Single Image Haze Removal Using Dark Channel Prior

Haze brings troubles to many computer vision/graphics applications. It reduces the visibility of

the scenes and lowers the reliability of outdoor surveillance systems; it reduces the clarity of the satellite

images; it also changes the colors and decreases the contrast of daily photos, which is an annoying problem

to photographers. Therefore, removing haze from images is an important and widely demanded topic in

computer vision and computer graphics areas. The main challenge lies in the ambiguity of the problem.

Haze attenuates the light reflected from the scenes, and further blends it with some additive light in the

atmosphere. The target of haze removal is to recover the reflected light (i.e., the scene colors) from the

blended light. This problem is mathematically ambiguous: there are an infinite number of solutions given the

blended light. How can we know which solution is true? We need to answer this question in haze removal.

Ambiguity is a common challenge for many computer vision problems. In terms of mathematics, ambiguity

is because the number of equations is smaller than the number of unknowns. The methods in computer

vision to solve the ambiguity can roughly categorized into two strategies. The first one is to acquire more

known variables, e.g., some haze removal algorithms capture multiple images of the same scene under

different settings (like polarizers).But it is not easy to obtain extra images in practice. The second strategy is

to impose extra constraints using some knowledge or assumptions .All the images in this thesis are best

viewed in the electronic version. This way is more practical since it requires as few as only one image. To

this end, we focus on single image haze removal in this thesis. The key is to find a suitable prior. Priors are

important in many computer vision topics. A prior tells the algorithm “what can we know about the fact

beforehand” when the fact is not directly available. In general, a prior can be some statistical/physical

properties, rules, or heuristic assumptions. The performance of the algorithms is often determined by the

extent to which the prior is valid. Some widely used priors in computer vision are the smoothness prior,

sparsity prior, and symmetry prior. In this thesis, we develop an effective but very simple prior, called the

dark channel prior, to remove haze from a single image. The dark channel prior is a statistical property of

outdoor haze-free images: most patches in these images should contain pixels which are dark in at least one

color channel. These dark pixels can be due to shadows, colorfulness, geometry, or other factors. This prior

provides a constraint for each pixel, and thus solves the ambiguity of the problem. Combining this prior with

a physical haze imaging model, we can easily recover high quality haze-free images.