Risk of Rain 2's UI uses Unity's UI System. As such, getting familiar with the Unity UI system is key.

Some mods may require a visual representation of data on screen for the user. For example, the amount of health left in the form of a health bar, the amount of ammo left, or maybe even a custom data.

Rects are put onto a Canvas, and the entire Canvas is rendered over everything in the game. The Canvas resizes to the screen and resizes the Rects in it as well to fit the new screen size.

Assuming you've setup your coding environment (if not, please follow this guide), let's get started.

1. Hook onto RoR2.HUD (or any particular UI elements') Awake function and store the HUD instance.
2. Create a GameObject. Add your new GameObject to the HUD container. Add a RectTransform component to it, and set anchorMin, anchorMax, sizeDelta and anchoredPosition accordingly.
3. Add an Image component (load your sprite into GetComponent<Image>().sprite).

Hook onto RoR2.HUD (or any particular UI elements') Awake function and store the instance.

As a general example, we will hook to HUD.

private HUD hud = null;

private void Awake()
{
On.RoR2.UI.HUD.Awake += MyFunc;
}

Now that we have hooked onto the HUD component, I just need to store the instance.

private void MyFunc(On.RoR2.UI.HUD.orig_Awake orig, RoR2.UI.HUD self)
{
    orig(self); // Don't forget to call this, or the vanilla / other mods' codes will not execute!
    hud = self;
    //hud.mainContainer.transform // This will return the main container. You should put your UI elements under it or its children!
    // Rest of the code is to go here
}

Don't forget to unhook OnDestroy!

private void OnDestroy()
{
On.RoR2.UI.HUD.Awake -= MyFunc;
}

Create a GameObject. Add your new GameObject to the UI element instance. Add a RectTransform component to it, and set anchorMin, anchorMax, sizeDelta and anchoredPosition accordingly.

This step is really important: it determines where the UI element is going to be and anchor it there. For now, understand this:

• The anchor positions are normalized from 0 to 1, and the bottom-left is (0, 0), top-right is (1, 1).
• Setting anchoredPosition is essentially moving the GameObject to translate by a certain amount, relative to its anchor.
• sizeDelta is the difference in size of the object relative to the anchors.

GameObject myObject = new GameObject("GameObjectName");
myObject.transform.SetParent(hud.mainContainer.transform);
RectTransform rectTransform = myObject.AddComponent<RectTransform>();
rectTransform.anchorMin = Vector2.zero;
rectTransform.anchorMax = Vector2.one;
rectTransform.sizeDelta = Vector2.zero;
rectTransform.anchoredPosition = Vector2.zero;
// You can safely omit the last line if the element is already where you want it to be.
// This collection of lines will fill the canvas with this Rect.

Creating GameObject with the name GameObjectName

Adding it to the HUD (typically a Canvas)

Adding a RectTransform

Setting its parameters

Add an Image component (load your sprite into GetComponent<Image>().sprite).

GameObjectReference.AddComponent<Image>();
GameObjectReference.GetComponent<Image>().sprite = Resources.Load<Sprite>("textures/itemicons/texBearIcon");
// Utilize the ResourcesAPI from R2API to load your image!

Adding a Image component

Setting the sprite

Get the Unity editor (it's free!), create a Canvas and start to create some Images, see what you can do with the Unity UI system. Feel free to experiment and try to recreate the steps you took in code.

• Unity UI documentation (Packages)
• Unity UI documentation (Manual)