DxO ClearView Plus Technology in DxO PhotoLab
What is haze?
Haze is related to light dispersion by air particles. Even when the weather is fine, the veil caused by haze can be strongly visible when objects are farther than about two miles: we then talk about dry haze. When haze is related to water particle diffusion, we talk about mist, or even fog if the visibility is less than 1 km.
Light diffusion by water can be observed in the air caused by spray.
Root causes of haze:
- Part of the light reflected by a photographed object (shown here as light reflected by a house represented with yellow arrows in figure A) toward the camera is lost because of the scattering process (light is redirected somewhere else, cf. multi-directional arrows) on air particles. Consequently, the object being photographed fades away.
- Part of the light coming from illumination (the sun in this illustration) is redirected toward the camera because of the scattering process caused by air particles, adding a parasite light to the picture (blue arrows figure A). Consequently, dark areas are no longer dark.
How to dehaze?
In theory, dehazing requires that we know the distance of each photographed object in the scene, the amount of diffusion, as well as the properties of illumination.
Strong assumptions must be made: the first principal is to assume that when there is no haze, a picture locally will have some dark areas (to be more precise, at least one of the three RGB color channels will have a low value).
→ Dehazing process should behave so that min (R, G, B) should locally be zero again (to compensate for illumination diffusion); and the signal is accordingly amplified to compensate for the photographed object losing light due to diffusion.
- Dark channel assumption on pictures without haze is valid for most pictures.
- Correction also requires finding the illumination properties (through image content analysis of brighter pixels), so that the correct veil color can be removed.
DxO ClearView is based on these principles and can remove haze in one click.
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