Texture2D readPixels: How to only read a section of the camera view?

Hi,

i am trying to get only a section of the camera view when i take a screenshot using readPixels. How can i do that? I have tried to change the rect left and top parameters but when i set them higher than 0 then i receive a grey texture.

Here is the part of the code

RenderTexture renderTexture = new RenderTexture(screenshotWidth, screenshotHeight, 32);
screenshotCamera.targetTexture = renderTexture;
screenshot = new Texture2D(screenshotWidth,screenshotHeight, TextureFormat.ARGB32, false);
screenshotCamera.clearFlags = CameraClearFlags.Color;
screenshotCamera.Render();
screenshotCamera.clearFlags = CameraClearFlags.Nothing;
RenderTexture.active = renderTexture;
screenshot.ReadPixels(new Rect(0, 0, screenshotWidth, screenshotHeight), 0, 0);
RenderTexture.active = null;
Destroy(renderTexture);

The screenshotCamera is a extra camera, i can´t use the main camera because i screenshot some elements on another layer.

Thanks for helping.