selected advanced topics

1G52IIP, School of Computer Science, University of Nottingham

Storing and Retrieving Images

Content-based Image/Video Indexing and Retrieval

Selected Advanced Topics

Problem

Find all images contain horses …..

Text-based technology Annotation: Each image is indexed

with a set of relevant text phrases, e.g.,

Retrieval: based on text search technology

Appropriate phrases to describe the content of this image include:

Mother, Child, Vegetable, Yellow, Green, Purple ….

Text-based technology - Drawbacks

Annotation - subjectivedifferent people may use different

phrases to describe the same or very similar image/content

Text-based technology - Drawbacks

Annotation - Laborious

It will take a lot of man-hours to label large image/video databases with 1m+ items

Content-based Technology

Using Visual Examples

r% g% b%

Using Visual Features

Content-based image indexing and retrieval (CBIR), is an image database management technique, which indexes the data items (images, or video clips) using visual features (e.g., color, shape, and texture) of the images or video clips.

A CBIR system lets users find pictorial information in large image and video databases based on visual cues, such as colour, shape, texture, and sketches.

The visual features, computed using image processing and computer vision techniques are used to represent the image contents numerically.

Image Content - a high level concept, e.g., this image is a sunset scene, a landscape scene, etc.

Numerical Content Representations - Low level numbers, often the same set of numbers can come from very different images, making the task very hard!

Techniques for Computing Visual Features/Representing Image Contents –

some are very sophisticated, and many are still not matured

hence the computational processes in some cases are automatic but in other cases are semi-automatic in the most difficult cases, it may have to be done

manually

Comparing Image Content/Retrieving Images based on Content

Simple approaches - compute the metric distance between low level numerical representations

Advanced Approaches - using sophisticated pattern recognition, artificial intelligence, neural networks, and interactive (relevant feed-back) techniques to compare the visual content (low level numerical features)

Content-based Technology - IBM QBIC System

The IBM’s QBIC (Query by Image and Video

Content) system is one of the early examples

of CBIR system developed in 1990s.

The system lets users find pictorial information

in large image and video databases based on

color, shape, texture, and sketches.

The User Interfaces Module Let user specify visual query by drawing, selecting from a color

wheel, selecting a sample image …

Display results as an ordered set of images

The Database Population and Database Query Modules Database population - process images and video to extract

features describing their content - colors, textures, shapes and camera and object motion, and store the features in a database

Database Query - let user compose a query graphically, extract features from the graphical query, input to a matching engine that finds images or video clips with similar features

The Data Model

Still image, or scene - full image Objects contained in the full image - subsets of an

image Videos - broken into clips called shots - sets of

contiguous frames Representative frames, the r-frames, are generated

for each shot R-frames are treated as still image - from which

features are extracted and stored in the database. Further processing of shots generates motion objects

- e.g., a car moving across the screen.

Queries are allowed on

Objects - e.g., Find images with a red round object Scenes - e.g., Find images that have approximately

30% red and 15% blue colors Shots - e.g., Find all shots panning from left to right A combination of above - e.g., Find images that have

30% red and contain a blue textured objects

Similarity Measures

Similarity queries are done against the database of pre-computed features using distance functions between the features

Examples include, Euclidean distance, City-block distance, ….

These distance functions are intended to mimic human perception to approximate a perceptual ordering of the database

But, it is often the case that a distance metric in a feature space will bear little relevance to perceptual similarity.

Content-based Technology - Basic Architecture

Meta data Imagery

color texture shape positions ….Record1

color texture shape positions ….Record n

texture

positions

Image DatabaseQuery

Similarity Measures

Colour - An effective Visual Cue

Colors can be a more powerful visual cue than you initially thought!

What soft drink

Which fruit?

Colour - An effective Visual Cue

In many cases, color can be very effective.

Here is an example

Results of content-based image retrieval using 4096-bin color histograms

Colour SpacesColour Models

RGB Model: This colour model uses the three NTSC primary colours to describe a colour within a colour image.

Yellow

Magenta White

Sometimes in Computer Vision, it isconvenient to use rg chromaticity space

r = R/(R+G+B)g= G/(R+G+B)

Colour SpacesYIQ Model: The YIQ models is used in commercial colour TV broadcasting, which is a re-coding of RGB for transmission efficiency and for maintaining compatibility with monochrome TV standard.

