow->height)


There is no need to compare.

weihsinyeh · 2025-10-29T06:36:52Z

src/draw-common.c

+                if (bottom_width <= 0)
+                    continue;
+
+                twin_coord_t dist_y = y - win_height;


twin_coord_t dist_y = y - bottom_start; this is clearer.

weihsinyeh · 2025-10-29T06:40:33Z

Only need to keep one variable for the same function.

jserv requested review from jouae, ndsl7109256 and weihsinyeh October 26, 2025 16:04

cubic-dev-ai bot reviewed Oct 26, 2025

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weihsinyeh reviewed Oct 28, 2025

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src/draw-common.c Outdated Show resolved Hide resolved

weihsinyeh reviewed Oct 28, 2025

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jserv force-pushed the box-shadow branch from 154befd to d365816 Compare October 29, 2025 02:55

jserv requested a review from weihsinyeh October 29, 2025 02:57

weihsinyeh reviewed Oct 29, 2025

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jserv commented Oct 26, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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weihsinyeh commented Oct 29, 2025

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jserv commented Oct 26, 2025 •

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