The recent rapid spread of digital video cameras and other diverse imaging devices facilitates easy accessibility of images in the form of digital data. Color image data derived from these devices consists of three color components red (R), green (G), and blue (B), corresponding to three primary colors of light. Some specific colors, that is, skin-tone, vegetation (green) and sky-sea (blue) generally tend to have emphasized visual effects in a color image. The user can easily identify, for example, even a slightly reddish tint or a slightly dim tint on the skin-tone in the color image, as well as, even slight differences among vegetation greens and sky-sea blues in the color image. These specific colors are visually recognizable in a distinctive manner from other colors.
Typically, color spaces such as HSV (Hue, Saturation, Value)/HSB (Hue, Saturation, Brightness) or HSI (Hue, Saturation, Intensity)/HSL (Hue, Saturation, Lightness) are more intuitive in manipulating color and are designed to approximate the way humans perceive and interpret color. The intuitive color spaces are related representations of points in an RGB color space, which attempt to describe perceptual color relationships more accurately than RGB. An intuitive correction technique is thus preferable for these colors in the color image. Conventionally, intuitive color enhancement/correction in these color spaces is performed by using one of the following techniques:                Varying saturation component of a specific color        Varying Hue component of a specific color        Independently varying Hue, saturation and intensity components of a specific color        
Varying saturation component of a specific color alone may introduce artifacts (e.g., unwanted visible effects). For example, if a color region of a video frame is dark then varying saturation alone may not show-up any significant visually perceivable improvements in the color. If a portion of a color region of a video frame is white-highlight, (e.g., an overexposed face with flash light) saturation correction alone may show-up as artifacts. These artifacts may show-up due to color enhancement with saturation correction in white-highlight portion of interest not being visually perceivable while in surrounding areas it is visually perceivable. This can bring non-uniformity in a color region and hence, may appear as an artifact. Further, Hue component changes of a specific color may lead to unnatural colors and may also result in undesirable color shifts.
Currently, all three HSI components are independently varied using either look-up tables or piecewise linear (PWL) approximation methods. However using the look-up table method may require large amount of memory for storing look-up tables associated with various gains and hence, limits number of gains due to limited memory. Also in the PWL method, the transfer characteristics curve may have segments with varying slopes and/or sharp transitions and may suffer from visible artifacts.
In addition, to above said artifacts and unnatural color shifts with independent saturation and Hue component corrections, independent correction on intensity component of a video frame may also bring-about additional artifacts or may nullify visually perceivable improvement achieved by varying saturation component independently for color enhancement. For example, if both saturation and intensity components of a video frame are low and/or high, any correction to intensity component independently may bring up undesirable gray shades, such as shadows or may cause reduced vividness of color of interest and so on.
Furthermore, any enhancements with non-converging transfer characteristics curve corrections or gain under user control may lead, to over-saturation due to excess gain and there-by, result in clipping. Also, dynamic correction in video may cause flicker due to gain switching not being adaptable to slow or fast moving image sequences.
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.
USPTO Backgrounds
USPTO Backgrounds