Rect relative to Render Texture - unity3d

Question

I am having a hard time figuring out how to find correct rect that represents a ui element area. I get the Ui element RectTransform. I tried to use it directly, using a function and RectTransformUtility, but nothing seems to wok.
Here is the code.
RenderTexture.active = scr;
//adjust rect position
//Rect rect = RectUtils.RectTransformToScreenSpace (rectTransform);
int width = System.Convert.ToInt32 (rect.width);
int height = System.Convert.ToInt32 (rect.height);
// Create a texture the size of the screen, RGB24 format
Texture2D tex = new Texture2D(width, height, TextureFormat.RGBA32, false);
RenderTexture.active = scr;
// Read screen contents into the texture
tex.ReadPixels(rect, 0, 0);
tex.Apply();
RenderTexture.active = null;
// Encode texture into PNG
byte[] bytes = tex.EncodeToPNG();
//todo destroy the texture
Object.Destroy (tex);
// For testing purposes, also write to a file in the project folder
File.WriteAllBytes(path, bytes);
I tried to create rect in different ways like:
Methood 1:
Vector2 size = Vector2.Scale(transform.rect.size, transform.lossyScale);
Rect rect = new Rect(transform.position.x, Screen.height - transform.position.y, size.x, size.y);
rect.x -= (transform.pivot.x * size.x);
rect.y -= ((1.0f - transform.pivot.y) * size.y);
return rect;
Method 2:
Vector2 temp = rectT.transform.position;
var startX = temp.x - width/2;
var startY = temp.y - height/2;
var tex = new Texture2D (width, height, TextureFormat.RGB24, false);
tex.ReadPixels (new Rect(startX, startY, width, height), 0, 0);
I am using Unity 5.5 Mac version.
when i pass to ReadPixels new Rect (0, 0, Screen.width, Screen.height) i see the whole RenderTexture in its defined dimensions 1920x1080
I want to use this as a baker of UI elements as I am having performance issue, and definitively no other solution to implement the required behavior.

Answer 1

I also struggled with this problem. Here's how I solved it.
First we need the position of RectTransform in screen and for this I used RectTransformUtility. Then since the rect x, y is the starting point at left bottom (note: in GUI x, y is at left top) we subtract respectively the half of width and height.
Vector2 rectPos = RectTransformUtility.WorldToScreenPoint (Camera.main, rectTransform.position);
float startX = rectPos.x - width / 2;
float startY = rectPos.y - height / 2;
Rect screenRect = new Rect (startX, startY, width, height);
Note: width height should be integer.
Edit: The above solution when we use Screen overlay and change resolution from standard to any other do not work. Even if I used the lossyscale I could not get consistent behavior.
One method of RecTransformUtility is really helpful for this situation (WorldToScreenPoint).
Vector3[] bounds = new Vector3[4];
rectTransform.GetWorldCorners (bounds);
Vector2 rectPos = RectTransformUtility.WorldToScreenPoint (camera, rectTransform.position);
Vector2 minPosition = RectTransformUtility.WorldToScreenPoint (camera, bounds[0]);
Vector2 maxPosition = RectTransformUtility.WorldToScreenPoint (camera, bounds[2]);
Vector2 size = maxPosition - minPosition;
float startX = rectPos.x - size.x / 2;
float startY = rectPos.y - size.y / 2;
return new Rect (startX, startY, size.x, size.y);