Unity immediate mode

Question

I have some existing C++ 2D game code that draws using immediate mode (i.e., each frame is drawn bottom-up one sprite at a time by my game code -- not using a scene graph or other retained mode concepts). I'm wondering if I can port this code to Unity without having to rework the game code to operate in terms of retained mode (adding objects to a scene, removing objects, etc.). I have read that Unity has an immediate mode GUI option. But in my case I want to use this for the game sprites themselves, not just the UI elements. I realize that for a 2D game there isn't a big difference between UI elements and game sprites. But I need to be able to do things like rotation and render-to-texture, so I wasn't sure if I'd run into trouble "misusing" the immediate mode GUI mechanism to handle the whole game. Alternatively, does Unity have a more general immediate mode option? (I did search for it and didn't see anything aside from the immediate mode GUI stuff.)

Answer 1

First of all, you should not be doing this.

The Unity scene graph exists for a reason. It's the very core concept which the engine architecture is built around. By rolling your own you are locking yourself out of approximately 99% of the features Unity provides to you. You won't really gain much over just keeping your codebase in C++. If this is just about portability, there are ways to make a C++ codebase portable to multiple platforms (but that's out of the scope of this question).

When you switch to a different technology to do the same thing you were already doing, then your first instinct is often to try to find a way to do everything in exactly the same way you used to do. But that's usually a bad approach, because you won't be using the tool in the way it was intended to be used. It's a bit like a carpenter switching from hammer and nails to screwdriver and screws, trying to drive the screws into the wood by repeatedly hitting them with the screwdriver, having a bad time and awful results and then wondering why everyone claims that screws are a viable alternative to nails.

A far better way to learn a new tool is to approach it with no preconception about how it is supposed to be used, familiarize yourself with its design choices and philosophies and embrace the differences rather than the similarities.

Still wanting to do it that way?

If you really want to manually draw sprites directly to the screen, then Unity can do that with the method Graphics.DrawTexture (the rest of the Graphics class might also come in handy for you).

It will draw a specific texture directly to specific screen coordinates, completely ignoring the camera position and settings, transform of the gameObject, z-buffering, post processing, UI-layout, and all the other nice features you pay for with your Unity license.

Create a Monobehaviour and implement the OnGui method to draw your sprites with Graphics.DrawTexture in the order you want. Here is the example which draws a sprite 2000 times per frame to random points in a 400x400 rectangle. Regarding performance: It runs at over 200 FPS on my machine:

using UnityEngine;

public class TextureRenderer : MonoBehaviour
{
    public Texture sprite;    
    public int numOfSprites = 2000;

    void OnGUI() {
        if (Event.current.type.Equals(EventType.Repaint)) {
            for (var i = 0; i < numOfSprites; i++) {    
                Graphics.DrawTexture(new Rect(
                                         Random.Range(1f, 400f), // x-position
                                         Random.Range(1f, 400f), // y-position
                                         64f, // width
                                         64f  // height
                                     ), 
                                     sprite);
            }
        }
    }
}