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Digital Images

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Title: Digital Images


1
Digital Images
  • Art 311
  • Dr. J. R. Parker

2
Key concepts 6 - Simulation
  • Concept of simulation is difficult.
  • A simulation is a functional representation of an
    object or process (a system).
  • It has the ability to produce the outputs from a
    system given a set of inputs, but it does not
    create he outputs using the same mechanism. It is
    based on a model of the system.

3
Key concepts 6 - Simulation
  • Why do it?
  • To get results when the system is not well
    understood
  • To get results when to do the real thing is
    dangerous
  • To get results quickly.
  • To allow multiple systems where one is
    expensive/hard to build.

4
Key concepts 6 - Simulation
  • Example?
  • NASA used simulations.
  • A computer game is a simulation
  • In virtual reality of any resolution we need a
    simulation of the underlying world.
  • Allows use of AI, 3D graphical models, sounds,
    interaction with simulated objects.

5
How are images represented?
  • Computer images (digital) are stored as a 2D
    array or grid of values.
  • The values are, of course, numbers. They are
    called picture elements, or pixels.
  • The numbers represent the darkness or lightness,
    or the color, of the image at that point.

6
How are images represented?
  • Pixels are relative values.
  • A value of 5 might be light or dark depending on
    the possible range of values, and the protocol.
  • Protocol smaller numbers are dark. 0 is darkest
    (black)
  • If the largest level is 8, then 5 is middle grey.
    If the largest is 255, then 5 is almost black.

7
How are images represented?
8
How are images represented?
9
How are images represented?
A dead pixel.
10
How are images represented?
  • Images are digitized from real scenes or photos.
    It is also called quantization the conversion
    of greys into discrete numeric values.
  • The degree of quantization describes the number
    of possible levels.
  • One byte (8 bits) allows 256 levels, from 0 to
    255. This is common. 32 is needed for image
    display.

11
How are images represented?
16x16 levels 256
levels per row
12
How are images represented?
This image has 8 distinct levels. We can see
contouring, where distinct regions meet.
13
How are images represented?
A few levels can convey a lot of information or
feeling.
14
How are images represented?
  • How many pixels are in an image is a measure of
    quality. It is the size of the 2D array (EG
    256x256 pixels)
  • This is often called resolution, but really
    resolution is about how big a pixel is what is
    its area in the real world? That is, how small
    an object can be seen.
  • Real-world example how small an object can be
    seen on Mars from an orbiter?

15
Resolution
  • 25 dpi (ppi) 6 dpi
    (ppi)

To be really useful we need over 200 dpi
16
Resolution
  • Here is a picture that is 1200 pixels wide by 800
    high.

How big do we print/display it?
17
Resolution
  • At 72 pixels per inch, this image would be
  • 1200/72 16.67 inches by 800/72
    11.11 inches.
  • If we do that we can see the pixels. The
    resolution is too low.
  • Thats why we cant reasonably project computer
    images as large as we like we can perceived the
    pixels if they are too big.

18
Resolution
Looks kind of blurred.
19
Resolution
  • Reasonable quality is 200 ppi.
  • Professional is 300 ppi minimum.
  • At that resolution, the image would be
  • Width 1200/300 4 inches
  • Height 800/300 2.667 inches
  • A typical photo is 4x6 what size do we need?
  • width/300 ppi 6 -gt width 300 6 1800
  • height/300 ppi 4 -gt height 300 4 1200
  • So we need 1800 x 1200 pixels, or 2,160,000 pixels

20
Resolution
  • We can do this. My cheesy HP camera can give me
    7.2 million pixels.
  • How about an 8x10?
  • DO THE WORK, FOLKS. You have 2 minutes.

21
Resolution
  • Answer
  • 300 pixels per inch x 8 inches wide 2400 pixels
  • 300 pixels per inch x 10 inches wide 3000
    pixels
  • Total number of pixels 2400 pixels wide x 3000
    pixels high 7,200,000 pixels
  • My cheesy camera wins!

22
Resolution
  • Heres a picture from my HP. 3072x2304

812 K JPEG
23
Resolution
  • Heres a picture from my Nikon. 2048x1536

585K JPEG
24
Resolution
  • In defense of the Nikon, it takes pictures on
    film which are then scanned by the photofinisher.
  • They could be scanned at a much higher
    resolution.
  • The optics are much better too.

25
Colour
  • Anyway, we now have resolution and quantization,
    and now should chat about colour.
  • Colour is stored (represented) how?
  • wait for an answer.

26
Colour
  • Right, the relative intensities of Red, Green,
    and Blue.
  • Just like on the TV screen, every pixel is
    specified as RGB values. Each one is 8 bits
    (0-255) so it takes 24 bits to store one pixel.
  • Our 7.2 megapixel image takes 21.6 million bytes
    to store in memory/on disk.
  • By the way, why RGB. Is there a physics thing
    involved, some kind of natural mathematical
    representation?
  • Nope.

27
Colour
  • It is because our retinas se these three colours.
    There is essentially a little coloured oil
    droplet in front of each cone in our retina.

28
Colour
  • Each animal has different retinal structure, and
    distinct colours of pigments.
  • This is a chicken 2 colors.

