Think of a portrait photo~on a dark portrait,details you know exsist,fine lines,uneven textures,lashes/brows blended would only just be visible in the most illuminated areas(foward points)outlines/edges less defined,full shape obscurred,background fading to inky black.Seems dark would lack stark detail,less 3 dimensional appearance,yet depth could remain in a raised relief,like a dessert sand dunes after sunset.Mind that this is just a guess based on what "dark" means to me concerning an image/picture.