From 5d2521843386a962078ac52396c41eeafae899b4 Mon Sep 17 00:00:00 2001
From: Jared Miller <jared@smell.flowers>
Date: Thu, 12 Mar 2026 10:16:14 -0400
Subject: [PATCH] Prevent ants from picking up the surface they walk on
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Angle noise (±12° from wander) caused the ahead-cell pickup check
to sample into the sand cell directly below the ant. Ants were
grabbing sand on their first frame of existence.

Fix: skip powder pickup when the ahead cell equals the cell directly
below (the walking surface). Food pickup is still allowed from any
adjacent cell. Diagonal dig pickup still works since the DIG priority
angles ants at ~40° which targets a diagonal neighbor, not directly
below.
---
 src/shaders/antsCompute.frag | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/src/shaders/antsCompute.frag b/src/shaders/antsCompute.frag
index 3a319b7..443392a 100644
--- a/src/shaders/antsCompute.frag
+++ b/src/shaders/antsCompute.frag
@@ -337,20 +337,27 @@ void main()	{
             float aheadMatId = texture(tWorld, aheadUv).x;
             int aheadMatInt = int(aheadMatId);
 
+            // food: pick up from any adjacent cell
             if (aheadMatInt == MAT_FOOD) {
                 isCarrying = 1.;
                 cargoMaterialId = aheadMatId;
                 angle += PI;
                 storage = getMaxScentStorage(vUv);
             } else if (aheadMatInt != MAT_AIR && aheadMatInt != MAT_HOME) {
-                vec4 props = texelFetch(uMaterialProps, ivec2(aheadMatInt, 0), 0);
-                float behavior = props.r;
-                float hardness = props.b;
-                if (behavior == BEHAVIOR_POWDER && hardness <= ANT_CARRY_STRENGTH) {
-                    isCarrying = 1.;
-                    cargoMaterialId = aheadMatId;
-                    angle += PI;
-                    storage = getMaxScentStorage(vUv);
+                // powder: don't grab the surface we're walking on — only dig
+                // when facing diagonally into material (DIG priority angles us there)
+                vec2 belowCell = roundUvToCellCenter(pos - vec2(0., cellSize));
+                bool isWalkingSurface = all(equal(aheadUv, belowCell));
+                if (!isWalkingSurface) {
+                    vec4 props = texelFetch(uMaterialProps, ivec2(aheadMatInt, 0), 0);
+                    float behavior = props.r;
+                    float hardness = props.b;
+                    if (behavior == BEHAVIOR_POWDER && hardness <= ANT_CARRY_STRENGTH) {
+                        isCarrying = 1.;
+                        cargoMaterialId = aheadMatId;
+                        angle += PI;
+                        storage = getMaxScentStorage(vUv);
+                    }
                 }
             }
         }