Trying to create a glowing halo around a 2D Sprite

I am trying to create a glowing halo around a 2D sprite. Mind you, I have no idea how to code a shader myself. I did find a Sprite Outline shader posted online by a fellow named Ryan Nielson, and in the comments somebody posted a modification to make the outline extend outward instead of inward. This seemed like a good start. The result reads like this:

Code (CSharp):

1. Shader "Sprites/Outline"
2. {
3.     Properties
4.     {
5.         [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
6.         _Color("Tint", Color) = (1,1,1,1)
7.         [MaterialToggle] PixelSnap("Pixel snap", Float) = 0
8.  
9.         // Add values to determine if outlining is enabled and outline color.
10.         [PerRendererData] _Outline("Outline", Float) = 0
11.         [PerRendererData] _OutlineColor("Outline Color", Color) = (1,1,1,1)
12.         [PerRendererData] _OutlineSize("Outline Size", int) = 1
13.     }
14.  
15.     SubShader
16.     {
17.         Tags
18.         {
19.             "Queue" = "Transparent"
20.             "IgnoreProjector" = "True"
21.             "RenderType" = "Transparent"
22.             "PreviewType" = "Plane"
23.             "CanUseSpriteAtlas" = "True"
24.         }
25.  
26.         Cull Off
27.         Lighting Off
28.         ZWrite Off
29.         Blend One OneMinusSrcAlpha
30.  
31.         Pass
32.         {
33.             CGPROGRAM
34.             #pragma vertex vert
35.             #pragma fragment frag
36.             #pragma multi_compile _ PIXELSNAP_ON
37.             #pragma shader_feature ETC1_EXTERNAL_ALPHA
38.             #include "UnityCG.cginc"
39.  
40.             struct appdata_t
41.             {
42.                 float4 vertex   : POSITION;
43.                 float4 color    : COLOR;
44.                 float2 texcoord : TEXCOORD0;
45.             };
46.  
47.             struct v2f
48.             {
49.                 float4 vertex   : SV_POSITION;
50.                 fixed4 color : COLOR;
51.                 float2 texcoord  : TEXCOORD0;
52.             };
53.  
54.             fixed4 _Color;
55.             float _Outline;
56.             fixed4 _OutlineColor;
57.             int _OutlineSize;
58.  
59.             v2f vert(appdata_t IN)
60.             {
61.                 v2f OUT;
62.                 OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
63.                 OUT.texcoord = IN.texcoord;
64.                 OUT.color = IN.color * _Color;
65.                 #ifdef PIXELSNAP_ON
66.                 OUT.vertex = UnityPixelSnap(OUT.vertex);
67.                 #endif
68.  
69.                 return OUT;
70.             }
71.  
72.             sampler2D _MainTex;
73.             sampler2D _AlphaTex;
74.             float4 _MainTex_TexelSize;
75.  
76.             fixed4 SampleSpriteTexture(float2 uv)
77.             {
78.                 fixed4 color = tex2D(_MainTex, uv);
79.  
80.                 #if ETC1_EXTERNAL_ALPHA
81.                 // get the color from an external texture (usecase: Alpha support for ETC1 on android)
82.                 color.a = tex2D(_AlphaTex, uv).r;
83.                 #endif //ETC1_EXTERNAL_ALPHA
84.  
85.                 return color;
86.             }
87.  
88.             fixed4 frag(v2f IN) : SV_Target
89.             {
90.                 fixed4 c = SampleSpriteTexture(IN.texcoord) * IN.color;
91.  
92.                 // If outline is enabled and there is a pixel, try to draw an outline.
93.                     if (_Outline > 0 && c.a == 0) {
94.                     float totalAlpha = 0.0;
95.  
96.                     [unroll(100)]
97.                     for (int i = 1; i < _OutlineSize + 1; i++) {
98.                         fixed4 pixelUp = tex2D(_MainTex, IN.texcoord + fixed2(0, i * _MainTex_TexelSize.y));
99.                         fixed4 pixelDown = tex2D(_MainTex, IN.texcoord - fixed2(0,i *  _MainTex_TexelSize.y));
100.                         fixed4 pixelRight = tex2D(_MainTex, IN.texcoord + fixed2(i * _MainTex_TexelSize.x, 0));
101.                         fixed4 pixelLeft = tex2D(_MainTex, IN.texcoord - fixed2(i * _MainTex_TexelSize.x, 0));
102.  
103.                         totalAlpha = totalAlpha + pixelUp.a + pixelDown.a + pixelRight.a + pixelLeft.a;
104.                     }
105.  
106.                     if (totalAlpha != 0) {
107.                     c.rgba = fixed4(1, 1, 1, 1) * _OutlineColor;
108.                     }
109.                 }
110.  
111.                 c.rgb *= c.a;
112.  
113.                 return c;
114.             }
115.             ENDCG
116.         }
117.     }
118. }

There are three issues that I've encountered here, though. The first one, of course, is that this is an outline, not a glow. I need the outline's alpha to fade out the farther away from the sprite it gets; that ought to create a decent glowing effect, I think? But I don't really know how to do that.

