create line on a texture

In case there is anyone still looking for a script that does this kind of thing:

 public static void DrawLine(this Texture2D tex, Vector2 p1, Vector2 p2, Color col)
 {
     Vector2 t = p1;
     float frac = 1/Mathf.Sqrt (Mathf.Pow (p2.x - p1.x, 2) + Mathf.Pow (p2.y - p1.y, 2));
     float ctr = 0;
     
     while ((int)t.x != (int)p2.x || (int)t.y != (int)p2.y) {
         t = Vector2.Lerp(p1, p2, ctr);
         ctr += frac;
         tex.SetPixel((int)t.x, (int)t.y, col);
     }
 }

use it like this: (don't forget to call .apply())

 public class LineDrawer : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
         Texture2D txr = new Texture2D (512, 512);
         txr.DrawLine (new Vector2 (0, 0), new Vector2 (512, 256), Color.red);
         txr.Apply ();
         
         byte[] bytes = txr.EncodeToPNG();
         File.WriteAllBytes(Application.dataPath + "/../SavedScreen.png", bytes);
     }
 }

I tested your code and it works with a few changes...

1. Make sure that the texture you are modifying was originally imported with Read/Write enabled (under advanced settings in the Inspector).

2. Be sure to call texture.Apply().

Here's an example showing the modifications I made...

//----------------------------------------------------------- using UnityEngine;

public class DrawLine : MonoBehaviour { public Texture2D MyTexture;

 // Use this for initialization
 void Start () 
 {

     Draw(0, 0, 100, 100);
     renderer.material.mainTexture = MyTexture;
 }
 
 // Update is called once per frame
 void Update () 
 {
 
 }

 void Draw(float x1, float y1, float x2, float y2)
 {
     float x,y;
     float dy = y2-y1;    
     float dx = x2-x1;    
     float m = dy/dx; 
     float dy_inc = -1;    
     
     if(  dy < 0 ) 
         dy = 1;    
     
     float dx_inc = 1;    
     if(  dx < 0 ) 
         dx = -1; 
         
     if( Mathf.Abs(dy) > Mathf.Abs(dx) )
     {
         for( y = y2; y < y1; y += dy_inc )
         {                
             x = x1 + ( y - y1 ) * m;
             Debug.Log("Setting Pixel at: x=" + x + ", y=" + y);
             MyTexture.SetPixel((int)(x), (int)(y), Color.black);       
         }
     }
     else
     {
         for( x = x1; x < x2; x +=  dx_inc  ) 
         {
             y = y1 + ( x - x1 ) * m;
             Debug.Log("Setting Pixel at: x=" + x + ", y=" + y);
             MyTexture.SetPixel((int)(x), (int)(y), Color.black);           
         }
     }
     MyTexture.Apply();
 }

   

}