Question Flipped RenderTexture Y coordinate when reading it with Texture2D.ReadPixels

I'm writing a simple color picker from a quad mesh that contains a RenderTexture.
When I read just a single pixel from the render texture, it's Y coordinate is flipped (on Windows DirectX and some other platforms too).
But when I read the whole Rect, the problem does not occur.

Here's the code:

Code (CSharp):

1. using UnityEngine;
2.  
3. public class ColorPicker : MonoBehaviour
4. {
5.     const int RTWidth = 3;
6.     const int RTHeight = 3;
7.  
8.     [SerializeField] private Texture2D _debugColorsTexture;
9.     [SerializeField] private Color _pickedColor;
10.     [SerializeField] private Color _pickedColorIncorrect;
11.  
12.     private Renderer _quadRenderer;
13.     private Texture2D _texture1x1;
14.     private Texture2D _textureFULL;
15.     private RenderTexture _renderTexture;
16.  
17.     private void Start()
18.     {
19.         _quadRenderer = GetComponent<Renderer>();
20.         _texture1x1 = new Texture2D(1, 1, TextureFormat.RGB24, false);
21.         _textureFULL = new Texture2D(RTWidth, RTHeight, TextureFormat.RGB24, false);
22.  
23.         _renderTexture = new RenderTexture(RTWidth, RTHeight, 0);
24.         _renderTexture.filterMode = FilterMode.Point;
25.  
26.         RenderTexture.active = _renderTexture;
27.         Graphics.Blit(_debugColorsTexture, _renderTexture);
28.         RenderTexture.active = null;
29.  
30.         _quadRenderer.material.SetTexture("_MainTex", _renderTexture);
31.     }
32.  
33.     private void Update()
34.     {
35.         Ray cameraRay = Camera.main.ScreenPointToRay(Input.mousePosition);
36.         if (Physics.Raycast(cameraRay, out RaycastHit hit, maxDistance: 100f))
37.         {
38.             Vector2 uv = hit.textureCoord;
39.  
40.             _pickedColor = PickColor(_renderTexture, uv);
41.             _pickedColorIncorrect = PickColorIncorrect(_renderTexture, uv);
42.         }
43.     }
44.  
45.     private Color PickColor(RenderTexture rt, Vector2 uv)
46.     {
47.         int x = Mathf.Min((int)(Mathf.Clamp01(uv.x) * RTWidth), RTWidth - 1);
48.         int y = Mathf.Min((int)(Mathf.Clamp01(uv.y) * RTHeight), RTHeight - 1);
49.  
50.         RenderTexture.active = rt;
51.         Rect textureRect = new Rect(0, 0, RTWidth, RTHeight);
52.         _textureFULL.ReadPixels(textureRect, 0, 0);
53.         _textureFULL.Apply();
54.         RenderTexture.active = null;
55.  
56.         return _textureFULL.GetPixel(x, y);
57.     }
58.  
59.     private Color PickColorIncorrect(RenderTexture rt, Vector2 uv)
60.     {
61.         int x = Mathf.Min((int)(Mathf.Clamp01(uv.x) * RTWidth), RTWidth - 1);
62.         int y = Mathf.Min((int)(Mathf.Clamp01(uv.y) * RTHeight), RTHeight - 1);
63.  
64.         RenderTexture.active = rt;
65.         Rect textureRect = new Rect(x, y, 1, 1);
66.         _texture1x1.ReadPixels(textureRect, 0, 0); // <--- Possible problem occurs here
67.         _texture1x1.Apply();
68.         RenderTexture.active = null;
69.  
70.         return _texture1x1.GetPixel(0, 0);
71.     }
72. }
73.  

At first I thought, that this problem is related to target Graphics API, because each API defines it's own texture coordinate space. But when I tested this project on 2 different Android devices (Huawei Honor 9, Huawei P20 Lite), they gave opposite Y values. My Unity version is 2021.3.16f1

Does anyone know why is the texture Y coordinate flipped, when using the PickColorIncorrect method?

Attaching .unitypackage if you want to take a look at this. Thanks in advance.

 

When SystemInfo.graphicsUVStartsAtTop is true, Unity has to render to render targets upside down. This is the case for DirectX but not OpenGL.

 

Thanks a lot! This solved my problem.