SetRenderTarget(null) clearing previous draw calls

Question

I'm not sure if everything's working as it should be or whether I'm just not doing something properly, but I couldn't find anything that discussed this so I thought I'd ask here.

I'm using Monogame 3.2 for Windows.

In my game I'm drawing sprites to the back buffer, then I set a render target and draw to that, then I switch back to the back with SetRenderTarget(null) so I can draw my render target but I find that all the drawing I did before I set the render target the first time has been cleared. Is this just how it works or have I done something wrong?

EDIT: The pink rectangle is the render target.

Here is example code:

        // Render some stuff first
        spriteBatch.Begin();
        spriteBatch.Draw(mPixel, new Rectangle(10, 10, 500, 50), Color.Red);
        spriteBatch.End();

        // Render to target
        GraphicsDevice.SetRenderTarget(renderTarget);
        GraphicsDevice.Clear(Color.Pink);
        spriteBatch.Begin();
        spriteBatch.Draw(mPixel, new Rectangle(10, 10, 500, 50), Color.Blue);
        spriteBatch.End();

        // Render back to backbuffer
        GraphicsDevice.SetRenderTarget(null);
        spriteBatch.Begin();
        spriteBatch.Draw(renderTarget, Vector2.Zero, Color.White);
        spriteBatch.End();

Here is the result, I expected the first red rectangle to remain behind the render target:

Answer 1

Having the exactly same issue here - the following code snipped works perfectly fine:

private void DoRenderSkybox (GameTime Time) {
  this.Device.SetRenderTarget(this.GridTexture);
  this.Device.SetRenderTarget(null);


  // compute a temporary transformation matrix containing
  // the combined world and projection transfromation
  Matrix WorldViewProjection = this.Camera.View * this.Camera.Projection;

  // set the render target to the back buffer in any case
  this.Device.SetRenderTarget(null);

  // assign the vertex - declaration and the vertex- and the index - buffer
  this.Device.SetVertexBuffer(this.SkyboxVertices);
  this.Device.Indices = this.SkyboxIndices;

  // choose the appropriate technique for the current render pass
  this.SceneEffect.CurrentTechnique = SceneEffect.Techniques["Skybox"];
  this.SceneEffect.CurrentTechnique.Passes[0].Apply();
  this.SceneEffect.Parameters["WorldViewProjection"].SetValue(WorldViewProjection);

  // finally render the sykbox with disabled depth stencil buffering
  this.Device.DepthStencilState = DepthStencilState.None;
  this.Device.DrawIndexedPrimitives( PrimitiveType.TriangleList, 0, 0, 36, 0, 12);
  this.Device.DepthStencilState = DepthStencilState.Default;


  //this.Device.SetRenderTarget(this.GridTexture);
  //this.Device.SetRenderTarget(null);
}

However, if I assign (and right away unassign) the render target at the end of the function like so:

    private void DoRenderSkybox (GameTime Time) {
  //this.Device.SetRenderTarget(this.GridTexture);
  //this.Device.SetRenderTarget(null);


  // compute a temporary transformation matrix containing
  // the combined world and projection transfromation
  Matrix WorldViewProjection = this.Camera.View * this.Camera.Projection;

  // set the render target to the back buffer in any case
  this.Device.SetRenderTarget(null);
  this.Device.Clear(Color.Black);

  // assign the vertex - declaration and the vertex- and the index - buffer
  this.Device.SetVertexBuffer(this.SkyboxVertices);
  this.Device.Indices = this.SkyboxIndices;

  // choose the appropriate technique for the current render pass
  this.SceneEffect.CurrentTechnique = SceneEffect.Techniques["Skybox"];
  this.SceneEffect.CurrentTechnique.Passes[0].Apply();
  this.SceneEffect.Parameters["WorldViewProjection"].SetValue(WorldViewProjection);

  // finally render the sykbox with disabled depth stencil buffering
  this.Device.DepthStencilState = DepthStencilState.None;
  this.Device.DrawIndexedPrimitives( PrimitiveType.TriangleList, 0, 0, 36, 0, 12);
  this.Device.DepthStencilState = DepthStencilState.Default;


  this.Device.SetRenderTarget(this.GridTexture);
  this.Device.SetRenderTarget(null);
}

nothing is being rendered - I got a purple screen! Does that make any sense to you!

P.S. I know the sample doesn't make much sense (like assigning and right away unassigning a render target). The real scenario here is much more complicated but I was able to narrow down the essence of the problem to this strange behaviour, which I can demonstrate in this very simple example!

EDIT: Just found the solution by further examining the API: Adding Params.RenderTargetUsage = RenderTargetUsage.PreserveContents; into the PresentationParameter structure solves the problem.

It looks to me that the content of the backbuffer is - like with render targets being destroyed whenever you switch to another render target.