EGAfilter5 editing

[BridgeHeadland]

For a few years, I've have had fun with an FX file called EGAfilter5, which I've found on VOGONS. It's like EGAfilter.fx, only you can set the saturation, as well as the brightness, plus it mimics SCI technology.
What I've noticed is that it tries to mimic the 640x200 mode, so that 400 pixels each vertical lines, at 1x1, turns into 200 pixels of 1x2, per vertical lines, as long as the resolution is of 400p or more. Does anyone know how to edit this shader?

As you can see, I have set the brightness at 1.0 and the saturation at 2.0. Here you see an image of Duke Nukem 3D with EGAfilter5 in the mentioned setting, where there are 300 1x2 pixels for each vertical lines (well, almost), instead of 600 1x1 pixels.

/*
   Downsamples to 16 colors using quantization.
   Uses 4-bit RGBI values for an "EGA"/"Tandy" look
   
   Author: VileR, leilei (saturation and dither stuff)
   License: public domain

	VERSION 5
*/


// 1.0 is the 'proper' value, 1.2 seems to give better results but brighter
// colors may clip.
#define color_enhance 1.0


// 1.0 is normal saturation, 2.0 is twice, 0 is gray scale
#define	color_saturation 2.0



// The name of this effect
string name : NAME = "EGAfilter";

// combineTechnique: Final combine steps. Outputs to destination frame buffer
string combineTechique : COMBINETECHNIQUE = "EGAfilter";

// Matrix Definitions for Scaler Effects
matrix World                : WORLD;
matrix View                 : VIEW;
matrix Projection           : PROJECTION;
matrix WorldView            : WORLDVIEW; // world * view
matrix ViewProjection       : VIEWPROJECTION; // view * projection
matrix WorldViewProjection  : WORLDVIEWPROJECTION; // world * view * projection

// Source Texture
texture SourceTexture : SOURCETEXTURE;

// Point Sampler
sampler SourcePointSampler = sampler_state {
 Texture   = (SourceTexture);
 MinFilter = POINT;
 MagFilter = POINT;
};  

// Vertex Shader Output
struct VS_OUTPUT_PRODUCT {
  float4  coord : POSITION;
  float2  TexCoord   : TEXCOORD0;
  float2  TexCoord2   : TEXCOORD1;
};

// Color lookup table (RGBI palette with brown fix)
const half3 rgbi_palette[19] = {
  half3(0.0,     0.0,     0.0),
  half3(0.0,     0.0,     0.66667),
  half3(0.0,     0.66667, 0.0),
  half3(0.0,     0.66667, 0.0),
  half3(0.0,     0.66667, 0.0),
  half3(0.0,     0.66667, 0.0),
  half3(0.0,     0.66667, 0.66667),
  half3(0.66667, 0.0,     0.0),
  half3(0.66667, 0.0,     0.66667),
  half3(0.66667, 0.33333, 0.0),
  half3(0.66667, 0.66667, 0.66667),
  half3(0.33333, 0.33333, 0.33333),
  half3(0.33333, 0.33333, 1.0),
  half3(0.33333, 1.0,     0.33333),
  half3(0.33333, 1.0,     1.0),
  half3(1.0,     0.33333, 0.33333),
  half3(1.0,     0.33333, 1.0),
  half3(1.0,     1.0,     0.33333),
  half3(1.0,     1.0,     1.0),
};


// Came off the wikipedia(R) article for Ordered_Dithering
float dithertable[16] = {
	 1, 9, 3,11,
	13, 5,15, 7,
	 4,12, 2,10,
        16, 8,14, 6,
};

// Image Dimentions 
float2 SourceDims : SOURCEDIMS = 256.0F;

// Compare vector distances and return nearest RGBI color
half3 nearest_rgbi (half3 original) {
  half dst;
  half min_dst = 2.0;
  int idx = 0;
  for (int i=0; i<19; i++) {
    dst = distance(original, rgbi_palette[i]);
    if (dst < min_dst) {
      min_dst = dst;
      idx = i;
    }
  }
  return rgbi_palette[idx];
}
float2 ps : TEXELSIZE;

// Vertex shader
VS_OUTPUT_PRODUCT S_VERTEX (float3 p : POSITION, float2 tc : TEXCOORD0)
{
  VS_OUTPUT_PRODUCT OUT = (VS_OUTPUT_PRODUCT)0;

	float dx = ps.x * 320;
	float dy = ps.y * 200;
  OUT.coord = mul(float4(p,1),WorldViewProjection);
  OUT.TexCoord = tc;
  return OUT;
}


float mod(float x, float y)
{
	return x - y * floor (x/y);
}


// Pixel shader
float4 S_FRAGMENT ( in VS_OUTPUT_PRODUCT VAR ) : COLOR
{
  half3 fragcolor = tex2D(SourcePointSampler, VAR.TexCoord).rgb;

	// leilei - dither 
	float2 res;
	float2 tex;
	tex.x = VAR.TexCoord.x;
	tex.y = VAR.TexCoord.y;
	res.x = SourceDims.x; 

	if (SourceDims.y < 900)
		res.y = SourceDims.y; 
	else
	res.y = SourceDims.y / 2; 

	int ohyes;
	int yeh;
	float2 ditheu = tex.xy * res.xy;
	ditheu.x = tex.x * res.x;
	ditheu.y = tex.y * res.y;

	int ditdex = 	int(mod(ditheu.x, 4.0)) * 4 + int(mod(ditheu.y, 4.0)); // 4x4!

	// looping through a lookup table matrix
	for (yeh=ditdex; yeh<(ditdex+16); yeh++) ohyes = dithertable[yeh-15] * 2.1 - 21;
	ohyes = floor(ohyes) - 6;

	fragcolor.rgb *= 255;
	fragcolor.rgb += ohyes;
	fragcolor.rgb /= 255;


	// leilei - end



	// leilei - saturation boost
  float fragsat = (fragcolor.r * 0.333) + (fragcolor.g * 0.333) + (fragcolor.b * 0.333);
  fragcolor.r = fragsat + (fragcolor.r - fragsat) * color_saturation;
  fragcolor.g = fragsat + (fragcolor.g - fragsat) * color_saturation;
  fragcolor.b = fragsat + (fragcolor.b - fragsat) * color_saturation;
	// leilei - end

  return half4(nearest_rgbi(fragcolor*color_enhance), 1);
}

technique EGAfilter
{
  pass P0
  {
    VertexShader = compile vs_3_0 S_VERTEX();
    PixelShader  = compile ps_3_0 S_FRAGMENT();
  }  
}