Make DeathEffect support transparent colors and custom texture

Celeste.Mod.mm/Patches/DeathEffect.cs

@@ -1,24 +1,50 @@
 using System;
+using System.Xml;
 using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
+using Monocle;
 using MonoMod;
 using MonoMod.Cil;
+using MonoMod.InlineRT;
+using MonoMod.Utils;
 
 namespace Celeste {
     class patch_DeathEffect : DeathEffect {
 
+        // no-op. MonoMod ignores this - we only need this to make the compiler shut up.
         public patch_DeathEffect(Color color, Vector2 offset)
             : base(color, offset) { }
 
+
+        public MTexture Texture;
+
+        /// <summary>
+        /// a temporary texture used to the next DeathEffect.Draw get call
+        /// </summary>
+        private static MTexture _texture;
+
         [MonoModIgnore]
         [PatchDeathEffectUpdate]
         public override extern void Update();
 
         [MonoModReplace]
         public override void Render() {
-            if (Entity != null)
+            if (Entity != null) {
+                _texture = Texture;
                 Draw(Entity.Position + Position, Color, Percent);
+            }
+        }
+
+        [MonoModIgnore]
+        [PatchDeathEffectDraw]
+        public static new void Draw(Vector2 position, Color color, float ease) { }
+
+        // Because of the auto naming rules of .Hook, we shouldn't name this method same as 'Draw'
+        public static void DrawWithTexture(MTexture texture, Vector2 position, Color color, float ease) {
+            _texture = texture;
+            Draw(position, color, ease);
         }
     }
 }
@@ -30,13 +56,71 @@ namespace MonoMod {
     [MonoModCustomMethodAttribute(nameof(MonoModRules.PatchDeathEffectUpdate))]
     class PatchDeathEffectUpdateAttribute : Attribute { }
 
-    static partial class MonoModRules {
+    [MonoModCustomMethodAttribute(nameof(MonoModRules.PatchDeathEffectDraw))]
+    class PatchDeathEffectDrawAttribute : Attribute {
+    }
 
+    static partial class MonoModRules {
         public static void PatchDeathEffectUpdate(ILContext context, CustomAttribute attrib) {
             ILCursor cursor = new ILCursor(context);
             cursor.GotoNext(instr => instr.OpCode == OpCodes.Ble_Un_S);
             cursor.Next.OpCode = OpCodes.Blt_Un_S;
         }
 
+        public static void PatchDeathEffectDraw(ILContext context, CustomAttribute attrib) {
+            TypeDefinition t_DeathEffect = MonoModRule.Modder.Module.GetType("Celeste.DeathEffect");
+            FieldReference m_DeathEffect_texture = t_DeathEffect.FindField("_texture");
+
+            TypeReference t_Color = MonoModRule.Modder.Module.ImportReference(MonoModRule.Modder.FindType("Microsoft.Xna.Framework.Color").Resolve());
+            MethodReference m_Color_get_Black = MonoModRule.Modder.Module.ImportReference(t_Color.Resolve().FindProperty("Black").GetMethod);
+            VariableDefinition v_outline = new VariableDefinition(t_Color);
+            context.Body.Variables.Add(v_outline);
+
+            MethodReference m_Color_get_A = MonoModRule.Modder.Module.ImportReference(t_Color.Resolve().FindProperty("A").GetMethod);
+            MethodReference m_Color_op_Multiply = MonoModRule.Modder.Module.ImportReference(t_Color.Resolve().FindMethod("op_Multiply"));
+
+            ILCursor cursor = new ILCursor(context);
+
+            // Color outline = Color.Black * (this.Color.A / 255f);
+            cursor.EmitCall(m_Color_get_Black);
+            cursor.EmitLdarga(1);
+            cursor.EmitCall(m_Color_get_A);
+            cursor.EmitConvR4();
+            cursor.EmitLdcR4(255f);
+            cursor.EmitDiv();
+            cursor.EmitCall(m_Color_op_Multiply);
+            cursor.EmitStloc(v_outline);
+
+            // Color.White => Color.White * (this.Color.A / 255f);
+            cursor.GotoNext(MoveType.After, instr => instr.MatchCall("Microsoft.Xna.Framework.Color", "get_White"));
+            cursor.EmitLdarga(1);
+            cursor.EmitCall(m_Color_get_A);
+            cursor.EmitConvR4();
+            cursor.EmitLdcR4(255f);
+            cursor.EmitDiv();
+            cursor.EmitCall(m_Color_op_Multiply);
+
+            // GFX.Game["characters/player/hair00"] => (_texture ?? GFX.Game["characters/player/hair00"]);
+            ILLabel Ifnull = cursor.DefineLabel();
+            cursor.GotoNext(MoveType.AfterLabel, instr => instr.MatchLdsfld("Celeste.GFX", "Game"));
+            cursor.EmitLdsfld(m_DeathEffect_texture);
+            cursor.EmitDup();
+            cursor.Emit(OpCodes.Brtrue_S, Ifnull);
+            cursor.EmitPop();
+            cursor.GotoNext(instr => instr.MatchStloc(out int i));
+            cursor.MarkLabel(Ifnull);
+
+            // Color.Black => outline;
+            for (int i = 4; i > 0; i--) {
+                cursor.GotoNext(instr => instr.MatchCall("Microsoft.Xna.Framework.Color", "get_Black"));
+                cursor.Remove();
+                cursor.EmitLdloc(v_outline);
+            }
+
+            // _texture = null;
+            cursor.GotoNext(instr => instr.MatchRet());
+            cursor.EmitLdnull();
+            cursor.EmitStsfld(m_DeathEffect_texture);
+        }
     }
 }