Fixed contrast calculation

Author: usercode

src/WPF.ImageEffects/Shaders/ImageShader.fx

@@ -7,7 +7,8 @@ float4 main(float2 uv : TEXCOORD) : COLOR
 	float4 texColor = tex2D(Input, uv);
 	float4 final = texColor;
 	final = texColor + brightness;
-	final = final * (contrast + 1.0);
+	//final = final * pow((contrast + 1.0) / 1.0, 2);
+	final = ((final - 0.5) * pow((contrast + 1.0) / 1.0, 2)) + 0.5;
 
 	final.a = texColor.a; //restore alpha value