SpriteMaker.cs

using System.IO;
using UnityEditor;
using UnityEngine;

namespace JamesFrowen.EditorScripts
{
    public static class SpriteMaker
    {
        [MenuItem("Tools/Sprite Maker/Make Circle 256x256")]
        public static void MakeCircle()
        {
            string path = EditorUtility.SaveFilePanel("Save Circle 256x256", Application.dataPath, "Circle", "png");
            if (string.IsNullOrEmpty(path))
            {
                return;
            }

            int radius = 128;
            float sqRadius = radius * radius;
            Texture2D texture = new Texture2D(radius * 2, radius * 2);
            for (int x = 0; x < texture.width; x++)
            {
                for (int y = 0; y < texture.height; y++)
                {
                    float dx = x - radius;
                    float dy = y - radius;
                    bool inCircle = (dx * dx + dy * dy) < sqRadius;
                    Color color = inCircle ? Color.white : Color.clear;
                    texture.SetPixel(x, y, color);
                }
            }

            byte[] bytes = texture.EncodeToPNG();

            File.WriteAllBytes(path, bytes);
            AssetDatabase.Refresh();
        }
    }
}