Digital Image Processing Image Enhancement Histogram Processing 2
- Slides: 45
Digital Image Processing Image Enhancement (Histogram Processing)
2 of 45 Contents Over the next few lectures we will look at image enhancement techniques working in the spatial domain: – What is image enhancement? – Different kinds of image enhancement – Histogram processing – Point processing – Neighbourhood operations
3 of 45 A Note About Grey Levels So far when we have spoken about image grey level values we have said they are in the range [0, 255] – Where 0 is black and 255 is white There is no reason why we have to use this range – The range [0, 255] stems from display technologies For many of the image processing operations in this lecture grey levels are assumed to be given in the range [0. 0, 1. 0]
4 of 45 What Is Image Enhancement? Image enhancement is the process of making images more useful. The reasons for doing this include: – Highlighting interesting detail in images, – Removing noise from images, – Making images more visually appealing.
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 5 of 45 Image Enhancement Examples
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 6 of 45 Image Enhancement Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 7 of 45 Image Enhancement Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 8 of 45 Image Enhancement Examples (cont…)
9 of 45 Spatial & Frequency Domains There are two broad categories of image enhancement techniques – Spatial domain techniques • Direct manipulation of image pixels – Frequency domain techniques • Manipulation of Fourier transform or wavelet transform of an image For the moment we will concentrate on techniques that operate in the spatial domain.
10 of 45 Image Histograms Frequencies The histogram of an image shows us the distribution of grey levels in the image Massively useful in image processing, especially in segmentation. Grey Levels
11 of 45 Image Histograms plots how many times (frequency) each intensity value in image occurs
12 of 45 Image Histograms • Many cameras display real time histogram of a scene. • Also easier to detect types of processing previously applied to image
13 of 45 Image Histograms • Histograms: only statistical information • No indication of pixels locations
14 of 45 Image Histograms • Different images can have same histogram • 3 images below have same histogram • Half of pixels are gray, half are white - Same histogram = same statistics Distribution of intensities could be different • Can we reconstruct image from histogram? NO
15 of 45 Image Histograms The histogram of a digital image with gray levels in the range [0, L-1] is a discrete function h (rk) = nk rk: the kth gray level nk: the number of pixels in the image having gray level rk
16 of 45 Normalized histogram Dividing each of its values by the total number of pixels in the image (n). p(rk) = nk/ n For k = 0, 1, ……, L-1. p(rk): Gives an estimate of the probability of occurrence of gray level rk. The sum of all components of a normalized histogram is equal to 1.
17 of 45 Image Contrast The contrast of a grayscale image indicates how easily objects in the image can be distinguished • High contrast image: many distinct intensity values. • Low contrast: image uses few intensity values.
18 of 45 Histograms and Contrast Good Contrast? Widely spread intensity values + large difference between min and max intensity values Low contrast Normal contrast High contrast
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 19 of 45 Histogram Examples
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 20 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 21 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 22 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 23 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 24 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 25 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 26 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 27 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 28 of 45 Histogram Examples (cont…)
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 29 of 45 Histogram Examples (cont…) A selection of images and their histograms Notice the relationships between the images and their histograms Note that the high contrast image has the most evenly spaced histogram
30 of 45 Contrast Stretching We can fix images that have poor contrast by applying a pretty simple contrast specification The interesting part is how do we decide on this transformation function?
31 of 45 Histogram Equalisation Spreading out the frequencies in an image (or equalising the image) is a simple way to improve dark or washed out images Can be expressed as a transformation of histogram Where: – rk: input intensity – sk: processed intensity – k: the intensity range (e. g 0. 0 – 1. 0)
32 of 45 Histogram Equalisation Let the variable r represent the gray levels of the image to be enhanced, with r=0 representing black and r=1 representing white. For any r, the following transformations produce a level s for every pixel value r in the original image. s = T(r) 0 <=r <=1 The transformation function T(r) satisfies the following condition: (a) T(r) is a single-valued and monotonically increasing in the interval 0 <=r <=1 (b) 0 <= T(r) <=1 for 0 <=r <=1
33 of 45 Histogram Equalisation • The requirement in (a) that T(r) be single valued is needed to guarantee that the inverse transformation will exist, and monotonicity condition preserves the increasing order from black to white in the output image. • i. e. A pixel which is darker in the original image should remain darker in the processed image, and a pixel which is brighter in the original should remain brighter in the processed image.
34 of 45 Histogram Equalisation • Condition (b) guarantees that the output gray levels will be in the same range as the input levels. • i. e. Ensure that the processed image that you get doesn’t lead to a pixel value which is higher than the max. intensity value that is allowed.
35 of 45 Histogram Equalisation The formula for histogram equalisation is given where – rk: input intensity – sk: processed intensity – k: the intensity range (e. g 0. 0 – 1. 0) – nj: the frequency of intensity j – n: the sum of all frequencies
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 36 of 45 Equalisation Transformation Function
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 37 of 45 Equalisation Examples 1
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 38 of 45 Equalisation Transformation Functions The functions used to equalise the images in the previous example
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 39 of 45 Equalisation Examples 2
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 40 of 45 Equalisation Transformation Functions The functions used to equalise the images in the previous example
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 41 of 45 Equalisation Examples (cont…) 3 4
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 42 of 45 Equalisation Examples (cont…) 3 4
Images taken from Gonzalez & Woods, Digital Image Processing (2002) 43 of 45 Equalisation Transformation Functions The functions used to equalise the images in the previous examples
44 of 45 Assignment 1. Histogram equalization proof. 2. Exposure? a. Underexposed b. Properly Exposed c. Overexposed
45 of 45 Summary We have looked at: – Different kinds of image enhancement – Histograms – Histogram equalisation Next time we will start to look at point processing and some neighbourhood operations
- Histogram processing in digital image processing
- Point processing operations in image processing
- Neighborhood processing in image processing
- A generalization of unsharp masking is
- Image processing
- Morphological
- Matlab histeq
- Histogram analysis in image processing
- Normalisasi histogram
- Translate
- What is image restoration in digital image processing
- Image compression model in digital image processing
- Key stages in digital image processing
- Fidelity criteria in digital image processing
- Image sharpening and restoration
- Geometric transformation in digital image processing
- Digital image processing diagram
- Image transform in digital image processing
- Maketform matlab
- Image restoration in digital image processing
- Digital foil enhancement
- Inverse log transformation in image processing
- Image enhancement in night vision technology
- Objective of image enhancement
- Gamma correction image processing
- Image enhancement in spatial domain
- Image enhancement in spatial domain
- Image enhancement in spatial domain
- Image enhancement in spatial domain
- Image enhancement
- Histogram
- In a dark image the components of histogram
- Minimum perimeter polygon in digital image processing
- Representation and description in digital image processing
- Double thresholding matlab
- Segmentation in digital image processing
- Relation between pixels in digital image processing
- Some basic intensity transformation functions
- Zooming and shrinking of digital images
- Imadjust
- If s is a subset of pixels pixels p and q are said to be
- Coordinate conventions in digital image processing
- Dam construction in image processing
- Digital image processing java
- Thresholding in digital image processing
- Segmentation in digital image processing