I was wondering if anybody could tell me how this photo effect was achieved? Or is it an effect or filter that is applied after the photo has been taken?
It looks almost negative, but it isn't.
My guess is that this is a composite of two or more pictures. One was taken at night when the lights were on. This is where the artificial lights in the picture come from. Another exposure was taken when there was more daylight. This is where the light sky and illumination of large flat areas that weren't bathed in artificial light came from.
I disagree this is from a single long exposure at night. Such a single exposure does not account for the fact that many large and flat surfaces are evenly illuminated, which they almost certainly are not from the night-time artifical lights. For examples, see the concrete wall of the building in the lower left corner, or the one facing towards us and to the right of the building at the bottom and little left of center. These are way to bright and too evenly lit for artifical lights to be the illumination source. There also seems to be no plausible source for aritifical lights onto those surfaces.
I'd say it's just the light pollution hitting the fog and smoke in the air. Exagerated by the long exposure needed to get the buildings that bright.
But you might want to ask the artist himself: http://caudroy.fr/gallery-category/projects/#hong-kong
This isn't 'high-key'. That term applies to photographs containing mostly things that are light-colored, such as snow, not the overall lightness or darkness of the photograph itself. An overexposed photo of a coal yard is still low-key, and an underexposed photo of a snow-covered field is still high key, even if the photos have a similar brightness. In summary: 'low key' refers to dark objects or surfaces, and 'high key' refers to light objects or surfaces, regardless of the exposure. A black cat in a coal yard is a low-key photo, no matter how you expose it.