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Dithering Image Effect - Unity Shader

Dithering Image Effect

We will be making this effect :
dithering effect done with shader
Dithering Effect
This effect can handle both color depth reduction ( reducing number of colors gets displayed ) as well as the dithering ( our main objective )
As this is an image effect go ahead and create a new image effect in Unity.
There is no vertex shader code as this is an image effect.
So now let's look at the properties that we have for the shader :
   _MainTex ("Texture", 2D) = "white" {}
That's all? well.. we have more but these are not exposed as public. So we do not declare them in the properties section.
These are the properties declared in the CG PROGRAM
sampler2D _MainTex;
int _ColourDepth;
float _DitherStrength;
We will come to see what _ColourDepth and _DitherStrength are later on.
Now we will look at the Dither Table that we will be using :
static const float4x4 ditherTable = float4x4
 -4.0, 0.0, -3.0, 1.0,
 2.0, -2.0, 3.0, -1.0,
 -3.0, 1.0, -4.0, 0.0,
 3.0, -1.0, 2.0, -2.0
Declare this table along with the rest of the properties ( or uniforms or whatever you wanna call it ).
We need this matrix as it describes the pattern that will be put on screen for dithering.
Dithering comes in handy in a lot of places, In future tutorials we will look into using dithering to make high quality gradients.
Now let's look at the magical fragment shader :
fixed4 frag (v2f i) : SV_TARGET
 /*(1)*/fixed4 col = tex2D(_MainTex, i.uv);
 /*(2)*/uint2 pixelCoord = i.uv*_ScreenParams.xy;
 /*(3)*/col += ditherTable[pixelCoord.x % 4][pixelCoord.y % 4] * _DitherStrength;
 /*(4)*/return round(col * _ColourDepth) / _ColourDepth;
We will go through each line and understand what it does.
  1. Just accessing the texture and retrieving the color of that texel at the uv co-ordinate.
  2. pixelCoord is the actual pixel on the screen as it gets converted from  ( 0 to 1, 0 to 1 ) space to (0 to width, 0 to height) space as it gets multiplied with _ScreenParams.xy
    *Note: pixelCoord is a unit2, as performing modulus with an int is slow. 
  3. Here we are acessing the ditherTable and according to our pixel coord we take a value and then make it smaller by multiplying with the _DitherStrength value ( 0 to 1) then add it to 'col'.
  4. In order to artificially reduce color depth we use the round operation to reduce the range of colors that get displayed. We use the _ColourDepth (int) value to determine how many colors show up.
Let's move on to making the C# file that goes along with this.
using UnityEngine;
[ExecuteInEditMode, ImageEffectAllowedInSceneView, RequireComponent(typeof(Camera))]
public class DitherEffect : MonoBehaviour
    public Material ditherMat;
    [Range(0.0f, 1.0f)]
    public float ditherStrength = 0.1f;
    [Range(1, 32)]
    public int colourDepth = 4;

    private void OnRenderImage(RenderTexture src, RenderTexture dest)
        ditherMat.SetFloat("_DitherStrength", ditherStrength);
        ditherMat.SetInt("_ColourDepth", colourDepth);
        Graphics.Blit(src, dest, ditherMat);
Nothing really complicated here...
We just pass in the required data for dither strength and color depth to the material.
If you are having trouble with this it's better to go over my previous shader tutorials.
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