Shaders : Blur only part of the screen

Question

I wish to blur only part of the screen (let's say half of it). From what I've found online I need to create a texture 'mask' that will make the shader only "work" on that mask. However, I have no idea of how to create that texture 'mask'. Looking at the inspector with the MobileBlur shader selected I can see there are two fields under "Default Maps" but changing them doesn't produce any results.

I was wondering where (and how) on the shader code can I create that texture 'mask'. Or maybe if you know some example/tutorial that could shed some light on this subject of texture masks that would be much appreciated. Thanks in advance!

Edit: After a while I encountered this tutorial series:
Unity Tutorial: A Practical Intro to Shaders - Part 1
which even though it did not touch the subject of texture masks, it allowed to understand shaders enough to get the problem solved. Still, if anyone can explain the texture maps part it would still be appreciated.

Answer 1

I found the correct solution finally! Here it is for those having the same problem:
Basically you create the texture you desire to apply your shader on. For me it was a circular gradient. White is where the shader will be apllied, black is where it isn't. On the texture don't forget to tick the box "Alpha from Grayscale".
Then on the shader you add the mask on your properties tab. The beggining should look something like this:

 Shader "Hidden/FastBlurCircular" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Mask ("Blur Mask", 2D) = "white" {}
         _Bloom ("Bloom (RGB)", 2D) = "black" {}
         
     }
     
     CGINCLUDE
 
         #include "UnityCG.cginc"
 
         sampler2D _MainTex;
         sampler2D _Mask;
         sampler2D _Bloom;

Then, after doing the desired calculations you will end up with a certain color for the pixel. Simply lerp that color with the original one using the alpha from the texture mask like so:

 //Some calculations that give you 'color'
 
 half4 originalColor = tex2D(_MainTex, uv) ;
 half4 maskColor = tex2D(_Mask, uv);
 
 return lerp(originalColor,color,maskColor.a);

I hope this helps someone with the same issue!

Shaders : Blur only part of the screen

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