I tried to change for the beginning just the brightness. Changed following line
return (tex_col - float4(0.5f, 0.5f, 0.5f, 0.0f)) * 1.2f + float4(0.5f, 0.5f, 0.5f, 0.0f);
to
return (tex_col - float4(0.5f, 0.5f, 0.5f, 0.0f)) * 5.2f + float4(0.5f, 0.5f, 0.5f, 0.0f);
I changed some other figures located below "GAMMA" and so on but it changed nothing :-(
What did I do wrong?