Paint a texture in Unity using shaders - unity3d

Question

I'm trying to solve how to paint a texture in Unity at runtime. But the idea is to have a Canvas, an Image (with a texture) and paint over it using GPU (shader pass). But I don't really know how to start, or where to look at.
There's a very interesting project but it does uses 3D Meshes, and solves the problem with Metallic, Smoothness integration. Also solves the hard edges problem, making the solution very difficult to start with for a beginner.
Mesh Texture painting in Unity Using Shaders and the repo https://github.com/IRCSS/TexturePaint.
My guess is that this shader is the basic and probably the foundation of how to solve this:
Shader "Unlit/TexturePaint"
{
SubShader
{
Tags { "RenderType"="Opaque" }
LOD 100
ZTest Always
ZWrite Off
Cull Off
Pass
{
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma exclude_renderers d3d9 d3d11 xbox360 ps3 flash
#include "UnityCG.cginc"
struct appdata
{
float4 vertex : POSITION;
float2 uv : TEXCOORD0;
};
struct v2f
{
float4 vertex : SV_POSITION;
float3 worldPos : TEXCOORD0;
float2 uv : TEXCOORD1;
};
float4 _Mouse;
float4x4 mesh_Object2World;
sampler2D _MainTex;
fixed4 _BrushColor;
float _BrushOpacity;
float _BrushHardness;
float _BrushSize;
v2f vert (appdata v)
{
v2f o;
float2 uvRemapped = v.uv.xy;
#if UNITY_UV_STARTS_AT_TOP
uvRemapped.y = 1. - uvRemapped.y;
#endif
uvRemapped = uvRemapped * 2 - 1;
o.vertex = float4(uvRemapped.xy, 0, 1);
o.worldPos= mul(mesh_Object2World, v.vertex);
o.uv = v.uv;
return o;
}
fixed4 frag (v2f i) : SV_Target
{
float4 col = tex2D(_MainTex, i.uv);
float size = _BrushSize;
float soft = _BrushHardness;
float f = distance(_Mouse.xz, i.worldPos.xz);
f = 1. - smoothstep(size * soft, size, f);
col = lerp(col, _BrushColor, f);
col = saturate(col);
return col;
}
ENDCG
}
}
Fallback Off
}