1. Technical Field
The present disclosure generally relates to an image contrast enhancement apparatus and a method thereof, and more particularly to an image contrast enhancement apparatus and a method thereof using the adaptive histogram adjustment.
2. Related Art
Under the environment in which surrounding light is not stable, such as the night, evening, or fog environment, the local contrast of the captured image may be weak. There are several method provided to solve the problem in which the contrast of the image is weak. For example, the technology provided by U.S. Pat. No. 7,573,533 B2 designs a transformation curve of an entire image to solve the problem mentioned above for the brightness distribution (such as high, middle, and low brightness) of each pixel in the entire image. For example, the technology provided by U.S. Pat. No. 7,453,524 B2 shifts levels of the YUV signal of the entire image, statistically analyzes the YUV signal of the entire image, and enhances the gray-level colors of the YUV signal of the entire image to solve the problem mentioned above. However, the two technologies for enhancing the entire image mentioned above have the drawback in which they only take a comprehensive consideration and can not enhance the local contrast.
Furthermore, for example, the technology provided by U.S. Pat. No. 7,386,186 B2 utilizes the edges of the image, and thus performs a non-liner transformation of the edges, such that the image is enhanced, and the problem mentioned above is solved. For example, the technology provided by U.S. Pat. No. 7,102,697 B2 remains the pixels having the high frequencies to solve the problem mentioned above by performing a statistical calculation on the spectrum distribution of the image. However, the two methods mentioned above have the drawback in which portions of the texture of the image may disappear.
Moreover, for example, the technology provided by TW Application 049097B adjusts the electrical signals before the image is completely captured, such that the image is enhanced and the problem mentioned above is solved. However, the method merely considers the light absorbing characteristic of each pixel in the image capturing apparatus, but ignores the whole content of the image.
The other technologies modified from the methods mentioned above are also provided. For example, the rubber band function known by people skilled in the art is used to adjust the histogram bins and to change the levels of the image. For example, the histogram equalization method can improve the contrasts of the images in the different applications. Furthermore, for example, the adaptive histogram equalization method and the gray-level grouping method based on the non-liner histogram can be used to automatically enhance contrast of the image. However, the methods mentioned above may have the problem of the block effect when the image is displayed.