From 1961cd353b59b17d0903e56471d330ebf3851db3 Mon Sep 17 00:00:00 2001
From: Thomas <festlandtommy@mailbox.org>
Date: Sun, 1 Mar 2026 18:33:45 +0000
Subject: [PATCH] Fix on_ground flickering on one-way platform tiles
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The platform collision check in resolve_tilemap_y required vel.y > 0
(strictly falling), so once landing zeroed vel.y the check would fail
next frame. Gravity would add a tiny downward velocity, the platform
would catch it again, and the cycle repeated — causing animation
jitter and repeated landing dust particles.

Change the condition to vel.y >= 0 so platforms resolve when resting.
Extend the ground-stick anti-flicker probe to also detect TILE_PLATFORM
tiles (was TILE_SOLID only), with a proximity guard to preserve
jump-through-from-below behavior.
---
 src/engine/physics.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/src/engine/physics.c b/src/engine/physics.c
index a230706..9c69c16 100644
--- a/src/engine/physics.c
+++ b/src/engine/physics.c
@@ -76,11 +76,14 @@ static void resolve_tilemap_y(Body *body, const Tilemap *map) {
         for (int tx = left; tx <= right; tx++) {
             uint32_t flags = tilemap_flags_at(map, tx, ty);
             if (!(flags & TILE_SOLID)) {
-                /* Check one-way platforms: only block when falling down */
-                if ((flags & TILE_PLATFORM) && body->vel.y > 0) {
+                /* Check one-way platforms: block when falling or resting on top.
+                 * vel.y >= 0 prevents on_ground flickering when standing still —
+                 * gravity adds a tiny downward vel each frame, the platform
+                 * resolves it back to zero, and the cycle would repeat. */
+                if ((flags & TILE_PLATFORM) && body->vel.y >= 0) {
                     float tile_top = tile_to_world(ty);
                     float body_bottom = body->pos.y + body->size.y;
-                    /* Only resolve if we just crossed into the tile */
+                    /* Only resolve if we just crossed into the tile or are resting on it */
                     if (body_bottom - body->vel.y * DT <= tile_top + 2.0f) {
                         body->pos.y = tile_top - body->size.y;
                         body->vel.y = 0;
@@ -134,17 +137,31 @@ void physics_update(Body *body, float dt, const Tilemap *map) {
     /* Ground-stick probe: if not detected as on_ground but vel.y is
      * near zero (just landed or standing), check one pixel below.
      * Prevents flickering between grounded/airborne each frame due to
-     * sub-pixel gravity accumulation. */
+     * sub-pixel gravity accumulation. Checks both solid tiles and
+     * one-way platforms. */
     if (!body->on_ground && body->vel.y >= 0 && body->vel.y < s_gravity * dt * 2.0f && map) {
         int left  = world_to_tile(body->pos.x);
         int right = world_to_tile(body->pos.x + body->size.x - 1);
         int probe = world_to_tile(body->pos.y + body->size.y + 1.0f);
+        float body_bottom = body->pos.y + body->size.y;
         for (int tx = left; tx <= right; tx++) {
-            if (tilemap_is_solid(map, tx, probe)) {
+            uint32_t pf = tilemap_flags_at(map, tx, probe);
+            if (pf & TILE_SOLID) {
                 body->on_ground = true;
                 body->vel.y = 0;
                 break;
             }
+            /* One-way platforms: only stick if feet are at or above the
+             * platform top. Prevents snapping to a platform the player
+             * is below (e.g. jumping up through it). */
+            if (pf & TILE_PLATFORM) {
+                float tile_top = tile_to_world(probe);
+                if (body_bottom <= tile_top + 2.0f) {
+                    body->on_ground = true;
+                    body->vel.y = 0;
+                    break;
+                }
+            }
         }
     }
 }