Scripting Reference
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Camera.ViewportToScreenPoint

function ViewportToScreenPoint (position : Vector3) : Vector3

Description

Transforms position from viewport space into screen space.

Viewport space is normalized and relative to the camera. The bottom-left of the camera is (0,0); the top-right is (1,1). The z position is in world units from the camera.

Screenspace is defined in pixels. The bottom-left of the screen is (0,0); the right-top is (pixelWidth,pixelHeight). The z position is in world units from the camera.

JavaScript 
    // Draw an image based on normalized view coordinates
    // rather than pixel positions.
    var bottomPanel: Texture2D;
    function VPToScreenPtExample() {
        var origin = Camera.main.ViewportToScreenPoint(new Vector3(0.25, 0.1, 0));
        var extent = Camera.main.ViewportToScreenPoint(new Vector3(0.5, 0.2, 0));
        
        GUI.DrawTexture(new Rect(origin.x, origin.y, extent.x, extent.y), bottomPanel);
    }
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Texture2D bottomPanel;
    void VPToScreenPtExample() {
        Vector3 origin = Camera.main.ViewportToScreenPoint(new Vector3(0.25F, 0.1F, 0));
        Vector3 extent = Camera.main.ViewportToScreenPoint(new Vector3(0.5F, 0.2F, 0));
        GUI.DrawTexture(new Rect(origin.x, origin.y, extent.x, extent.y), bottomPanel);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public bottomPanel as Texture2D

    def VPToScreenPtExample():
        origin as Vector3 = Camera.main.ViewportToScreenPoint(Vector3(0.25F, 0.1F, 0))
        extent as Vector3 = Camera.main.ViewportToScreenPoint(Vector3(0.5F, 0.2F, 0))
        GUI.DrawTexture(Rect(origin.x, origin.y, extent.x, extent.y), bottomPanel)