Add spray brush, paint interval, and per-channel paint opacity

Spray brush mode: bakes a procedural scattered-dot mask per stamp (random
position, size, alpha within the brush disc) and paints it through the
existing single-stamp pipeline; dot centers clamped to (0.5 - dotR) so the
spray footprint matches the solid brush radius.

Paint interval: float on MeshPainter; 0 keeps the continuous interpolated
stroke, >0 emits a single timed dab every x.xx seconds.

Color picker alpha: horizontal alpha bar on both HSV pickers, runtime-built
in Rebind (no prefab re-bake), passed through to brushColor.a / emissionColor.a.

Metallic/smoothness + occlusion paint opacity: new opacity fields applied in
both CPU and GPU backends (GPU MSO draw split into MS and occlusion sub-stamps
for independent opacity), with cloned alpha sliders wired into the HUD.

Also includes the prefab layout tweaks and scene adjustments.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/HsvColorPicker.cs

@@ -3,27 +3,35 @@ using UnityEngine;
 using UnityEngine.UI;
 
 /// <summary>
-/// Compact runtime HSV colour picker: a saturation/value square, a hue bar and a live swatch.
-/// The square is a pure-hue base with two static gradient overlays on top (white-out ->
-/// saturation, black-out -> value), so only the base tint changes per hue - no per-frame
+/// Compact runtime HSV colour picker: a saturation/value square, a hue bar, an alpha bar and a
+/// live swatch. The square is a pure-hue base with two static gradient overlays on top (white-out
+/// -> saturation, black-out -> value), so only the base tint changes per hue - no per-frame
 /// texture rebuilds. Hue is never lost while dragging the square (the square only edits S/V,
-/// the bar only edits H).
+/// the bar only edits H, the alpha bar only edits A).
 ///
 /// <see cref="Build"/> creates the child structure (stable names, no sprites or hooks); it can
 /// be baked into a prefab. <see cref="Rebind"/> then (re)assigns the runtime gradient sprites
 /// (which a prefab cannot store) and (re)hooks the drag relays - it runs for both the code-built
-/// picker and one instantiated from a prefab, so the two are behaviourally identical.
+/// picker and one instantiated from a prefab, so the two are behaviourally identical. For a prefab
+/// baked before the alpha bar existed it also builds that bar on the fly (and grows the picker's
+/// reserved layout height), so no re-bake is required.
 /// </summary>
 public class HsvColorPicker : MonoBehaviour
 {
-    /// <summary>Raised with the new colour when the user moves the square or hue bar.</summary>
+    /// <summary>Raised with the new colour when the user moves the square, hue bar or alpha bar.</summary>
     public event Action<Color> Changed;
 
     private float _h, _s, _v, _a = 1f;
 
+    // Gap between the stacked square / hue / alpha rows (matches the layout in Build).
+    private const float RowGap = 6f;
+    private static readonly Color AlphaBackdrop = new Color(0.5f, 0.5f, 0.5f, 1f);
+
     private Image _hueBase;          // pure-hue background of the SV square
     private RectTransform _svHandle;
     private RectTransform _hueHandle;
+    private Image _alphaGrad;        // colour-tinted transparency ramp of the alpha bar
+    private RectTransform _alphaHandle;
     private Image _swatch;
 
     /// <summary>Current colour (HSV -> RGB, with the preserved alpha).</summary>
@@ -46,9 +54,10 @@ public class HsvColorPicker : MonoBehaviour
     }
 
     /// <summary>
-    /// Builds the picker structure (square + hue bar + swatch) under this object, top-left aligned.
-    /// Layout only - no gradient sprites and no event hooks, so the result is safe to bake into a
-    /// prefab. Call <see cref="Rebind"/> afterwards (at runtime) to make it functional.
+    /// Builds the picker structure (square + hue bar + alpha bar + swatch) under this object,
+    /// top-left aligned. Layout only - no gradient sprites and no event hooks, so the result is
+    /// safe to bake into a prefab. Call <see cref="Rebind"/> afterwards (at runtime) to make it
+    /// functional.
     /// </summary>
     public void Build(float squareSize, float hueHeight)
     {
@@ -68,7 +77,7 @@ public class HsvColorPicker : MonoBehaviour
 
         // Hue bar below the square.
         var hue = PaintHud.NewRect("Hue", transform);
-        TopLeft(hue, new Vector2(0f, -(squareSize + 6f)), new Vector2(squareSize, hueHeight));
+        TopLeft(hue, new Vector2(0f, -(squareSize + RowGap)), new Vector2(squareSize, hueHeight));
         var hueImg = hue.gameObject.AddComponent<Image>();
         hueImg.type = Image.Type.Simple;
 
@@ -78,12 +87,41 @@ public class HsvColorPicker : MonoBehaviour
         hueHandle.sizeDelta = new Vector2(4f, hueHeight + 4f);
         PaintHud.AddImage(hueHandle.gameObject, Color.white).raycastTarget = false;
 
+        // Alpha bar below the hue bar.
+        BuildAlphaBar(sv, hue);
+
         // Swatch, to the right of the square.
         var sw = PaintHud.NewRect("Swatch", transform);
         TopLeft(sw, new Vector2(squareSize + 10f, 0f), new Vector2(34f, 34f));
         PaintHud.AddImage(sw.gameObject, Color.white).raycastTarget = false;
     }
 
