This is a raw paste of the screenshot utility I use for debugging purposes in Masteroid. This demonstrates how to render to a buffer (render target) and save that buffer as a png in the FlatRedBall game engine.

Masteroid Screenshot Utility for FlatRedBall

using FlatRedBall;
using FlatRedBall.Graphics;
using Microsoft.Xna.Framework.Graphics;
using System;
using System.IO;
using System.Text.RegularExpressions;

namespace Masteroid.Utilities
{
    public class ScreenshotUtility
    {
        private static ScreenshotUtility instance;

        public static ScreenshotUtility Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new ScreenshotUtility();
                }

                return instance;
            }
        }

        public string SavePath { get; set; }

        private ScreenshotUtility()
        {
            SavePath = Environment.GetFolderPath(Environment.SpecialFolder.MyPictures);
        }

        public void SaveScreenshotPng(Camera camera = null)
        {

            if (!FlatRedBallServices.IsInitialized)
            {
                throw new Exception("FlatRedBall must be fully initialized before you can take a screenshot!");
            }

            camera = camera ?? Camera.Main;
            var gfx = FlatRedBallServices.GraphicsDevice;
            var width = FlatRedBallServices.GraphicsOptions.ResolutionWidth;
            var height = FlatRedBallServices.GraphicsOptions.ResolutionHeight;
            var buffer = new RenderTarget2D(
                FlatRedBallServices.GraphicsDevice,
                width,
                height);

            // get existing render target
            var existingTargets = gfx.GetRenderTargets();

            // render
            gfx.SetRenderTarget(buffer);
            Renderer.DrawCamera(camera, null);

            // restore render targets
            gfx.SetRenderTargets(existingTargets);

            var path = Path.Combine(SavePath, GetFilename());

            if (File.Exists(path))
            {
                throw new Exception($"Existing pathname generated: {path}");
            }

            using (Stream stream = System.IO.File.Create(path))
            {
                buffer.SaveAsPng(stream, width, height);
            }
        }

        private string GetFilename()
        {
            var timestamp = Regex.Replace(DateTime.Now.ToString("G"), "[^0-9]", "_");
            return $"masteroid_{timestamp}.png";
        }
    }
}