Sprite disappear shader

Question

i am working on Unity 2D platform game. i am using a shader i found here, but when i flip my sprite (scale -1), the sprite disappeared, i have little understand in shader coding, And i didn't found any help on Google, can any one help fixing the shader?. here is the shader:

Properties {
    [PerRendererData] _MainTex ("Main texture", 2D) = "white" {}
    _DissolveTex ("Dissolution texture", 2D) = "gray" {}
    _Threshold ("Threshold", Range(0., 1.01)) = 0.
}

SubShader {

    Tags { "Queue"="Transparent" }
    Blend SrcAlpha OneMinusSrcAlpha

    Pass {

        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag

        #include "UnityCG.cginc"

        struct v2f {
            float4 pos : SV_POSITION;
            float2 uv : TEXCOORD0;
        };

        sampler2D _MainTex;

        v2f vert(appdata_base v) {
            v2f o;
            o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
            o.uv = v.texcoord;
            return o;
        }

        sampler2D _DissolveTex;
        float _Threshold;

        fixed4 frag(v2f i) : SV_Target {
            float4 c = tex2D(_MainTex, i.uv);
            float val = tex2D(_DissolveTex, i.uv).r;

            c.a *= step(_Threshold, val);
            return c;
        }
        ENDCG
    }
}

Answer 1

The sprite is rendered as a rectangle with two triangle of positive index order: clockwise.

You can change the Shading Mode to Shaded Wireframe to inspect that.

And if you flip the sprite (scale -1), the two triangles of the sprite will become counterclockwise. And, counterclockwise triangles will not be rendered by GPU. That's why your sprite disappeared. See also what is a clockwise face in openGL.

If you want to see your sprite even when it is flipped, you can

1. create two sprites with the same texture: a normal one and a flipped one:

   MySprite
    |--Normal
    |--Flipped (scaled -1)
   

   Then, scale MySprite GameObject to -1 instead.

2. Turn off backface culling in your shader.

   Just add Cull Off to the subshader. See also this.

   SubShader {
   
       Tags { "Queue"="Transparent" }
       Blend SrcAlpha OneMinusSrcAlpha
   
       Pass {
       Cull Off
       ...