結果

グラデーションの具合の調整をUnityの中でやるために，
グラデーション画像の生成をUnity側でやります．

1. Textureを生成する．

public class GradationTexCreater{

    public GradationTexCreater(){

    }

    public Texture2D Create(Gradient gradient){
        return GradientToTexture (gradient);
    }

    /// <summary>
    /// グラデーションをそのまま画像にする．
    /// </summary>
    public static Texture2D GradientToTexture(Gradient gradient, int width = 256, int height = 1)
    {
        var texture = CreateTexture(width, height);
        for (int h = 0; h < texture.height; h++)
        {
            for (int w = 0; w < texture.width; w++)
            {
                float rate = (float)w / (float)texture.width;
                Color color = gradient.Evaluate(rate);
                texture.SetPixel(w, h, color);
            }
        }

        texture.Apply();
        return texture;
    }
    /// <summary>
    /// グラデーションを円形グラデーションのテクスチャにする．．
    /// </summary>
    public static Texture2D GradientToRadialGradationTex(Gradient gradient, int width = 256, int height = 1)
    {
        var texture = CreateTexture(width, height);
        for (int h = 0; h < texture.height; h++)
        {
            for (int w = 0; w < texture.width; w++)
            {
                float u = (float)w / (float)texture.width;
                float v = (float)h / (float)texture.height;
                //Color color = gradient.Evaluate(u);
                Color color = new RadialGradationColorCalclator(gradient).Evaluate(u, v);
                texture.SetPixel(w, h, color);
            }
        }

        texture.Apply();
        return texture;
    }

    /// <summary>
    /// 円形グラデーションのカラーを取得するためのクラス
    /// </summary>
    public class RadialGradationColorCalclator{
        public Color backGroundColor = Color.black;
        public float radius = 0.9f; // グラデーションの伸びる割
        public Gradient gradient;

        public RadialGradationColorCalclator(Gradient gradient){
            this.gradient = gradient;
        }

        // uvでカラーを取得 // 左下が0,0 to 1,1
        public Color Evaluate(float u, float v){
            Vector2 r = RemapMinus1To1 (u, v); // -1 -> 1
            float l = r.magnitude;
            if (l < radius) {
                float t = l / radius;
                return gradient.Evaluate (t);
            } else {
                return backGroundColor;
            }
        }

        Vector2 RemapMinus1To1(float u, float v){
            return new Vector2 (u - 0.5f, v - 0.5f) * 2.0f;
        }
    }


    /// <summary>
    /// グラデーション用の設定でTexture2Dを作成
    /// </summary>
    static Texture2D CreateTexture(int width, int height)
    {
        // Alpha使うのでRGB32にする. 使わないならRGB24
        Texture2D texture = new Texture2D(width, height, TextureFormat.RGBA32, false);
        texture.wrapMode = TextureWrapMode.Clamp;
        return texture;
    }
}

Gradientから画像を生成します．
Gradientについては以下の記事に書きました．
https://qiita.com/Teach/items/3bc616d9f8115e80c31e

Texture2F作成の仕組みは以下です．
1. CreateTextureでwidthとheightを指定して生成
2. widthとheightの分だけFor分を回し，各pixelごとにSetPixelで色をつける．

フラグメントシェーダーに近い色の付け方です．

2. pngで保存する．

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System.IO;


#if UNITY_EDITOR
using UnityEditor;
#endif

public class GradationApplier : MonoBehaviour {

    public Gradient gradient;
    public string fileName = "gradation";

    [HideInInspector]
    public Texture2D texCache;
    public UnityEngine.UI.RawImage rawImage;
    public int height = 128;
    public int width = 128;
    public bool isRadial = false;
    public bool setGradationTexToMaterial = false;

    void OnValidate(){
        if (height < 0)
            height = 0;
        if (width < 0)
            width = 0;

        if(isRadial)
            texCache = GradationTexCreater.GradientToRadialGradationTex (gradient, width, height);
        else
            texCache = GradationTexCreater.GradientToTexture (gradient, width, height);
        if(rawImage !=null)
            rawImage.texture = texCache;
        if(texCache != null)
            texCache.name = fileName;

        if (setGradationTexToMaterial && texCache != null)
            this.GetComponent<SpriteRenderer> ().sharedMaterial.SetTexture ("_GradationTex", texCache);
    }
}