331.0523.0212.0

321.0275.0596.0

114.0587.0299.0

In YIQ, the luminance (Y) and colour information (I and Q) are de-coupled.

YCbCr Model

Y = 0.299R + 0.587G + 0.114BCb = -0.169R - 0.331G + 0.500BCr = 0.500R - 0.419G - 0.081B

Perceived Color Differences

One problem with the RGB colour system is that colorimetric distances between the individual colours don't correspond to perceived colour differences.

For example, in the chromaticity diagram, a difference between green and greenish-yellow is relatively large, whereas the distance distinguishing blue and red is quite small. r = R/(R+G+B)

g= G/(R+G+B)

CIELAB CIE (Commission Internationale

de l'Eclairage) solved this problem in 1976 with the development of the Lab colour space. A three-dimensional color space was the result. In this model, the color differences which you perceive correspond to distances when measured colorimetrically. The a axis extends from green (-a) to red (+a) and the b axis from blue (-b) to yellow (+b). The brightness (L) increases from the bottom to the top of the three-dimensional model.

With CIELAB what you see is what you get (in theory at least).

Colour Histogram

Given a discrete colour space defined by some colour axes (e.g., red, green, blue), the colour histogram is obtained by discretizing the image colours and counting the number of times each discrete colour occurs in the image.

The image colours that are transformed to a common discrete colour are usefully thought of as being in the same 3D histogram bin centered at that colour.

Colour Histogram Construction

Step 1

Colour quantization (discretizing the image colours)

Step 2

Count the number of times each discrete colour occurs in the image.

Colour Quantization

A true colour, 24-bit/pixel image (8 bit - R, 8 bit - G, 8 bit -B), will have 224 = 16777216 bins !

That is, each image will have to be represented by over 16 million numbers

computationally impossible in practice not necessary

Colour quantization - reduce the number of (colours) bins

Simple Colour Quantization

Simple Colour Quantization (Non-adaptive)

Divide each colour axis into equal length sections (different axis can be divided differently).

Map (quantize) each colour into its corresponding bin

Simple Colour Quantization

0 31 63 95 127 159 191 223 255

Colour Bin Colour Bin

(123,23,45) (3, 0, 1 ) (122, 28, 46) (3, 0, 2)

(132, 29,50) (4, 0, 1) (122, 172, 27) (3, 5, 0)

(121,26,48) (x, x, x) (142, 28, 46) (x, x, x)

Example: In RGB space, quantize each image colour into one of 8x8x8 = 512 colour bins

Advanced Colour Quantization Adaptive Colour quantization (Not

required)

Vector Quantization K-means clustering K representative colours The colour histogram consists of K bins,

each corresponding to one of the representative colours.

A pixels is classified as belonging to the nth bin if the nth representative colour is the one (amongst all the representative colours) that is closest to the pixel.

A pixel is a point in the 3D colour space

Representative colours

Colour Histogram Construction - An Example

A 3 x 3, 24-bit/pixel image has following RGB planes

Construct an 8-bin colour histogram (using simple colour quantization, treating each axis as equally important).

23 24 7711 24 6922 12 12

213 24 7711 232 23922 12 12

23 24 7712 24 6922 123 123

Bin (0,0,0) = Bin (0,0,1) = Bin (0,1,0) = Bin (0,1,1) =

Bin (1,0,0) = Bin (1,0,1) = Bin (1,1,0) = Bin (1,1,1) =

Colour Histogram Construction - An Example

Quantized Colour Planes

Count the number of times each discrete colour occurs in the image.

0 0 00 0 00 0 0

1 0 00 1 10 0 0

0 0 00 0 00 0 0

Bin (0,0,0) = 6 Bin (0,0,1) = 0 Bin (0,1,0) = 3 Bin (0,1,1) = 0

Bin (1,0,0) = 0 Bin (1,0,1) = 0 Bin (1,1,0) = 0 Bin (1,1,1) = 0

Colour Based Image Indexing

# of pixels

10000100103000

Color Distribution = (10,0,0,0,100,10,30,0,0)

The histogram of colours in an image defines the image colour distribution

Colour based Image Retrieval

Images are similar if their histograms are similar!

Colour Distribution = (10,0,0,0,100,10,30,0,0)

Colour Distribution = (0,40,0,0,0,0,0,0,110,0)

Dissimilar

Similar!

