Encode deposit material ID in discretize pass

src/shaders/antsDiscretize.frag

@@ -5,9 +5,11 @@ in vec2 vUv;
 in float vIsCarryingFood;
 in float vScentFactor;
 in float vIsCellCleared;
+in float vDepositMaterialId;
 
 out vec4 FragColor;
 
 void main()	{
-    FragColor = vec4(vIsCarryingFood * vScentFactor, (1. - vIsCarryingFood) * vScentFactor, vIsCellCleared, 1);
+    // encode deposit material ID in alpha: divide by 255 to fit in UnsignedByte channel
+    FragColor = vec4(vIsCarryingFood * vScentFactor, (1. - vIsCarryingFood) * vScentFactor, vIsCellCleared, vDepositMaterialId / 255.);
 }

src/shaders/antsDiscretize.vert

@@ -8,6 +8,7 @@ out vec2 vUv;
 out float vIsCarryingFood;
 out float vScentFactor;
 out float vIsCellCleared;
+out float vDepositMaterialId;
 
 uniform sampler2D tDataCurrent;
 uniform sampler2D tDataLast;
@@ -38,6 +39,13 @@ void main()	{
     vScentFactor = storage / SCENT_MAX_STORAGE;
     vIsCellCleared = isCellCleared;
 
+    // detect deposit: was carrying, now not — read cargo ID from extended state
+    float isDepositing = float(wasCarrying == 1. && isCarrying == 0.);
+    vec4 extSample = texture(tDataExtCurrent, vec2(sampleX, sampleY) / dataTextureSize);
+    // cargoMaterialId is kept non-zero on the deposit frame (zeroed next frame for food drops,
+    // and stays as-is for powder deposits until the ant picks something else up)
+    vDepositMaterialId = isDepositing * extSample.g;
+
     gl_Position = vec4(
         (position.xy * cellSize + floor(offset * WORLD_SIZE) / WORLD_SIZE + cellSize * 0.5) * 2. - 1.,
         0,