Inconsistencies rendering in a RenderTexture in a Coroutine

Questions & Answers
1

I’ve been trying for the past few days to render a texture in a coroutine and stumbled onto quite weird behaviours:

For example if the coroutine do not wait before rendering or wait for 1 or 3 frames it does so correctly.
But, it it wait for 2 or 4+ frames, it just does not render at all what I’m trying to render.

Does anyone has any idea why this is happening ?

Below is the script and Shader used for that

MyScript.cs:

using System.Collections;
using UnityEngine;

public class MyScript : MonoBehaviour
{
	public MeshRenderer myRenderer;
	public RenderTexture myRenderTexture;
	public Material myMaterial;

	[Space] public Mode myTestMode;

	public enum Mode
	{
		Seconds = 0,
		NbYields = 1,
	}

	public float mySecondsWaiting;
	public int myMaxIter;
	public bool myDoInUpdate;

	private Texture2D MyTexture2D;

	private void Start()
	{
		StartCoroutine(MyCoroutine());
	}

	private void Update()
	{
		if (myDoInUpdate)
			MyFunction();
	}

	private void MyFunction(Camera cam = null)
	{
		// Draw to Texture
		myMaterial.SetTexture("_MainTex", null);
		//m_Material.SetColor("_DrawColor", m_DrawColor);

		RenderTexture previous = RenderTexture.active;
		RenderTexture.active = myRenderTexture;

		GL.Clear(true, true, Color.clear, 0);
		GL.PushMatrix();
		GL.LoadIdentity();
		GL.LoadOrtho();

		myMaterial.SetPass(0);

		Mesh newM = new Mesh
		{
			vertices = new[] {Vector3.zero, Vector3.right, Vector3.forward},
			triangles = new[] {0, 1, 2},
			uv = new[] {Vector2.zero, Vector2.right, Vector2.up}
		};
		newM.RecalculateNormals();
		newM.RecalculateTangents();

		Graphics.DrawMeshNow(newM, Vector3.forward, Quaternion.Euler(-90, 0, 0));

		GL.PopMatrix();
		GL.Flush();

		RenderTexture.active = previous;

		myRenderer.sharedMaterial.mainTexture = myRenderTexture;

		Destroy(newM);
	}

	private IEnumerator MyCoroutine()
	{
		switch (myTestMode)
		{
			case Mode.Seconds:
				yield return new WaitForSeconds(mySecondsWaiting);
				break;

			case Mode.NbYields:
				for (int i = 0; i < myMaxIter; ++i)
					yield return null;
				break;
		}

		MyFunction();
		yield break;
	}
}

MyShader.shader:

Shader "Painter"
{
	Properties
	{
		_MainTex("Texture", 2D) = "white" {}
		_DrawColor("Color", Color) = (1.0, 1.0, 1.0, 1.0)
	}
	SubShader
	{
		Tags { "RenderType" = "Transparent"  }

		// No culling or depth
		Cull Off ZWrite Off ZTest Always

		CGINCLUDE

		#include "UnityCG.cginc"

		// Main input texture
		sampler2D _MainTex;

		//x: 1.0 / width
		//y: 1.0 / height
		//z: width
		//w: height
		uniform half4 _MainTex_TexelSize;
		float4 _PixelSize;

		// Basic drawing settings
		float4 _DrawColor;

		struct appdata
		{
			float4 vertex : POSITION;
			float2 uv : TEXCOORD0;
		};

		struct v2f
		{
			float2 uv : TEXCOORD0;
			float4 vertex : SV_POSITION;
		};

		v2f vert(appdata v)
		{
			v2f o;
			o.vertex = UnityObjectToClipPos(v.vertex);
			o.uv = v.uv;
			return o;
		}

		ENDCG
		
		// Simple color
		Pass
		{
			Blend SrcAlpha OneMinusSrcAlpha

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			fixed4 frag(v2f i) : SV_TARGET {
				return _DrawColor;
			}
			ENDCG
		}
	}
}

EDIT: Tested in 2018.4.17f1

EDIT: Tested in 2019.2.19f1, found another weird behavior that if the material which will receive the main texture is selected, the texture is correctly rendered.

EDIT: Tested in 2019.3.2f1, texture is not rendered if frames are skipped, not rendered if material is selected.

2

Forgot that GL calls are completely immediate.
Switched to CommandBuffer calls fixed it.

i.e. :

CommandBuffer buffer = new CommandBuffer();

buffer.SetRenderTarget(myRenderTexture);
buffer.ClearRenderTarget(true, true, Color.clear);
buffer.SetViewProjectionMatrices(Matrix4x4.identity, Matrix4x4.Ortho(0, 1, 0, 1, -1, 100));

buffer.DrawMesh(m, Matrix4x4.Rotate(Quaternion.Euler(-90, 0, 0)), myMaterial, 0, 3));

Graphics.ExecuteCommandBuffer(buffer);

myRenderer.sharedMaterial.mainTexture = myRenderTexture;