MonoGame.Samples/XNA2DShaderExamples/Content/effects/HighContrast.fx

sampler s0;

float4 PixelShaderFunction(float2 coords: TEXCOORD0) : COLOR0
{
    float4 color = tex2D(s0, coords);

	if      (color.r > .65) color.r = 1;
	else if (color.r < .35) color.r = 0;

    return color;
}

technique Technique1
{
    pass Pass1
    {
        PixelShader = compile ps_2_0 PixelShaderFunction();
    }
}