Formalizing Similarity

Colour Distribution = (10,0,0,0,100,10,30,0,0) 1H

Similarity(Image 1, Image 2) = D (H1, H2)

where D( ) is a distance measure between vectors (histograms) H1 and H2

Metric DistancesA distance measure D( ) is a good measure if it is a metric!

D(a,b) is a metric if

D(a,a) = 0 (the distance from a to itself is 0

D(a,b) = D(b,a) (the distance from a to b = distance from b to a)

D(a,c) <= D(a,b) + D(b,c) ( triangle inequality [ the straight line distance is always the least!] )

D(a,b) + D(b,c) should be no smallerthan D(a,c)

Common Metric Distance measures

iii HH

21 ),min(HI(H1, H2) =

H1 = (10, 0, 0, 0, 100, 10, 30, 0, 0)

H2 = ( 0, 40, 0, 0, 0, 6, 0, 110, 0)

Similarity = HI(H1, H2) = 0 + 0 + 0 + 0 + 0 + 6 + 0 + 0 = 6

Histogram Intersection, HI

Euclidean or straight-line distance or L2-norm, D2

21221212 ),( HHHHHHDi

ii Root-mean square

H1 = (10, 0, 0)

H2 = ( 0, 40, 0)

Similarity = D2(H1, H2) = sqrt(100 + 1600 +0) = 41.23

Manhattan or city-block or L1-norm, D1

2121211 ),( HHHHHHDi

ii sum of absolute

differences

H1 = (10, 0, 0)

H2 = ( 0, 40, 0)

Similarity = D1(H1, H2) = (10 + 40 +0) = 50

Histogram Intersection vs City Block Distance

Theorem: if H1 and H2 are colour histograms and the total countin each is N (there are N-pixels in an image) then:

2121 5.0 HHHHN (Histogram Intersection inversely proportional to a metric distance!)

ii HHHH ),min( 2121 (by definition)

1),max(),min( bababa

Histogram Intersection vs City Block Distance

),min(),max( bababa (3)

Substituting (2) and (3) in (1)

i i iiiiii

HHHHHH

212121

),min(

),max(),min(

2121 ),min(2 HHNNHH iii

2121 5.0 HHHHN

BuildHistogram

HistogramDatabase

Colour Histogram Database

How well does Color histogram intersection work ?

66 test histograms in the database

31 query images

Recognition rate almost 100%

Indeed, because color indexing worked so well it is athe heart of almost all image database systems

Swain Original Test:

Google Image Search

After clicking this colour patch

Problems with color histogram matching

1. Color Quantization problem:

Because, thetwo images haveslightly different color distributionstheir histograms havenothing in common!

0 intersection!

Sources of quantization error: noise, illumination, camera

2. The resolution of a color histogram

Colour Distribution = (0,40,0,0,0 … ,0,0,110,0)

For the best results, Swain quantized colourspace into 4096 distinct colours => Each colourdistribution is a 4096-dimensional vector.

=> Histogram intersection costs O(4096) operations(some constant * 4096)

4096 comparisons per database histogram => histogram intersectionwill be very slow for large databases

Many newer methods work well using 8 - 64 D features

3. The colour of the light

Under a yellowish light all image colours aremore yellow than they ought to be

All four images have the same color distribution - need to take into account spatial relationships!

4. The structure of colour distribution

Problem solution => Use statistical moments

1st order statistics

=Mean BGR ,,

=Variance/Covariance GBRBRGBGR ,,,,, 222

2nd order statistics

22 )(/1 Ri

iR RN ))((/1 GiRi

iRG GRN

Statistical similarity

Colour Distribution = (50,50,50)

Colour Distribution = (20,70,40)

....][...)]()([)]()([ 222221

221 GRGGRR IIII Statistical similarity =

Compare mean RGBs(In general compare all statistical measures)

(Euclidean distance between corresponding statistical measures)

Histogram vs Statistical Similarity

histogram

Low order stats

Low and highorder stats

Completeness ofrepresentation

#params/Match speed

Sensitivity to Quantization error

Many/slow

Few/fast

Many/moderate

complete

incomplete

complete(or over complete)

sensitive

insensitive

sensitive

Advanced Topics

Fast Indexing Interactive/Relevant Feedback Reducing the Semantic Gap Visualization, Navigation, Browsing Internet scale image/video retrieval

Flickr – billions of photos Youtube – billions of videos …

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