The second issue is that the aura doesn't extend in all directions, it only extends on the X and Y axes independently. That creates some concave corners in certain places, like so:


The third issue is that the aura gets cropped. By changing a setting and adding a margin around my sprite, I managed to fix this in the editor, but at runtime, the sprite has some kind of bounding box that gets cropped in based on the sprite's image.


Again, I don't really know how to code a shader, so I'm not really sure how to fix these issues. Any help would be greatly appreciated.

 

With help from a friend, the shader has been modified as such:

Code (CSharp):

1. Shader "Sprites/Outline"
2. {
3.     Properties
4.     {
5.         [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
6.         _Color("Tint", Color) = (1,1,1,1)
7.         [MaterialToggle] PixelSnap("Pixel snap", Float) = 0
8.  
9.         // Add values to determine if outlining is enabled and outline color.
10.         [PerRendererData] _Outline("Outline", Float) = 0
11.         [PerRendererData] _OutlineColor("Outline Color", Color) = (1,1,1,1)
12.         [PerRendererData] _OutlineSize("Outline Size", int) = 1
13.     }
14.  
15.     SubShader
16.     {
17.         Tags
18.         {
19.             "Queue" = "Transparent"
20.             "IgnoreProjector" = "True"
21.             "RenderType" = "Transparent"
22.             "PreviewType" = "Plane"
23.             "CanUseSpriteAtlas" = "True"
24.         }
25.  
26.         Cull Off
27.         Lighting Off
28.         ZWrite Off
29.         Blend One OneMinusSrcAlpha
30.  
31.         Pass
32.         {
33.             CGPROGRAM
34.             #pragma vertex vert
35.             #pragma fragment frag
36.             #pragma multi_compile _ PIXELSNAP_ON
37.             #pragma shader_feature ETC1_EXTERNAL_ALPHA
38.             #include "UnityCG.cginc"
39.  
40.             struct appdata_t
41.             {
42.                 float4 vertex   : POSITION;
43.                 float4 color    : COLOR;
44.                 float2 texcoord : TEXCOORD0;
45.             };
46.  
47.             struct v2f
48.             {
49.                 float4 vertex   : SV_POSITION;
50.                 fixed4 color : COLOR;
51.                 float2 texcoord  : TEXCOORD0;
52.             };
53.  
54.             fixed4 _Color;
55.             float _Outline;
56.             fixed4 _OutlineColor;
57.             int _OutlineSize;
58.  
59.             v2f vert(appdata_t IN)
60.             {
61.                 v2f OUT;
62.                 OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
63.                 OUT.texcoord = IN.texcoord;
64.                 OUT.color = IN.color * _Color;
65.                 #ifdef PIXELSNAP_ON
66.                 OUT.vertex = UnityPixelSnap(OUT.vertex);
67.                 #endif
68.  
69.                 return OUT;
70.             }
71.  
72.             sampler2D _MainTex;
73.             sampler2D _AlphaTex;
74.             float4 _MainTex_TexelSize;
75.  
76.             fixed4 SampleSpriteTexture(float2 uv)
77.             {
78.                 fixed4 color = tex2D(_MainTex, uv);
79.  
80.                 #if ETC1_EXTERNAL_ALPHA
81.                 // get the color from an external texture (usecase: Alpha support for ETC1 on android)
82.                 color.a = tex2D(_AlphaTex, uv).r;
83.                 #endif //ETC1_EXTERNAL_ALPHA
84.  
85.                 return color;
86.             }
87.  
88.             fixed4 frag(v2f IN) : SV_Target
89.             {
90.                 fixed4 c = SampleSpriteTexture(IN.texcoord) * IN.color;
91.  
92.                 // If outline is enabled and there is a pixel, try to draw an outline.
93.                     if (_Outline > 0 && c.a == 0) {
94.                     float totalAlpha = 0.0;
95.                     float distance = 0.0;
96.  
97.                     [unroll(100)]
98.                     for (int i = 1; i < _OutlineSize + 1; i++) {
99.                         fixed4 pixelUp = tex2D(_MainTex, IN.texcoord + fixed2(0, i * _MainTex_TexelSize.y));
100.                         fixed4 pixelDown = tex2D(_MainTex, IN.texcoord - fixed2(0,i *  _MainTex_TexelSize.y));
101.                         fixed4 pixelRight = tex2D(_MainTex, IN.texcoord + fixed2(i * _MainTex_TexelSize.x, 0));
102.                         fixed4 pixelLeft = tex2D(_MainTex, IN.texcoord - fixed2(i * _MainTex_TexelSize.x, 0));
103.                         if(pixelUp.a + pixelDown.a + pixelRight.a + pixelLeft.a > 0) { distance = i; break; }
104.                     }
105.  
106.                     if (distance > 0) {
107.                         float totalAlpha = 1 - ((distance * 1.0f)/_OutlineSize);
108.                         c.rgba = fixed4(1, 1, 1, totalAlpha) * _OutlineColor;
109.                     }
110.                 }
111.  
112.                 c.rgb *= c.a;
113.  
114.                 return c;
115.             }
116.             ENDCG
117.         }
118.     }
119. }

Now the alpha gradates as the distance from the sprite increases, more like a proper glow. I'm still struggling with the other issues, though. I tried to fix the corners by adding checks along the diagonals, but that gave the corners a strange feathered look.