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Multimedia Systems

Image I

(Acquisition and Representation)

Hamid R. Rabiee

Mahdi Amiri

March 2015

Sharif University of Technology

Course Presentation

Multimedia Systems, Mahdi Amiri, Image I

Image RepresentationColor Depth

1bit

2bit

4bit

8bit

24bit

The number of bits used

to represent the color of

a single pixel.

bits per pixel (bpp).

1bit: Monochrome

24bit: Truecolor

Image RepresentationIndexed Color, Palette

It is a form of vector quantization compression.

A 2-bit indexed

color image

Color table

(the palette)

8-bit (256-color)

Indexed image and

its own palette

8-bit Grayscale

image and palette


Image Representation, PaletteDisadvantages

Limited set of simultaneous colors per image.

If the original color palette for a given indexed image is

lost, it can be nearly impossible to restore it.

4-bit8-bit24-bit Incorrect

palette


Image RepresentationHalftone

A technique that simulates continuous tone imagery through the use

of dots, varying either in size, in shape or in spacing.

How the human eye

would see this sort of

arrangement from a

sufficient distance.

Halftone

dots

Three examples of color halftoning with CMYK separations. From left to

right: The cyan separation, the magenta separation, the yellow separation, the

black separation, the combined halftone pattern and finally how the human eye

would observe the combined halftone pattern from a sufficient distance.

Colo

r Halfto

nin

g


Image Representation, DitheringDefinition

An intentionally applied form of noise used to randomize

quantization error.

Etymology: …Mechanical computers performed more accurately

when flying on board the aircraft, and less well on ground!

Application: Increasing color depth without adding new bits

1-bit

black and white thresholding

24-bit 1-bit,

with Floyd-Steinberg dithering


Image Representation, DitheringFloyd–Steinberg Algorithm

Distribute the quantization residual to neighboring

pixels that have not yet been processed.

Pseudocode:

for each y from top to bottom

for each x from left to right

oldpixel := pixel[x][y]

newpixel := find_closest_palette_color(oldpixel)

pixel[x][y] := newpixel

quant_error := oldpixel – newpixel

pixel[x+1][y] := pixel[x+1][y] + 7/16 * quant_error

pixel[x-1][y+1] := pixel[x-1][y+1] + 3/16 * quant_error

pixel[x][y+1] := pixel[x][y+1] + 5/16 * quant_error

pixel[x+1][y+1] := pixel[x+1][y+1] + 1/16 * quant_error

Distribution matrix


Image Representation, DitheringColor Banding Artifact

Dithering prevents large-scale patterns such as "banding" in images.

Web-safe color palette

with no dithering

Web-safe color palette

with Floyd–Steinberg

dithering


Image ResolutionImage Resolution

Image Resolution, Most Common Display Resolutions

Asp

ect Ratio


HD

3rd gen. of

iPad (QXGA)

Full-HD

Google's Nexus 10

(WQXGA)

Image ResolutionDigital TV Resolutions


en.wikipedia.org/wiki/4K_resolution

The term 4K refers to the

horizontal resolution of these

formats, which are all on the

order of 4,000 pixels.

4K Ultra high definition television (UHD)

is a resolution of 3840 pixels × 2160 pixels

(8.3 megapixels, aspect ratio 16:9).

8K UHD which is 7680 pixels × 4320

pixels (33.2 megapixels).

16:9 resolutions in comparison.

Image ResolutionUHDTV, HDTD, SDTV


http://en.wikipedia.org/wiki/Ultra_high_definition_television

Chart showing resolutions for 8K UHDTV, 4K

UHDTV, 1080p HDTV, and 480i SDTV.

SDTV: Standard-definition television.

HDTV: high-definition television.

UHDTV: Ultra HDTV.

Diagram of the CIE 1931 color space that shows the Rec. 2020

(UHDTV) color space in the outer triangle and Rec. 709 (HDTV)

color space in the inner triangle. Both Rec. 2020 and Rec. 709 use

Illuminant D65 for the white point.

