示例：

工程截图

代码

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using DrawingGraphics = System.Drawing.Graphics;
using DrawingFont = System.Drawing.Font;
using DrawingFontStyle = System.Drawing.FontStyle;
using DrawingColor = System.Drawing.Color;

using UnityEngine;
using UnityFont = UnityEngine.Font;
using UnityColor = UnityEngine.Color;

/// <summary>
/// 合并文字到贴图 (仅考虑Windows平台)
/// </summary>
public class TextTextureMerge : MonoBehaviour
{
    [ContextMenuItem("Test", "OnTestContextMenuItem")]
    public Material material;
    public string familyName = "宋体";
    public string text = "Test";
    public int fontSize = 24;
    public UnityColor fontColor;
    //文字在贴图中的坐标
    public int dstX = 0;
    public int dstY = 0;

    // 仅测试调用
    void OnTestContextMenuItem()
    {
        if (material == null || material.mainTexture == null)
            return;
        Texture2D dst = material.mainTexture as Texture2D;
        Texture2D tex = DrawString(text, dst, dstX, dstY, fontColor, fontSize, familyName);
        material.mainTexture = tex;
    }

    // 绘文字到Texture的指定位置
    public static Texture2D DrawString(string text, Texture2D dst, int dstX, int dstY, UnityColor color, int fontSize = 24, string familyName = "宋体")
    {
        DrawingColor draw_color = DrawingColor.FromArgb((int)(color.a * 255f), (int)(color.r * 255f), (int)(color.g * 255f), (int)(color.b * 255f));
        Texture2D src = DrawString(text, draw_color, fontSize);
        Texture2D tex = MergeTexture(src, dst, dstX, dstY, color);
        return tex;
    }

    // 文字转Texture
    public static Texture2D DrawString(string text, DrawingColor color, int fontSize = 24, string familyName="宋体")
    {
        string str = text;
        DrawingGraphics g = DrawingGraphics.FromImage(new Bitmap(1, 1));
        DrawingFont font = new DrawingFont(familyName, fontSize, DrawingFontStyle.Regular, GraphicsUnit.Pixel);
        SizeF sizeF = g.MeasureString(str, font); //测量出字体的高度和宽度
        Brush brush; //笔刷，颜色
        //brush = Brushes.Lime;
        brush = new SolidBrush(color);
        PointF pf = new PointF(0, 0);
        Bitmap img = new Bitmap(Convert.ToInt32(sizeF.Width), Convert.ToInt32(sizeF.Height), PixelFormat.Format32bppArgb);
        g = DrawingGraphics.FromImage(img);
        g.DrawString(str, font, brush, pf);
        //输出图片
        MemoryStream ms = new MemoryStream();
        img.Save(ms, ImageFormat.Png);
        byte[] bytes = ms.ToArray();

        Texture2D tex = new Texture2D(img.Width, img.Height, TextureFormat.RGBA32, false);
        tex.LoadImage(bytes);
        tex.Apply();

        return tex;
    }

    // 合并Texture
    public static Texture2D MergeTexture(Texture2D src, Texture2D dst, int dstX, int dstY, UnityColor color)
    {
        //克隆一个副本
        Texture2D tex = new Texture2D(dst.width, dst.height, TextureFormat.RGBA32, false);
        tex.SetPixels(0, 0, dst.width, dst.height, dst.GetPixels());
        //tex.SetPixels(dstX, dstY, src.width, src.height, src.GetPixels());
        for (int y = 0; y < src.height; y++)
        {
            for (int x = 0; x < src.width; x++)
            {
                UnityColor src_color = src.GetPixel(x, y);
                if (Mathf.Approximately(src_color.a, 0f))
                    continue;
                tex.SetPixel(dstX + x, dstY + y, color);
            }
        }
        tex.Apply();
        return tex;
    }
}

Android平台文字转图片（将下面的工具类封装成jar或aar，在Unity中调用即可）

package com.deltalpha.image;
import android.graphics.Bitmap;
import android.graphics.Bitmap.CompressFormat;
import android.graphics.Paint;
import android.graphics.Canvas;
import android.text.TextPaint;
import android.util.Log;

import java.io.ByteArrayOutputStream;

public class TextToImage
{
    public int imageWidth;
    public int imageHeight;

    // Text转Bitmap
    public Bitmap generateBitmap(String text, int textColor, int textSize, int alpha)
    {
        // 设置基本属性
        TextPaint textPaint = new TextPaint();
        textPaint.setColor(textColor);//0xff0000
        textPaint.setTextSize(textSize);
        textPaint.setAntiAlias(true);//抗锯齿
        textPaint.setAlpha(alpha);//0-255

        // 获取文字尺寸
        Paint.FontMetrics fm = textPaint.getFontMetrics();
        float textWidth = textPaint.measureText(text);
        float textHeight = fm.descent - fm.ascent;
        imageWidth = (int)Math.ceil(textWidth);
        imageHeight = (int)Math.ceil(textHeight);

        // 生成图片
        Bitmap bitmap = Bitmap.createBitmap(imageWidth, imageHeight, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        canvas.drawText(text, 0f, textHeight, textPaint);

        return bitmap;
    }

    // Text转Bitmap bytes （Unity中调用此接口获取图片数据）
    public byte[] generatePngBytes(String text, int textColor, int textSize, int alpha)
    {
        Bitmap bitmap = generateBitmap(text, textColor, textSize, alpha);
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        bitmap.compress(CompressFormat.PNG, 100, baos);
        return baos.toByteArray();
    }

    // 打印日志
    public void printLog(final String message){
        Log.d("alpha", message);
    }
}