Intelligent Vision Systems Image Geometry and Acquisition ENT 496 Ms. HEMA C.R. Lecture 2.

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Intelligent Vision Systems

Image Geometry and AcquisitionImage Geometry and Acquisition

ENT 496ENT 496

Ms. HEMA C.R.Ms. HEMA C.R.

Lecture 2.

2Hema-ENT 496 -Lecture 2

Road MapRoad Map

Image Formation

Image Geometry

Image Sampling

Image Quantization

Image Acquisition

Image Definitions

Image FormationImage Formation Image formation in the eye and the

camera Understanding function of the human

eye provides insight into Intelligent Vision System solutions

Biological vision is the process of using light reflected from the surrounding world as a way of modifying behavior

Image Formation in the eye

Light enters through cornea Passes through aqueous humor, the

lens and vitreous humor Finally forms an image on the retina Lens adjusts and focus image

directly on retina Retina is a complex tiling of

photoreceptors known as rods and cones

When stimulated by light they produce electrical signals that are transmitted to the brain by the optic nerve

Refer: http://homepages/inf.ed.ac.uk/rbf/CVonline/

Image formation :Pin hole camera

Camera is

analogous to

the eye Pin hole camera

has a small hole

through which

light enters before forming an inverted image Pin hole cameras are modeled by placing

image plane between focal point of the camera and the object so that image is not inverted

Image GeometryImage Geometry

Image Formation has two divisions

– Geometry of image formation

– Physics of light [brightness of point]

Image geometry determines where a world point is projected on the image plane

Object point is represented by x, y and z 3D co-ordinates.

Image plane is parallel to x and y axis [world] at a distance f [focal length]

y (x,y,z)

(x`, y`)

Object Point

22 yxr 22 yxr

y (x,y,z)

(x`,y`)

Object Point

Mapping of three dimensions onto two dimension is called perspective projection

Image SamplingImage Sampling Continuous images are sampled to convert them to

digital form Each image sample is called a pixel [picture

element] Sampling is the process of representing a continuous

signal by a set of samples taken at discrete intervals of time [sampling interval]

Continuous SignalSampled Signal

Sampling Frequency Tf s 1

Original Image

Sampled Image

Reconstructed Image

Image SamplingImage Sampling

Quantization is the process of converting analog pixel intensities to discrete valued integer numbers

Quantization involves assigning a single value to each sample values in such a way that the image reconstructed from quantized values is good

Image QuantizationImage Quantization

550 x 413 pixels 49868 colors

8 colors

Image QuantizationImage QuantizationFrom 600x400, 32 bits image

To 600x400, 2 bits image (4 colors)

To 600x400, 4 bits image (16 colors)

Image AcquisitionImage Acquisition Image acquisition is the first stage of a vision

system Acquired Image is dependent on

– Nature of sensing deviceVidicon, CCD, infra red , grayscale , color

– Properties of the deviceSensitivity, resolution, lenses, stability , focus

– The lighting of the sceneShadows, excessive reflection, poor contrast

– The environmentDust, fog, humidity

– The reflective properties of the objectsTexture, color, specularity

Image AcquisitionImage Acquisition

Two Dimensional Images Three Dimensional Images

Acquisition [capture] of 2D Images

– Monochrome or ColorAnalog Cameras Digital CCD CamerasDigital CMOS CamerasVideo Cameras [Analog and

Digital]

Methods of acquisition for 3D

– Laser Ranging Systems

– Structured Lighting Methods

– Moire Fringe Methods

– Shape from Shading Methods

– Passive Stereoscopic Methods

– Active Stereoscopic Methods

Image CapturingImage Capturing A basic image capture system

contains a lens and a detector. Film detects far more visual information than is possible with a digital system.

With digital imaging, the detector is a solid state image sensor called a charge coupled device...CCD for short

On an area array CCD, a matrix of hundreds of thousands of microscopic photocells creates pixels by sensing the light intensity of small portions of the image

Image CaptureImage Capture To capture images in color, red,

green and blue filters are placed over the photocells.

Film scanners often use three linear array image sensors covered with red, green and blue filters.

Each linear image sensor, containing thousands of photocells, is moved across the film to capture the image one-line-at-a-time.

Image DefinitionsImage Definitions

Pixel – A sample of the image intensity quantized to an integer value

Image – A two dimensional array of pixels Pixel

– Row and column indices [ i, j] are integer values

– Pixels have intensity values 0 to 255 grayscale images RGB value [vector value] color

images

Pixel ArrayPixel Array

Pixel [4,4] ↓i →j

Pixel Concept MapPixel Concept Map

PixelPixel The quality of a scanned

image is determined by pixel size, or spatial resolution; and by pixel depth, or brightness resolution

This relates to the two basic steps in the digital capture process:

In step one, sampling determines pixel size and brightness value.

In step two quantization determines pixel depth

Image File FormatsImage File Formats

Images are stored in a computer in one of the following formats, depending on the application of the images stored.

– Tagged Image Format [.tif]– Portable Network Graphics [.png]– Joint Photographic Experts Group

[.jpeg, .jpg]– Bitmap [.bmp]– Graphics Interchange Format [.gif]– Raster Images [.ras]– Postscript [.ps]

End of Lecture 2

Intelligent Vision Systems

Intelligent Vision Systems Image Geometry and Acquisition ENT 496 Ms. HEMA C.R. Lecture 2.

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