ThreadedTexture2D.cs

using UnityEngine;
using FreeImageAPI;
using System.Threading;
using System;

public class ThreadedTexture2D : MonoBehaviour
{
    // The actual Texture
    public Texture2D tex;

    // Dimensions of image to be loaded (must match size on disk)
    public int importHeight = 4096;
    public int importWidth = 4096;

    // Raw texture data
    private byte[] texData;

    // is thread done?
    private bool isDone = false;

    // Actual thread
    private Thread thread;

    // EVENTS:
    // Loading has started but hasn't finished yet
    public event Action OnLoading;
    // The Texture2D is now ready to use
    public event Action<Texture2D> OnLoaded;
    
    private void Awake()
    {
        // Converted texture data, 32bpp BGRA32
        texData = new byte[importHeight * importWidth * 4];

        // Texture2D to load it into, also BGRA32
        tex = new Texture2D(importWidth, importHeight, TextureFormat.BGRA32, false, false);
        tex.filterMode = FilterMode.Point;
    }

    public void Load(string path)
    {
        // Trigger event to signify the texture is currently being loaded
        OnLoading?.Invoke();

        // Create thread
        thread = new Thread(OnThread);

        // Make sure it's a background thread so it's aborted
        thread.IsBackground = true;

        // Start thread to load png texture async
        thread.Start(path);
    }

    private void OnThread(object path)
    {
        // Load png into FreeImage bitmap
        FIBITMAP fiBitmap = FreeImage.Load(FREE_IMAGE_FORMAT.FIF_PNG, (string)path, FREE_IMAGE_LOAD_FLAGS.DEFAULT);

        // Write it into texData as 32bppp BGRA32. Params 5, 6, 7 have no effect all on 32bpp formats
        FreeImage.ConvertToRawBits(texData, fiBitmap, (int)FreeImage.GetPitch(fiBitmap), 32, FreeImage.FI16_555_BLUE_MASK, FreeImage.FI_RGBA_RED_MASK, FreeImage.FI_RGBA_BLUE_MASK, false);

        // Can be verified with FreeImage.GetDIBSize(fiBitmap);
        FreeImage.Unload(fiBitmap);

        // Thread is done
        isDone = true;
    }

    private void Update()
    {
        // When thread has finished begin creating textures
        if (isDone) CreateTextures();
    }

    private void CreateTextures()
    {
        // Load texData into tex2D as raw
        tex.LoadRawTextureData(texData);

        // Send texture to GPU
        tex.Apply(false, true);

        // Reset isDone
        isDone = false;

        // Trigger event to signify texture is ready
        OnLoaded?.Invoke(tex);
    }
}

Wow this is really neat!

I was trying to implement this myself, but got stuck at building my project.
Could you tell me which Library your FreeImageAPI is coming from?
Currently I'm trying to get a custom for mono build of the FreeImageNET project to run, but I struggle with IL2CPP builds :/

Thanks beforehand!