Initial version. Uses external library for image resizing. Fixed memory consumption, but slow.

Assets/Scripts/PhotoFrame.cs

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+using System.Collections;
+using System.IO;
+using UnityEngine;
+using UnityEngine.Networking;
+using UnityEngine.UI;
+using Image = UnityEngine.UI.Image;
+using Color = UnityEngine.Color;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp;
+
+public class PhotoFrame : MonoBehaviour
+{
+    [SerializeField]
+    GameObject mainUI;
+
+	[SerializeField]
+	RectTransform imageContainer;
+    [SerializeField]
+    Image[] images;
+
+    [SerializeField]
+    InputField url;
+
+	private int currentImage;
+
+	private Image CurrentImage
+	{
+		get { return images[currentImage]; }
+	}
+
+	private Image NextImage
+	{
+		get { return images[(currentImage + 1) % images.Length]; }
+	}
+
+	public void Start()
+	{
+		currentImage = 0;
+		ActivateUI(true);
+	}
+
+	private void ActivateUI(bool v)
+	{
+		mainUI.gameObject.SetActive(v);
+		for (int i = 0; i < images.Length; i++)
+		{
+			images[i].gameObject.SetActive(!v);
+		}
+	}
+
+	Coroutine mainCoroutine;
+
+	public void OnRunButtonClicked()
+	{
+		ActivateUI(false);
+		mainCoroutine = StartCoroutine(SwitchImages());
+	}
+
+	public void OnStopButtonClicked()
+	{
+		if (mainCoroutine != null)
+		{
+			StopCoroutine(mainCoroutine);
+			mainCoroutine = null;
+		}
+		for (int i = 0; i < images.Length; i++)
+		{
+			if (images[i].sprite != null)
+			{
+				Destroy(images[i].sprite.texture);
+				Destroy(images[i].sprite);
+				images[i].sprite = null;
+			}
+		}
+		ActivateUI(true);
+	}
+
+	private IEnumerator SwitchImages()
+	{
+		yield return null;
+		var loader = new ImageLoader(url.text);
+		while (loader.Status == LoaderStatus.Initializing) yield return null;
+		Color opaque = new Color(1, 1, 1, 1);
+		Color transparent = new Color(1, 1, 1, 0);
+		CurrentImage.color = transparent;
+		NextImage.color = transparent;
+		while (true)
+		{
+			int imageIndex = Random.Range(0, loader.images.Count);
+			string imageUrl = loader.images[imageIndex];
+
+			var path = Application.persistentDataPath + "/source." + System.IO.Path.GetExtension(imageUrl);
+
+			var request = UnityWebRequest.Get(imageUrl);
+			request.downloadHandler = new DownloadHandlerFile(path);
+			yield return request.SendWebRequest();
+
+			if (request.isNetworkError || request.isHttpError)
+			{
+				yield return new WaitForSeconds(1f);
+				continue;
+			}
+
+			SetImageTexture(NextImage, path);
+			request.downloadHandler.Dispose();
+			request.Dispose();
+
+			float currentStart = CurrentImage.color.a;
+			float currentFinish = transparent.a;
+			float nextStart = NextImage.color.a;
+			float nextFinish = opaque.a;
+			float startTime = Time.time;
+			while (Time.time < startTime + 2f)
+			{
+				float ct = Time.time;
+				float x = (ct - startTime) / 2f;
+				float currentA = currentStart + x * (currentFinish - currentStart);
+				Color cC = new Color(CurrentImage.color.r, CurrentImage.color.g, CurrentImage.color.b, currentA);
+				CurrentImage.color = cC;
+				float nextA = nextStart + x * (nextFinish - nextStart);
+				Color nC = new Color(NextImage.color.r, NextImage.color.g, NextImage.color.b, nextA);
+				NextImage.color = nC;
+				yield return null;
+			}
+			yield return new WaitForSeconds(5f);
+
+			currentImage = (currentImage + 1) % images.Length;
+			Resources.UnloadUnusedAssets();
+			System.GC.Collect();
+		}
+	}
+
+	private void SetImageTexture(Image nextImage, string path)
+	{
+		if (nextImage.sprite != null)
+		{
+			Destroy(nextImage.sprite.texture);
+			Destroy(nextImage.sprite);
+			nextImage.sprite = null;
+		}
+
+		float imageWidth = imageContainer.rect.width;
+		float imageHeight = imageContainer.rect.height;
+		//
+		int maxHeight, maxWidth;
+		if (imageWidth < imageHeight)
+		{
+			// scale to height
+			maxHeight = 2048;
+			maxWidth = (int)(2048 / imageHeight * imageWidth);
+		}
+		else
+		{
+			// scale to width
+			maxWidth = 2048;
+			maxHeight = (int)(2048 / imageWidth * imageHeight);
+		}
+		Texture2D tex = new Texture2D(2, 2);
+		tex.LoadImage(File.ReadAllBytes(path));
+		float factor = Mathf.Min((float) maxWidth / tex.width, (float)maxHeight / tex.height);
+		int newWidth = (int)(tex.width * factor);
+		int newHeight = (int)(tex.height * factor);
+		Destroy(tex);
+		var newPath = Scale(path, newWidth, newHeight);
+		tex = new Texture2D(2, 2);
+		tex.LoadImage(File.ReadAllBytes(newPath));
+		//TextureScale.Bilinear(tex, newWidth, newHeight);
+		//TextureScale.Point(tex, newWidth, newHeight);
+		//Color[] cs = tex.GetPixels();
+		var sprite = Sprite.Create(tex, new Rect(0, 0, tex.width, tex.height), Vector2.zero);
+		nextImage.sprite = sprite;
+		nextImage.rectTransform.anchorMin = new Vector2((maxWidth - (float)newWidth) / maxWidth / 2, (maxHeight - (float)newHeight) / maxHeight / 2);
+	}
+
+	private string Scale(string path, int newWidth, int newHeight)
+	{
+		var resultPath = Path.GetDirectoryName(path) + "/target." + Path.GetExtension(path);
+		using (SixLabors.ImageSharp.Image image = SixLabors.ImageSharp.Image.Load(path))
+		{
+			image.Mutate(x => x.Resize(newWidth, newHeight));
+			image.Save(resultPath);
+		}
+		return resultPath;
+	}
+}