#if UNITY_EDITOR
[CustomEditor(typeof(GradationApplier))]
public class GradationApplierEditor : Editor{

    GradationApplier applier;

    void OnEnable(){
        applier = target as GradationApplier;
    }

    public override void OnInspectorGUI ()
    {
        DrawDefaultInspector ();
        if (GUILayout.Button ("Export")) {
            ExportTexture (applier.texCache);
        }
    }

    public static void ExportTexture(Texture2D tex){
        string path = GetSavePath (tex.name);
        Directory.CreateDirectory (Path.GetDirectoryName (path));
        File.WriteAllBytes(path, tex.EncodeToPNG());
        AssetDatabase.Refresh();
    }

    static string GetSavePath(string fileName){
        var path2 = string.Format ("Export/{0}.png", fileName);
        return Path.Combine (Application.dataPath, path2);
    }
}

#endif

ExportTextureで画像を保存します．

3. シェーダーで合成する

Shader "TSprites/Effect"
{
    Properties
    {
        [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
        [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
        [HideInInspector] _RendererColor ("RendererColor", Color) = (1,1,1,1)
        [HideInInspector] _Flip ("Flip", Vector) = (1,1,1,1)
        [PerRendererData] _AlphaTex ("External Alpha", 2D) = "white" {}
        [PerRendererData] _EnableExternalAlpha ("Enable External Alpha", Float) = 0
        _GradationTex("Gradation Texture", 2D) = "white"{}
    }

    SubShader
    {
        Tags
        {
            "Queue"="Transparent"
            "IgnoreProjector"="True"
            "RenderType"="Transparent"
            "PreviewType"="Plane"
            "CanUseSpriteAtlas"="True"
        }

        Cull Off
        Lighting Off
        ZWrite Off
        Blend SrcAlpha OneMinusSrcAlpha

        Pass
        {
        CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0
            #pragma multi_compile_instancing
            #pragma multi_compile _ PIXELSNAP_ON
            #pragma multi_compile _ ETC1_EXTERNAL_ALPHA
            #include "UnitySprites.cginc"

            sampler2D _GradationTex;
            float _val_u;

            struct appdata_ts
            {
                float4 vertex : POSITION;
                float2 texcoord0 : TEXCOORD0;
                float4 color : COLOR;
            };

            struct v2f_ts
            {
                float4 pos : SV_POSITION;
                float2 uv0 : TEXCOORD0;
                fixed4 color : COLOR;
                UNITY_VERTEX_OUTPUT_STEREO  
            };

            v2f_ts vert(appdata_ts IN){
                v2f_ts OUT;
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);

                OUT.pos = UnityObjectToClipPos(IN.vertex);
                OUT.uv0 = IN.texcoord0;

                #ifdef PIXELSNAP_ON
                OUT.pos = UnityPixelSnap (OUT.pos);
                #endif
                OUT.color = IN.color;
                return OUT;
            }

            fixed4 frag(v2f_ts IN) : SV_Target{
                // Default
                  fixed4 mainColor = SampleSpriteTexture(IN.uv0);
                  // グラデーションの色を求める
                  float2 gradation_uv = float2(mainColor.r, 0);
                  fixed4 finalColor = tex2D(_GradationTex, gradation_uv);

                  finalColor *= IN.color;
                  float finalAlpha = mainColor.a * finalColor.a;
                  return fixed4(finalColor.rgb, finalAlpha);
            }
        ENDCG
        }
    }
}

SpriteRenderer用のシェーダーです．

まとめ

Photoshop等を使わずUnityだけで作れるという点で，スクリプトから画像を作るのは非常に役立つと思いました．

いろいろなことの自動化に応用できそうで，非常に勉強になりました．

結果

1. Textureを生成する．

2. pngで保存する．

3. シェーダーで合成する

まとめ

参考