sprite geometry clipping for water effect

Robust.Client/GameObjects/Components/Renderable/SpriteComponent.cs

@@ -214,6 +214,14 @@ public ShaderInstance? PostShader
             set;
         }
 
+        /// <summary>
+        /// Fraction of every normal sprite layer cropped from its bottom edge before rendering.
+        /// This is applied before layer and post shaders, allowing effects such as water immersion
+        /// without consuming the sprite's single post-shader slot.
+        /// </summary>
+        [ViewVariables(VVAccess.ReadWrite)]
+        public float BottomClipFraction { get; set; }
+
         /// <summary>
         ///     Whether to pass the screen texture to the <see cref="PostShader"/>.
         /// </summary>

Robust.Client/GameObjects/EntitySystems/SpriteSystem.Render.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Numerics;
 using Robust.Client.Graphics;
 using Robust.Client.Graphics.Clyde;
@@ -146,6 +147,20 @@ private void RenderLayer(Layer layer, DrawingHandleWorld drawingHandle, ref Matr
         var layerColor = layer.Owner.Comp.color * layer.Color;
         var textureSize = texture.Size / (float) EyeManager.PixelsPerMeter;
         var quad = Box2.FromDimensions(textureSize / -2, textureSize);
+        UIBox2? subRegion = null;
+
+        var bottomClip = Math.Clamp(layer.Owner.Comp.BottomClipFraction, 0f, 0.999f);
+        if (bottomClip > 0f)
+        {
+            // Crop the texture as well as its quad so the visible portion keeps its original scale.
+            // Texture sub-regions use a top-left origin while world quads use a bottom-left origin.
+            quad.Bottom = MathHelper.Lerp(quad.Bottom, quad.Top, bottomClip);
+            subRegion = new UIBox2(
+                0f,
+                0f,
+                texture.Width,
+                texture.Height * (1f - bottomClip));
+        }
 
         if (layer.UnShaded)
         {
@@ -158,7 +173,7 @@ private void RenderLayer(Layer layer, DrawingHandleWorld drawingHandle, ref Matr
             layerColor = new(new Vector4(-1) - layerColor.RGBA);
         }
 
-        drawingHandle.DrawTextureRectRegion(texture, quad, layerColor);
+        drawingHandle.DrawTextureRectRegion(texture, quad, layerColor, subRegion);
 
         if (layer.Shader != null)
             drawingHandle.UseShader(null);