PixelPeekScript.cs

/*
 * Copyright 2022 Marcin Swiderski
 * 
 * The MIT Licence (MIT)
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

using UnityEditor;
using UnityEngine;
using UnityEngine.InputSystem;
using UnityEngine.Rendering;

[ExecuteAlways]
public class PixelPeekScript : MonoBehaviour
{
#if UNITY_EDITOR
    private RenderTexture _renderTexture;
    private Texture2D _peekTexture;
    private Color _peekPixel;
    
    private void OnEnable()
    {
        RenderPipelineManager.beginCameraRendering += OnBeginCameraRendering;
        RenderPipelineManager.endCameraRendering += OnEndCameraRendering;
        _peekTexture = new Texture2D(1, 1);
        _peekPixel = Color.black;
    }

    private void OnDisable()
    {
        RenderPipelineManager.beginCameraRendering -= OnBeginCameraRendering;
        RenderPipelineManager.endCameraRendering -= OnEndCameraRendering;
        ReleaseRenderTexture();
        _peekTexture = null;
    }

    private void OnGUI()
    {
        float rf = _peekPixel.r;
        float gf = _peekPixel.g;
        float bf = _peekPixel.b;
        
        int ri = (int)(rf * 255.0f);
        int gi = (int)(gf * 255.0f);
        int bi = (int)(bf * 255.0f);

        float flt = rf + gf / 255.0f;

        string rgbH = $"RGB(hex): {ri << 16 | gi << 8 | bi << 0:x6}";
        string rgbI = $"RGB(int): {ri}, {gi}, {bi}";
        string rgbF = $"RGB(float): {rf}, {gf}, {bf}";
        string rgFenc = $"Float(enc): {flt}";

        GUIStyle style = new GUIStyle(EditorStyles.textArea);
        style.fontSize = 24;
        GUILayout.Label($"{rgbH}\n{rgbI}\n{rgbF}\n{rgFenc}", style);
    }

    private void OnBeginCameraRendering(ScriptableRenderContext context, Camera cam)
    {
        if (cam == GetComponent<Camera>()) {
            EnsureRenderTexture(cam.pixelWidth, cam.pixelHeight);
            cam.targetTexture = _renderTexture;
        }
    }
    
    private void OnEndCameraRendering(ScriptableRenderContext context, Camera cam)
    {
        if (_renderTexture != null && cam.targetTexture == _renderTexture) {
            cam.targetTexture = null;
            Graphics.Blit(_renderTexture, (RenderTexture)null);

            Vector2 cursor = Mouse.current.position.ReadValue();
            int screenXMax = cam.pixelWidth - 1;
            int screenYMax = cam.pixelHeight - 1;
            int pixelX = Mathf.Clamp((int)cursor.x, 0, screenXMax);
            int pixelY = screenYMax - Mathf.Clamp((int)cursor.y, 0, screenYMax);
            
            RenderTexture currentRT = RenderTexture.active;
            RenderTexture.active = _renderTexture;
            _peekTexture.ReadPixels(new Rect(pixelX, pixelY, 1, 1), 0, 0);
            _peekPixel = _peekTexture.GetPixel(0, 0);
            RenderTexture.active = currentRT;
        }
    }

    private void EnsureRenderTexture(int width, int height)
    {
        if (_renderTexture == null || _renderTexture.width != width || _renderTexture.height != height) {
            ReleaseRenderTexture();
            _renderTexture = RenderTexture.GetTemporary(width, height, 24);
        }
    }

    private void ReleaseRenderTexture()
    {
        if (_renderTexture != null) {
            RenderTexture.ReleaseTemporary(_renderTexture);
            _renderTexture = null;
        }
    }
#endif // UNITY_EDITOR
}

Just attach it to your main camera and enable/disable it in the hierarchy when needed. Tested with URP.