Glow around point in ScreenSpace

Hello, everyone!

I'm trying to write a shader for UI elements that would highlight them around given point (global shader variable) - the closer the pixel is to the screen point, the more _GlowColor is added to it.

However, I can't get it working. This is my shader code:

Spoiler: ShaderCode

Code (CSharp):

1.  
2. Shader "Custom/UI/UI - Glow around point"
3. {
4.     Properties
5.     {
6.         [PerRendererData]_MainTex("Static texture (can be empty)", 2D) = "white" {}
7.         _Color("Main color", Color) = (0,0,0,0)
8.         _GlowColor("Glow color", Color) = (0,0,0,0)
9.         _GlowDistance("Glow distance", float) = 5
10.  
11.         [Space(30)]
12.  
13.         _StencilComp("Stencil Comparison", Float) = 8
14.         _Stencil("Stencil ID", Float) = 0
15.         _StencilOp("Stencil Operation", Float) = 0
16.         _StencilWriteMask("Stencil Write Mask", Float) = 255
17.         _StencilReadMask("Stencil Read Mask", Float) = 255
18.         _ColorMask("Color Mask", Float) = 15
19.     }
20.  
21.         SubShader
22.     {
23.             Tags
24.         {
25.             "Queue" = "Transparent"
26.             "IgnoreProjector" = "True"
27.             "RenderType" = "Transparent"
28.             "PreviewType" = "Plane"
29.         }
30.  
31.             Stencil
32.         {
33.             Ref[_Stencil]
34.             Comp[_StencilComp]
35.             Pass[_StencilOp]
36.             ReadMask[_StencilReadMask]
37.             WriteMask[_StencilWriteMask]
38.         }
39.  
40.             ColorMask[_ColorMask]
41.             Cull Off
42.             ZWrite Off
43.             ZTest[unity_GUIZTestMode]
44.             Blend SrcAlpha One
45.  
46.             Pass
47.         {
48.             CGPROGRAM
49.             #pragma vertex vert
50.             #pragma fragment frag
51.  
52.             #include "UnityCG.cginc"
53.  
54.             uniform sampler2D _MainTex;
55.          
56.             uniform Vector _PointPosition;
57.  
58.             uniform float4 _MainTex_ST;
59.             uniform float4 _Color;
60.             uniform float4 _GlowColor;
61.  
62.             uniform float _GlowDistance;
63.  
64.             uniform float _GlowIntensity;
65.  
66.             struct appdata
67.             {
68.                 float4 vertex : POSITION;
69.                 float2 texcoord : TEXCOORD0;
70.                 float4 color : COLOR;
71.             };
72.  
73.             struct v2f
74.             {
75.                 float2 uv : TEXCOORD0;
76.                 float4 vertex : SV_POSITION;
77.                 float4 color : COLOR;
78.             };
79.  
80.             v2f vert(appdata v)
81.             {
82.                 v2f o;
83.                 o.vertex = UnityObjectToClipPos(v.vertex);
84.                 o.color = v.color;
85.                 o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
86.  
87.                 return o;
88.             }
89.  
90.             fixed4 frag(v2f i) : SV_Target
91.             {
92.  
93.                 fixed4 color = _Color * tex2D(_MainTex, i.uv*_MainTex_ST)*i.color;
94.  
95.                 float dist = distance(_PointPosition, i.vertex) / _GlowDistance;
96.  
97.                 color.rgb += (_GlowColor*_GlowIntensity) / dist;
98.  
99.                 return color;
100.             }
101.             ENDCG
102.         }
103.     }
104.         Fallback "Mobile/VertexLit"
105. }
106.  

And in a test script that is attached to a point I do this, to get the coordinates of the point game object:

Code (CSharp):

1.  
2.     void Update () {
3.         Vector4 screenPos = Camera.main.WorldToScreenPoint((gameObject.transform.position));
4.         Shader.SetGlobalVector("_PointPosition", screenPos);
5.         Shader.SetGlobalFloat("_GlowIntensity", (Mathf.Sin((Mathf.PI*Time.time))+1)*0.5f);
6.  
7.         Debug.Log("target is " + screenPos.x + " pixels from the left");
8.         Debug.Log("target is " + screenPos.y + " pixels from the bottom");
9.     }
10.    

Everything seems to work, but for some reason the y coordinate of screen gets mirrored around the screen center... I can't figure out why for few hours already, so I'm asking for help: what am I missing? Here's a video of what's happening: http://take.ms/ECHpw

 

https://docs.unity3d.com/Manual/SL-PlatformDifferences.html

 

This looks totally like the case, but for some reason, flipping y-uv-coordinate doesn't change anything as well as y-vertex coordinate. What fixed my issue was flipping points Y coordinate:

Code (CSharp):

1.  var pos = gameObject.transform.position;
2.         pos.y *= -1;
3.         Vector4 screenPos = Camera.main. WorldToScreenPoint((pos));
4.         Shader.SetGlobalVector("_PointPosition", screenPos);

Is it a platform-specific dependency? Doesn't look like that...
And it wasn't working if I flipped that coordinate inside shader...