Render material to full screen quad

Hi,

I'm trying to render a material to a full-screen quad, and then saving the image to a Texture2D.

This is my current implementation, which works :

 public Material material;
 public int width = 1024, height = 1024;
 public Texture2D noiseTex;
 private RenderTexture heightmap;
 
 void Start()
 {
     heightmap = new RenderTexture(width, height, 24);
     noiseTex = new Texture2D(width, height, TextureFormat.ARGB32, false);
 }
 
 void Update()
 {
     GetHeightMap();
 }
 
 void GetHeightMap()
 {
     // Create an orthographic camera that points to the created plane below
     GameObject renderCamera = new GameObject();
     renderCamera.AddComponent<Camera>();
     renderCamera.GetComponent<Camera>().backgroundColor = Color.black;
     renderCamera.GetComponent<Camera>().orthographic = true;
     renderCamera.GetComponent<Camera>().orthographicSize = 5;
     renderCamera.GetComponent<Camera>().transform.localPosition = new Vector3(9999, 10009, 9999);
     renderCamera.GetComponent<Camera>().transform.localRotation = Quaternion.Euler(new Vector3(90.0f, 0.0f, 0.0f));
 
     // Create a plane that occupies all the created camera's view and assign the material
     GameObject renderPlane = GameObject.CreatePrimitive(PrimitiveType.Plane);
     renderPlane.transform.localPosition = new Vector3(9999, 9999, 9999);
     renderPlane.transform.localRotation = Quaternion.Euler(new Vector3(0, 0, 0));
     renderPlane.GetComponent<MeshRenderer>().material = material;
 
     RenderTexture.active = heightmap;
 
     // Render a frame and save it to a Texture2D
     renderCamera.GetComponent<Camera>().targetTexture = heightmap;
     renderCamera.GetComponent<Camera>().Render();
     noiseTex.ReadPixels(new Rect(0.0f, 0.0f, heightmap.width, heightmap.height), 0, 0);
     renderCamera.GetComponent<Camera>().targetTexture = null;
     noiseTex.Apply();
 
     // Clean up
     RenderTexture.active = null;
     DestroyImmediate(renderPlane);
     DestroyImmediate(renderCamera);
 }

Now, I know this is not the best way to achieve this goal. I tried with the following method, which I think is the standard way to render a full-screen material, but the output Texture2D is always blank (completely black) :

 void GetHeightMap2()
 {
     GameObject renderCamera = new GameObject();
     renderCamera.AddComponent<Camera>();
     renderCamera.camera.backgroundColor = Color.white;
     renderCamera.camera.orthographic = true;
     renderCamera.camera.orthographicSize = 5;

     RenderTexture.active = heightmap;

     renderCamera.camera.targetTexture = heightmap;
 
     GL.PushMatrix();
     for (int i = 0; i < material.passCount; i++)
     {
         material.SetPass(i);
         GL.LoadOrtho();
         GL.Viewport(new Rect(0, 0, 1024, 1024));
 
         GL.Begin(GL.QUADS);
         GL.TexCoord2(0, 0);
         GL.Vertex3(0.0F, 0.0F, 0);
         GL.TexCoord2(0, 1);
         GL.Vertex3(0.0F, 1.0F, 0);
         GL.TexCoord2(1, 1);
         GL.Vertex3(1.0F, 1.0F, 0);
         GL.TexCoord2(1, 0);
         GL.Vertex3(1.0F, 0.0F, 0);
         GL.End();
     }
     GL.PopMatrix();
 
     // Read pixels
     noiseTex.ReadPixels(new Rect(0.0f, 0.0f, heightmap.width, heightmap.height), 0, 0);
     noiseTex.Apply();
 
     // Clean up
     renderCamera.camera.targetTexture = null;
     RenderTexture.active = null;
     DestroyImmediate(renderCamera);
 }

Shouldn't the two methods be equivalent? What am I missing?

Thanks for your help!