Help with Mesh based top-down 2D shadows like Rimworld and Prison Architect

Hey there,

I'm trying to implement a 2D top down shadow using meshes. The image below explains it better. It's from the game Rimworld.



Rimwold creator explaining how he did it: https://ludeon.com/blog/2013/08/sun-shadows/#comment-1945.

I gave my try on it, but I'm still trying to learn shaders. I managed to change a cube's "front" face (grabbing all the vertices that composes it) and moving them using an oscillation formula. I squashed all the cubes vertices in the Z dimension to the same value.

As the example (image) above, I want them to blend with the ground, but I don't want them to blend with each other. Is that possible?

I'm using a simple unlit transparent shader:

Code (CSharp):

1. Shader "Unlit/ShadowShader"
2. {
3.     Properties
4.     {
5.         _Color("Color", Color) = (1, 1, 1, 1)
6.         _MoveAmount("Move Amount", Float) = 0
7.         _Length("Length", Float) = 1
8.         _BottomVerticesYOffset("Bottom Vertices Y Offset", Float) = 1.5
9.     }
10.     SubShader
11.     {
12.         Tags{ "Queue" = "Transparent" "RenderType" = "Transparent" }
13.         LOD 100
14.  
15.         ZWrite Off
16.         Blend SrcAlpha OneMinusSrcAlpha
17.  
18.         Pass
19.         {
20.             CGPROGRAM
21.             #pragma vertex vert
22.             #pragma fragment frag
23.  
24.             #include "UnityCG.cginc"
25.  
26.             struct appdata
27.             {
28.                 float4 vertex : POSITION;
29.             };
30.  
31.             struct v2f
32.             {
33.                 float4 vertex : SV_POSITION;
34.             };
35.  
36.             float _MoveAmount;
37.             float _Length;
38.             float _BottomVerticesYOffset;
39.             float4 _Color;
40.  
41.             v2f vert (appdata v, uint vertexId : SV_VertexID)
42.             {
43.                 v2f o;
44.  
45.                 if (vertexId != 2 && vertexId != 3 && vertexId != 0 && vertexId != 1 && vertexId != 8 && vertexId != 9 && vertexId != 13 && vertexId != 14 && vertexId != 16 && vertexId != 17 && vertexId != 22 && vertexId != 23)
46.                     o.vertex = UnityObjectToClipPos(float4(v.vertex.x + sin(_MoveAmount) * _Length, (v.vertex.y + _BottomVerticesYOffset) - cos(_MoveAmount) * _Length, 0, 0));
47.                 else
48.                     o.vertex = UnityObjectToClipPos(float4(v.vertex.x, v.vertex.y, 0, 0));
49.  
50.                 return o;
51.             }
52.  
53.             fixed4 frag (v2f i) : SV_Target
54.             {
55.                 return _Color;
56.             }
57.             ENDCG
58.         }
59.     }
60. }
61.  

Results in this (in a cube):



This is still a messy code. The important part, I believe is it being transparent and the blend mode strategy.

To recap I would like the "shadows" not to blend with each other, just with the "ground".

I want to understand why this won't work, if that's the case, and what concepts should I look into that could give the same result.

If it's possible to follow what I'm already doing, what am I missing?

Best regards!

 

So, I was searching for the same problem that someone could have had before, but I was having a bad time in finding the words. After some time I stumbled upon this one:

https://computergraphics.stackexcha...tween-two-overlapping-semi-transparent-shapes

It talks about Stencil buffer to check pixels on subsequent draw calls.

It solves the problem, but I don't get it. I want to understand.

The stencil is declared like this:

Code (CSharp):

1. Stencil {
2.     Ref 0
3.     Comp Equal
4.     Pass IncrSat
5.     Fail IncrSat
6. }

I understood that the Ref is just assigning a reference, but for what I didn't understand.
The Comp for me made sense, sort of. What I understood is that it checks the buffer that was rendered in the "opaque" render queue. If it is not (already blended a "shadow" in a previous draw call), then it discards, if it is Equal then run the pass.

The IncrSat for Pass and Fail I don't get it.

(I read https://docs.unity3d.com/Manual/SL-Stencil.html)

 

i was looking into the same thing, and i think what they did is modify the unity standard 2d sprite shadows to fit their needs. Check out the sourcecode for ShadowCaster2D