Histogram and Its Processing -...
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Histogram and Its Processing 3rd Lecture on Image Processing Martina Mudrová 2004
Transcript of Histogram and Its Processing -...
Histogramand Its Processing
3rd Lectureon Image Processing
Martina Mudrová 2004
Definition
What a histogram is?
= vector of absolute numbers occurrence of every colour in the picture[H(1),H(2),…H(c)]
• 1 vector H is used for intensity images• 3 vectors H1,H2,H3 are used in case of colour pictures (for R,G,B components,
each of them is processed separatelly according to the same scheme)
c ≤ m.n c…number of coloursm,n…image size∑
=
=c
inmiH
1.)(
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Histogram Properties
• histogram is a statistical value describing the probability of occurency each colour• it says nothing about colour layout in the picture
Two different pictures:
Histogram both of them – bimodal
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Another Ways to Histogram Expression
• normalizing the range of x-axis to the <0,1>• displaying a cumulative sum of histogram values
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Histogram Use I
• histogram provides basic information about brightness level in the picture– further histogram processing can improve picture quality
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this picture has very low level of contrast- histogram bars are only in the middle part
this picture is too bright-histogram bars are almost in the upper part
this picture is too dark-histogram bars are almost in the low part
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Histogram Use II
• enables threshold level T selection for picture colour reduction- especially in case of bimodal histogram- application in the shape recognition, granulometry,…
yL pro x TH pro x T
=<≥
⎧⎨⎩
x… input intensityy… new intensityT… threshold level
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Histogram Use III
• serves for cameras calibration
– there are taken the pictures with known histogram, the comparison of given and obtained histogram serves for settingsof camera parameters
Example of testing picture used for cameras calibrations and tests
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Histogram Equalization
What do I mean by this?
• Make a histogram as flat as is possible
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Ideal histogram
• The algorithm of equalization is based on statistical methods:
D … the optimal intensity valuen,m ... Image size (in pixels)MAX... Maximal intensity value MAX
mnD .=
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Example of Histogram Equalization
Original Image Image after histogram equalizationoriginal po ekvalizaci.
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1 0 0 0
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h i s t o g r a m
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0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 00
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1 0x 1 0 4 k u m u l a c e c a r
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Histogram Adjustment
What happens if I change the histogram bars position?
0 1l h
b t
y = xγ
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1. Shifting the histogram2. Stretching the histogram
A. Linear correction: g = 1
B. Non-Linear correction: g <> 1
x…input histogram
y…output histogram
Gamma correction
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input value x
outp
ut v
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yg=0.5g=2
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Shifting Histogram Bars to the Right
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• Increasing the brightness shifts the histogram to the right (towards white).
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Shifting the Histogram Bars to the Left
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• Decreasing the brightness shifts the histogram to the left (towards black).
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Another Possibility of Brightness Control ?
Can I simple add any constant to every pixel’s value?original original+0.3
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his togram po uprave
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his togram originalu
! This way can lead to the destroying of “highlight” detailswithout possibility of their recovering by means of following histogram operations
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Loosing Shadow Details
Can I simple subtract any constant to every pixel’s value?
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! This way can lead to the destroying of “shadow” detailswithout possibility of their recovering by means of following histogram operations
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Histogram Dilatation and Stretching - Contrast
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Histogram dilatation causes contrast increasing
Histogram stretching causes contrast decreasing
! Be careful about loosing “shadow” and “highlight” details during the histogram dilatation, any following histogram stretching can not recover them – information can be beyond redemption
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Gamma Correction
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y = x γ
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x…input histogram
γxy =0<γ <1…a picture becomes lighterγ >1… a picture becomes darker γ =1…linear correction
y…output histogram
Gamma correction
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! Some digital cameras work accordingto another equation:
γ1
xy =16
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Gamma Correction Use
Histogram Adjustment Parameters
Image Histogram
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original image
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5 0shifting histogram bars from<0.25,1> to <0, 0.75>, γ =1
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shifting histogram bars from<0.25,1> to <0, 0.75>, γ =0.2
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Advanced Histogram Operations
- Operations with a LUT (Look-up table)
-1 new color bar is assigned to 1,2 or more original colors- non-inversible operation- is used for interesting artificial (artistic) effects (substitution of water colour paintingf from realistic photos…)
- New histogram with another shape can be imposed to the original picture
- …
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Matlab Commands for Histogram Operation
imhist
histeq
imadjust
(brighten)
(contrast)
stretchlim
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Example
Histogram Equalizationoriginal
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histogram originalu
po ekvalizaci.
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histogram
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% Image Adjustment (1)% Histogram Equalization
cleardelete(get(0,'children'));
[x,map]=imread('busek.bmp');i=ind2gray(x,map);subplot(321), imshow(i)title('original')subplot(323),imhist(i)title('histogram originalu'),j=histeq(i,32);subplot(322), subimage(j )axis off,title('po ekvalizaci.')subplot(324),imhist(j)title('histogram '),subplot(325),plot(cumsum(imhist(i)))title('kumulace car'),subplot(326),plot(cumsum(imhist(j)))
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Example
Linear Histogram AdjustmentOriginal Image Histogram of Original Image% Histogram adjustment (2)
cleardelete(get(0,'children'))
figure(1)[x,map]=imread('../busek.bmp');i=ind2gray(x,map); j=imadjust(i,[0 1],[0 0.8],1); subplot(221),imshow(i)title('original')subplot(222),imhist(i,128)title('histogram originalu')subplot(223),imshow(j)title(' po uprave')subplot(224),imhist(j)title('histogram po uprave')
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Histogram of Modified ImageModified Image
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