Opaque pragma sampler lambert shadow feature surface albedo alpha property material Tessellation vertex fragment program tangent saturate irradiance scattering absorption refraction transimission reflection specular diffuse exitance emissive ambient gloss shininess texel coordinates bump displacement spherical harmonics spot swizzle Apparently declaration exposure ratio distortion horizontal vertical Multiplier Magnitude Frequency Restraints approximate gaps fences vegetation stencil luminance saturation brightness contrast gradient accumulation