+    /// <summary>
+    /// Builds the alpha bar (grey backdrop + colour-tinted transparency ramp + drag relay + handle)
+    /// directly below the hue bar, sized/positioned from the existing SV square and hue bar so it
+    /// works for both a fresh build and an older baked prefab. Sprites/hooks are applied by
+    /// <see cref="Rebind"/>.
+    /// </summary>
+    private RectTransform BuildAlphaBar(RectTransform sv, RectTransform hue)
+    {
+        float width = sv.sizeDelta.x;
+        float barHeight = hue.sizeDelta.y;
+        float y = hue.anchoredPosition.y - (barHeight + RowGap);
+
+        var alpha = PaintHud.NewRect("Alpha", transform);
+        TopLeft(alpha, new Vector2(0f, y), new Vector2(width, barHeight));
+        PaintHud.AddImage(alpha.gameObject, AlphaBackdrop); // raycast target for the drag relay
+
+        PaintHud.AddOverlay(alpha, "AlphaGrad"); // tinted alpha ramp, sprite assigned in Rebind
+
+        alpha.gameObject.AddComponent<UiPointerSurface>();
+
+        var handle = PaintHud.NewRect("Handle", alpha);
+        handle.sizeDelta = new Vector2(4f, barHeight + 4f);
+        PaintHud.AddImage(handle.gameObject, Color.white).raycastTarget = false;
+        return alpha;
+    }
+
     /// <summary>
     /// Re-finds the child widgets, (re)assigns the runtime gradient sprites and (re)hooks the drag
     /// relays. Idempotent (unsubscribe-then-subscribe), so it is safe whether the structure was just
@@ -110,7 +148,7 @@ public class HsvColorPicker : MonoBehaviour
             }
         }
 
-        var hue = transform.Find("Hue");
+        var hue = transform.Find("Hue") as RectTransform;
         if (hue)
         {
             if (hue.TryGetComponent(out Image hueImg)) hueImg.sprite = PaintHud.HueSprite();
@@ -123,12 +161,48 @@ public class HsvColorPicker : MonoBehaviour
             }
         }
 
+        // Alpha bar: build it if the (older) prefab lacks it, then (re)assign sprite + relay.
+        var alpha = transform.Find("Alpha") as RectTransform;
+        bool builtAlpha = false;
+        if (!alpha && sv is RectTransform svRect && hue)
+        {
+            alpha = BuildAlphaBar(svRect, hue);
+            builtAlpha = true;
+        }
+        if (alpha)
+        {
+            var grad = alpha.Find("AlphaGrad");
+            if (grad && grad.TryGetComponent(out Image gradImg))
+            {
+                gradImg.sprite = PaintHud.AlphaSprite();
+                _alphaGrad = gradImg;
+            }
+            _alphaHandle = alpha.Find("Handle") as RectTransform;
+
+            if (alpha.TryGetComponent(out UiPointerSurface alphaPointer))
+            {
+                alphaPointer.Picked -= OnAlphaPicked;
+                alphaPointer.Picked += OnAlphaPicked;
+            }
+
+            if (builtAlpha) GrowReservedHeight(alpha.sizeDelta.y + RowGap);
+        }
+
         var swatch = transform.Find("Swatch");
         if (swatch) _swatch = swatch.GetComponent<Image>();
 
         Refresh();
     }
 
+    /// <summary>Grows this picker's LayoutElement so a runtime-added alpha bar does not overlap the
+    /// widget below it; the panel's ContentSizeFitter then expands to match. No-op without a LayoutElement.</summary>
+    private void GrowReservedHeight(float extra)
+    {
+        if (!TryGetComponent(out LayoutElement le)) return;
+        if (le.minHeight > 0f) le.minHeight += extra;
+        if (le.preferredHeight > 0f) le.preferredHeight += extra;
+    }
+
     private void OnSvPicked(Vector2 p)
     {
         _s = p.x;
@@ -144,6 +218,13 @@ public class HsvColorPicker : MonoBehaviour
         Changed?.Invoke(Value);
     }
 
+    private void OnAlphaPicked(Vector2 p)
+    {
+        _a = p.x;
+        Refresh();
+        Changed?.Invoke(Value);
+    }
+
     private void Refresh()
     {
         if (_hueBase) _hueBase.color = Color.HSVToRGB(_h, 1f, 1f);
@@ -157,6 +238,12 @@ public class HsvColorPicker : MonoBehaviour
             _hueHandle.anchorMin = _hueHandle.anchorMax = new Vector2(_h, 0.5f);
             _hueHandle.anchoredPosition = Vector2.zero;
         }
+        if (_alphaGrad) _alphaGrad.color = Color.HSVToRGB(_h, _s, _v); // tint the transparency ramp to the current colour
+        if (_alphaHandle)
+        {
+            _alphaHandle.anchorMin = _alphaHandle.anchorMax = new Vector2(_a, 0.5f);
+            _alphaHandle.anchoredPosition = Vector2.zero;
+        }
         if (_swatch) _swatch.color = Color.HSVToRGB(_h, _s, _v);
     }