Image Enhancements Flashcards
What is the ‘aim’ (or goal) for image enhancement?
The aim of image enhancement is to alter the distribution of DNs in each image channel in the image to increase the contrast, making the image interpretation easier.
Define Contrast Stretching
2 Different types?
Contrast stretching is designed to expand the original brightness values in an image to increase the contrast
Linear and Non-Linear
Linear Stretch
A Linear stretch increases the contrast while preserving the original radiance relationship between the pixels.
Non-Linear Stretch
Increases the contrast without preserving the original radiance relationship between pixels.
- different portions of the image may be stretched/compressed.
Spatial Filtering
the process of extracting the information from only the spatial domain (not spectral).
Convolution Filters
Example?
convolution filters multiply the kernel by a matrix of filtering coefficients
- example low pass filter
Non-convolution Filters
Example
Only work with numbers within the kernel.
- Example median filter
What is a Kernel
Kernels are smaller square subsets of the image on which the filtering operation is based.
Spatial Filtering - Spatial Frequency
The number of times a certain pixel occurs within an image.
Low frequency distribution
values do not change significantly over the range of the image.
High frequency distribution
values change drastically over the range of the image.
Low Pass Filter
filter is designed to remove or de-emphasize the imagery with a high spatial frequency
-smoothes the image.
How can you remove the effects of blurring from a low pass filter?
can use unequal weightings for the kernel.
High Pass Filter
designed to enhance abrupt changes in data within the image.
- fine detail reduced or removed in extreme cases.
- This tends to highlight edges and other sharp boundaries
Fast Fourier Transform (FFT)
A Fast Fourier Transform breaks an image down into two components: frequency and direction
- We can use this information to remove specific types of information depending on frequency
(to use low pass or high pass filter)
FFT: Pass procedure
only the information that occurs within the mask is retained.
FFT: Cut Procedure
only the information that occurs outside the mask is retained.
Edge Detection Filters: Non-directional filters.
Non-directional filters do not focus on a particular direction within the image but rather deal with linear features in all directions.
Examples: LaPlacian Filter
Edge Detection Filters: Directional Filters
Directional filters are only interested in accenting the features in one direction, or a limited number of directions.
Examples: Sobel Filters
Compass Gradient Filter
•A compass gradient filter enhances linear features according to compass directions
Sobel Filter
This is a nonlinear filter in that it combines the pixel values in a nonlinear fashion.
Intensity-Hue-Saturation
Only makes appearance look better = eye candy
Spectral Ratioing: Band-ratios
Band-ratios are commonly used to enhance or amplify the signal.
- enhance differences in brightness values from identical surface material
Principle Component Analysis - 3 functions
1) The first is to reduce the dimensionality of the data set.
2) The second reason for applying a PCA is to decorrelate the data.
3) for choosing the most appropriate data set(s) for use in a specific application.
Tasselled Cap Transformation
The result of the transformation is a series of new data sets, where each represents a unique reflectance component.
