Get transparency of one pixel

Question

Hi. I have C# function like this:

 void Update () {
     
     if (!Input.GetMouseButtonDown(0))
     {
         return;
     }
     
     var ray = Camera.main.ScreenPointToRay(Input.mousePosition);
     
      RaycastHit[] hits;
             hits = Physics.RaycastAll(ray);

         for (int i=0; i < hits.Length; i++)
         {
             Debug.Log(hits[i].transform);
         }
 }

The function catchs mouse click position and lists which of object have we clicked. It's works fine, but now I want to add checking, whether the texture PNG is transparency in clicked point. Simply - when we click in invisible part of picture, function avoids this object and notices just another objects which was clicked.

I have no idea how to check one exact point properties (whats more - from raycast coordinates) and how to find the alpha channel. Could somebody prompt me something?

Thanx!

Answer 1

What you are asking for is not not normal in a 3D environment, and difficult to do. Shaders often use multiple textures. Sometimes transparency is in the texture, sometimes it is done with a mask, sometimes it is determined by the blending of multiple textures. Sometimes a texture is really an atlas of textures. In addition, an object may have a transparent area that the ray first hits, but would hit a non-transparent area as it pass through the back of the mesh. And in the end, the collider that is reporting through your Raycast and the mesh/texture are different components.

If your object/mesh is a plane, and if you are using a simple shader, and if your material offsets are set to their defaults, you might be able to do it by:

Translate the RaycastHit.point to local coordinates of the object
Translate the coordinates to a UV system (0 to 1) based on the object size.
Use Texture2D.GetPixel() or Texture2D.GetPixelBilinear()
Examine the alpha value of the color returned

As a limited alternative, note that you can make changes to the collider. You can change the sizes. You can add multiple colliders of different types to a game object. You can put colliders on empty child objects. I don't know your application, but you can create a tighter bounding for your image by manipulating the collider.

Answer 2

I'm sorry, that so late, but I eventually resolve the problem. You've right, robertbu, it was possible for so easy situation as mine - my game is quite simply, I use one texture (set in MeshRender), the transparency is wrote in picture and in Shader (Transparent/Diffuse) and every object is a plane (cause actually it is 2D game :p ).

 using UnityEngine;
 using System.Collections;
 
 public class RayCasting : MonoBehaviour {
         
     void Update () {
         
         if (!Input.GetMouseButtonDown(0))
         {
             return;
         }
         
         var ray = Camera.main.ScreenPointToRay(Input.mousePosition);
         RaycastHit[] hits = Physics.RaycastAll(ray);
         
         Vector3 v;
         bool[] transparent = new bool[hits.Length];
         
         for (int i=0; i < hits.Length; i++)
         {
             Texture2D pic = hits[i].transform.gameObject.renderer.material.mainTexture as Texture2D;
             // - Translate the RaycastHit.point to local coordinates of the object
             v = hits[i].transform.worldToLocalMatrix.MultiplyPoint(hits[i].point);
             
             // - Translate the coordinates to a UV system (0 to 1) based on the object size.
             float xPic = 10 - (v.x + 5);
             float yPic = v.y + 5;
             
             // - Use Texture2D.GetPixel() or Texture2D.GetPixelBilinear()
             // - Examine the alpha value of the color returned
             if(pic.GetPixel((int)((xPic/10)*pic.width),(int)((yPic/10)*pic.height)).a).a < 0.5)
                 transparent[i] = true;    //pixel is transparent
                 Debug.Log(hits[i].transform);
         }
         
         int res = 0;
         if(hits.Length != 0)
             res = exam(hits, transparent);

         //if front checking function found something
         //we remove it
         if(res != hits.Length)
             Destroy(hits[res].transform.gameObject);
     }

     // function which check, what is on front
     int exam(RaycastHit[] hits, bool[] t)
     {
         float tmp = 100;
         int resRay = hits.Length;
         
         for(int i = 0; i < hits.Length; i++)
         {
             if((hits[i].transform.position.z < tmp) && !t[i])
             {
                 tmp = hits[i].transform.position.z;
                 resRay = i;
             }
         }
         return resRay;
     }
 }

Funny, but the hardest was computing the coordinates from "LocalMatrix" to pixels of texture - they was set in completely different way. Maybe wiser would be to think about this UV system, but eventually it work :) So thanks, robertbu, you told me it's possible, so I give you plus.

Get transparency of one pixel

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