I am trying to understand the concept of homomorphic filtering, I have read several online pages about this topic such as http://en.wikipedia.org/wiki/Homomorphic_filtering However, I noticed a scarce online resources that discuss this topic. Therefore, I have two questions.
First, I am wondering, how one can decide that an image can be enhanced using homomorphic filtering? In other words, what is the type of images that require homomorphic filtering?
Second, is homomrphic filtering always used to attenuate the illunimation (low frequency) and increase the reflectance (high frequency)? Or it can be also used to do the opposite (attenuate reflectance and highlight illunmiation)?
After further research, I found the answer,
1- Images that are useful to be enhanced using homomorphic filtering, are images where the illumination is distributed unequally causing the objects in the image to appear in a dark color. In general, these images are dark and so their details are hidden.
2- According to this source, Homomorphic filtering is a frequency domain filtering process that compresses the brightness (from the lighting condition) while enhancing the contrast (from the reflectance properties of the object).