Multimedia Systems
Image I
(Acquisition and Representation)
Hamid R. Rabiee
Mahdi Amiri
March 2015
Sharif University of Technology
Course Presentation
Page 1 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
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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
Multimedia Systems, Mahdi Amiri, Image I
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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
Multimedia Systems, Mahdi Amiri, Image I
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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
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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
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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
Multimedia Systems, Mahdi Amiri, Image I
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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
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Image ResolutionImage Resolution
Image Resolution, Most Common Display Resolutions
Asp
ect Ratio
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HD
3rd gen. of
iPad (QXGA)
Full-HD
Google's Nexus 10
(WQXGA)
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Image ResolutionDigital TV Resolutions
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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.
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Image ResolutionUHDTV, HDTD, SDTV
Multimedia Systems, Mahdi Amiri, Image I
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.
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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
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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
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72 ppi 150 ppi
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Image ResolutionPixels per inch (ppi)
The average human eye can only detect 300 ppi.
iPhone 5/5s
4"
1136x640
326 ppi
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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
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Image ResolutionPixels per inch (ppi)
iPhone 4, 4s
3.5"
640x960
326 ppi
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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
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Image VisionHistogram
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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
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Image VisionContrast
Multimedia Systems, Mahdi Amiri, Image I
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
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Image VisionHistogram Equalization
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Histogram equalization is a method in image processing of
contrast adjustment using the image's histogram.
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Image File FormatsRaster and Vector Graphics
Raster Graphics (Bitmap)
.BMP, .JPG, .PNG, .GIF
Vector Graphics
.CGM, .SVGBoth
.AI, .CDR, .PSD, .TIFF
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Image RepresentationPanorama
Stitching images captured
above Milad Tower
Example Software:
“Hugin” and “AutoStitch”
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Image RepresentationAutoStitch Process
Example algorithm: SIFT Keypoint detection and matching.
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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)
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Image AcquisitionHDR Movie Demo
Play HDR movie demo
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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
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Page 26
Image AcquisitionFocus Bracketing
Focus stacked image
A sequence of 5 incrementally focused images
Example Software:
“CombineZP”
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Bracketing is the general technique of taking several shots of the same subject using different camera settings.
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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
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