Lecture 2 – 4
Graphics and Image Data Representations
(Chapter 3)
3.1 Graphics/Image Data Types (Lecture 2)
3.2 Popular File Formats (Lecture 3)
3.3 Further Exploration (Lecture 4)
(material will be provided and discussed)
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Graphics and Image Data Representations-Multimedia Applications-Lecture Slides, Slides of Multimedia Applications
This lecture was delivered by Dr. Paresh Sapan at Biju Patnaik University of Technology, Rourkela. This lecture is part of lecture series on Multimedia Applications course. It includes: Graphics, Image, Data, Representations, Types, Popular, File, Formats, Macromedia, Director, Monochrome
Typology: Slides
2011/2012
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Lecture 2 – 4
Graphics and Image Data Representations
(Chapter 3)
3.1 Graphics/Image Data Types (
Lecture 2
)
3.2 Popular File Formats (
Lecture 3
)
3.3 Further Exploration (
Lecture 4
)
(material will be provided and discussed)
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3.1 Graphics/Image Data Types
The number of file formats used in multimedia continues toproliferate. For example, Table 3.1 shows a list of some fileformats used in the popular product Macromedia Director.
Table 3.1: Macromedia Director File Formats
File Import
File Export
Native
Image
Palette Sound Video Anim. Image Video
.BMP, .DIB,
.PAL
.AIFF .AVI
.DIR
.BMP .AVI
.DIR
.GIF, .JPG,
.ACT
.AU
.MOV .FLA
.MOV .DXR
.PICT, .PNG,
.MP
.FLC
.EXE
.PNT, .PSD,
.WAV
.FLI
.TGA, .TIFF,
.GIF
.WMF
.PPT
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Fig. 3.1: Monochrome 1-bit Lena image.
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8-bit Gray-level Images
Each pixel has a gray-value between 0 and 255.
Each pixel
a value of 10, and a bright one might be 230.is represented by a single byte; e.g., a dark pixel might have
Bitmap
: The two-dimensional array of pixel values that rep-
resents the graphics/image data.
Image resolution
refers to the number of pixels in a digital
image (higher resolution always yields better quality).
Fairly high resolution for such an image might be 1
×
(^) 200, whereas lower resolution might be 640
×
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Plane 0
Plane 7
Bitplane
Fig. 3.2: Bit-planes for 8-bit grayscale image.
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Multimedia Presentation
to 255), so a 640Each pixel is usually stored as a byte (a value between 0
×
(^) 480 grayscale image requires 300 kB of
storage (
×
Fig. 3.3 shows the Lena image again, but this time in grayscale.
When an image is printed, the basic strategy of
dithering
is
printers.to provide ability to print multi-level images on 2-level (1-bit)used, which trades intensity resolution for spatial resolution
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Dithering
Dithering
is used to calculate patterns of dots such that
printer.and more filled at darker pixel values, for printing on a 1-bitvalues from 0 to 255 correspond to patterns that are more
tern, say 2The main strategy is to replace a pixel value by a larger pat-
×
(^) 2 or 4
×
(^) 4, such that the number of printed dots
halftone printingapproximates the varying-sized disks of ink used in analog, in
(e.g., for newspaper photos).
- Half-tone printing is an analog process that uses smaller
newspaper printing.or larger filled circles of black ink to represent shading, for
- For example, if we use a 2
×
dither matrix
(
)
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range 0..4 by (integer) dividing by 256we can first re-map image values in 0..255 into the new
Then, e.g., if
the pixel value is 0 we print nothing, in a 2
×
2 area of
dots.printer output. But if the pixel value is 4 we print all four
The rule is:
If the intensity is
the dither matrix entry then print
an
on
dot at that entry location: replace each pixel by
an
n
×
n
matrix of dots.
image, since replacing each pixel by a 4Note that the image size may be much larger, for a dithered
×
4 array of dots,
makes an image 16 times as large.
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BEGIN
for
x
= 0 to
x max
// columns
for
y
= 0 to
y max
// rows
i
x mod n
j
y mod n
I
x, y
) is the input,
O
x, y
) is the output,
D
is the dither matrix.
if
I
x, y
> D
i, j
O
x, y
else
O
x, y
END
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(a)
(b)
(c)
Fig. 3.4: Dithering of grayscale images.
(a): 8-bit grey image “lenagray.bmp”.
(b): Dithered version of
the image. (c): Detail of dithered version.
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24-bit Color Images
bytes, usually representing RGB.In a color 24-bit image, each pixel is represented by three
This format supports 256
(^) ×
(^256)
(^) ×
(^) 256 possible combined colors, or a
total of 16,777,216 possible colors.
However such flexibility does result in a storage penalty: A 640
(^) ×
(^480)
compression.24-bit color image would require 921.6 kB of storage without any
An important point
: many 24-bit color images are actually
pixel used to store anstored as 32-bit images, with the extra byte of data for each
alpha
value representing special effect
information (e.g., transparency).
Fig. 3.5 shows the image
forestfire.bmp.
, a 24-bit image
in
Microsoft
Windows
BMP
format.
Also
shown
are
the
for this image.grayscale images for just the Red, Green, and Blue channels,
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(a)
(b)
(c)
(d)
c, d): R, G, and B color channels for this imageimages. (a): Example of 24-bit color image “forestfire.bmp”. (b,Fig. 3.5 High-resolution color and separate R, G, B color channel
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Fig. 3.6: 3-dimensional histogram of RGB colors in “forestfire.bmp”.
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Fig. 3.7 shows the resulting 8-bit image, in GIF format.
Fig. 3.7 Example of 8-bit color image.
ones:Note the great savings in space for 8-bit images, over 24-bit
a 640
×
480 8-bit color image only requires 300 kB
without any compression applied).of storage, compared to 921.6 kB for a color image (again,
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