Image ResolutionMegapixel (MP)

One million pixels

To express:

The number of pixels in an image

The number of image sensor elements of digital cameras

The number of display elements of digital displays

2048×1536 sensor elements, or QXGA display

3.1 MP (2048 × 1536 = 3,145,728)

8 MP Phone

Camera

160 MP

Camera


Image ResolutionPixels per inch (ppi)

Pixels per inch (PPI) or pixel density is a measurement of the

resolution of devices in various contexts; typically computer displays,

image scanners, and digital camera image sensors.

18 ppi


72 ppi 150 ppi


The average human eye can only detect 300 ppi.

iPhone 5/5s

4"

1136x640

326 ppi


List of displays by pixel density

http://en.wikipedia.org/wiki/List_of_displays_by_pixel_density

HTC One

4.7"

1920x1080

468 ppi

Galaxy Note 3

5.7"

1080x1920

388 ppi

iPad 4/Air

9.7"

2048x1536

264 ppi

Lumia 920

4.5"

1280x768

332 ppi

Xperia Z

5"

1920x1080

443 ppi


iPhone 4, 4s

3.5"

640x960

326 ppi


iPad 1, iPad 2

9.7"

1024x768

132 ppi

Nokia N95

2.6"

240x320

153 ppi

Google Nexus One

3.7"

480x800

254 ppi

iPad 3

9.7"

2048x1536

264 ppi

Nokia Lumia 800

3.7"

800x480

252 ppi

Samsung I9100 Galaxy S II

4.27"

480x800

219 ppi

Pixels per inch for a

few more devices.

Galaxy Note II

5.55"

720x1280

267 ppi

Image VisionHistogram


Plots the number of pixels for each tonal value. By looking at the histogram for a specific

image a viewer will be able to judge the entire tonal distribution at a glance.

Intensity (tonal value)

Count (Number of

pixels for each

different intensity

value)

Image

histogram

Image VisionContrast


Typ. histogram of

a low contrast image

Typ. histogram of

a high contrast image

Contrast is the difference in visual properties that

makes an object distinguishable from other objects

and the background.

Formula

Image VisionHistogram Equalization


Histogram equalization is a method in image processing of

contrast adjustment using the image's histogram.

Image Vision


Histogram equalization, Example

Image File FormatsRaster and Vector Graphics

Raster Graphics (Bitmap)

.BMP, .JPG, .PNG, .GIF

Vector Graphics

.CGM, .SVGBoth

.AI, .CDR, .PSD, .TIFF


Image RepresentationPanorama

Stitching images captured

above Milad Tower

Example Software:

“Hugin” and “AutoStitch”


Image RepresentationAutoStitch Process

Example algorithm: SIFT Keypoint detection and matching.


Image RepresentationLarge Screen Projection


Image AcquisitionHigh-Dynamic-Range (HDR)

HDR, Accurately

representing the range

of intensity levels

found in real scenes

4 Images captured with

different Exposure

Values (EV or stop)


Image AcquisitionHDR Movie Demo

Play HDR movie demo


../Resources/Image/The HDR Movie function demo1.mp4

Image Acquisition, HDRAlgorithm: Tone Mapping

To overcome the limited dynamic

range of current standard digital

imaging techniques

Tone mapped HDR image

A simple version of tone mapping:

Mean Value Mapping

This is

Exposure Bracketing


Image AcquisitionFocus Bracketing

Focus stacked image

A sequence of 5 incrementally focused images

Example Software:

“CombineZP”


Bracketing is the general technique of taking several shots of the same subject using different camera settings.

Image AcquisitionFocus Bracketing

The resulting focus stacked image with an

extended depth of field

The three source image

slices at three focal depths

Contributions in the final

"focus stacked" image

Example Application:

Microscopy


Thank You

1. http://sharif.edu/~Rabiee/

2. http://www.dml.ir/

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