How to reproduce:
# Open the user-attached project (BlitBug.zip)
# In the “Project” panel open and edit the “TestShader” file
# Modify line 49 by changing {{float2(dx, 0)}} to {{float2(-dx, 0)}}
# In the Editor enter Play Mode
# Switch to the Scene view and observer the “Render Texture Preview“ Game Object

Expected result: the Game Object’s texture is a single solid color

Actual result: the Game Object’s texture has three dark lines going vertically and one diagonal line (when using Vulkan API there are flickering dark squares too)

Reproducible with: 2021.3.6f1, 2022.1.7f1, 2022.2.0a17, 2023.1.0a2

Could not test with: 2020.3.36f1 (due to shader compilation errors)

Reproducible on: Ubuntu 20.04 and Windows 10

Note: The issue is present on both OpenGLCore and Vulkan graphics APIs

Screenshot of the lines visible when using the OpenGLCore API:

!DemoScreenshot.png!

 
h2. Note after investigation:

This is an undefined behaviour to set source and destination texture to the same one according to the [documentation|https://docs.unity3d.com/ScriptReference/Graphics.Blit.html].

In the user's C# script:
{code:java}
_material.SetTexture(Shader.PropertyToID("_BaseMap"), RenderTexture.active);
Graphics.Blit(RenderTexture.active, camera.targetTexture, _material); {code}
_RenderTexture.active_ and _camera.targetTexture_ are actually the same texture. It's also used in the blit material as a shader property.