29
Colour
  • Right, the relative intensities of Red, Green,
    and Blue.

30
Colour
  • As artists, you must know something about how
    colours mix.
  • EQ red and blue mix to purple
  • On a computer, we know exactly how much.

31
Colour
  • Yellow red green
  • 255.255.128 255.255.64 255.255.32
    255.255.0

Magenta red blue 255.128.255 255.64.255
255.0.255
Cyan green blue 128.255.255
64.255.255 0.255.255
32
Colour
  • Changing a color value by one does not make much
    difference.
  • 128/129 128/119

But it depends on the colour. Our eyes are more
sensitive to some colours than to others.
128/129 128/119
33
Colour
  • http//homepages.ulb.ac.be/dgonze/INFO/htmlcolors
    .png
  • This site shows you the colors as a table.

34
Colour
  • There are lots of colours (How many?)

35
Colour
  • Colours have names. HTML names, for example

36
Colour
  • There are many "neon" colors.
  • They have a common feature. They contain a
    chemical substance that absorbs ultraviolet
    radiation just at shorter wavelengths that
    visible light (about 400 nm) and re-emit light in
    the visible range of wavelengths (400-700 nm) nm
    nanometers. Mixed with other dyes and pigments
    gives the "dazzling" colors. There are many web
    sites dealing with "fluorescence"
  • Cant render them on a normal screen.

37
Colour
  • So, a pixel can be 3 bytes, one for each colour
  • R, G, B.
  • Red 126 green 33 blue 100
  • Stored as a single number
  • 126 (25633) (65536 100)
  • Why? These are powers of 2.
  • 100 33 126 in hex is 64 21
    7E
  • And thus in binary
  • 01100100 00100001 01111110

38
Colour
  • In the colour table, the colour chocolate was
    defined as
  • D2691E
  • Which is to say
  • RED D2 210
  • GREEN 69 105
  • BLUE 1E 31
  • This is one reason we learned Hex colours are
    specified using it.

39
Colour
  • Converting from colour into grey scale is
    relatively simple.
  • 1. Can average the RGB values.
  • So chocolate becomes 210 105 31 356
  • 346/3 119 as a grey level.
  • 2. Could simply choose R or G or B to represent
    grey. This is not recommended.

40
Colour
Frequency? Not really useful For design and
computer stuff.
41
Colour
  • Use a weighted average. This is useful for
    specific conversions, like NTSC television
    images.
  • grey 0.213R 0.715G 0.072B
  • NTSC is bad at representing blues.

42
Colour
  • There are other ways to represent colour.
  • HIS or HSV (Hue/Saturation/Intensity)
  • Hue is the colour
  • Saturation is degree of colour
  • Intensity is brightness

43
Colour
  • Hue goes from 0.0 to 1.0 or 0 to 255
  • 255 is red, and is right next to 0 (circular)

0
44
Colour
45
Colour
There are some basic rules for design using color
that take advantage of the hue circle
46
Colour
The high contrast of complementary colors creates
a vibrant look especially when used at full
saturation. This color scheme must be managed
well so it is not jarring. Complementary colors
are really bad for text
47
Colour
Analogous color schemes use colors that are next
to each other on the color wheel. They create
serene and comfortable designs. Choose one color
to dominate, a second to support. The third color
is used (along with black, white or gray) as an
accent.
48
Colour
A triadic color scheme uses colors that are
evenly spaced around the color wheel. Triadic
color harmonies tend to be quite vibrant, even
with unsaturated versions of your hues. The
colors should be carefully balanced - let one
color dominate and use the two others for accent.
49
Colour
The split-complementary color scheme is a
variation of the complementary color scheme. In
addition to the base color, it uses the two
colors adjacent to its complement. Often a good
choice for beginners, because it is difficult to
mess up.
50
Colour
Complementary Triad
split-complementary
51
Colour
Analogous colours in a web page
52
Colour
Complementary colours in a web page
53
Colour
Split-complementary colours in a web page
54
Image Files
  • How are images stored on a computer?
  • As computer files, containing numbers.
  • All such files must hold pixel values, and must
    define the
  • size of the image (rows and columns).
  • There are other things possible too
  • Number of grey/color levels
  • identification of instrument and/or photographer
  • compression

55
Image Files
PNM format. Can be PBM (black/white) PGM (grey)
PPM (colour) P1 This is an example bitmap of
the letter "J" 6 10 0 0 0 0 1 0 0 0 0 0 1 0 0
0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0
1 0 0 0 1 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0
0
56
Image Files
Header P1 ASCII, bi-level columns rows
PBM format. P1 This is an example bitmap of
the letter "J" 6 10 0 0 0 0 1 0 0 0 0 0 1 0
0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1
0 1 0 0 0 1 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0
0 0
Pixel values
57
Image Files
This is a good format for sending by mail. We can
also examine the image using a text editor. P1
black/white ascii p4 black/white binary P2 grey
ascii p5 grey binary P3 colour ascii p6 color
binary P3 3 2 255 255 0 0 0 255 0 0 0 255
255 255 0 255 255 255 0 0 0
58
Image Files
  • GIF Graphics Interchange Format (1987)
  • Interesting from a number of perspectives.
  • The image data is compressed (Using
    Lempel-Ziv-Welch
  • (LZW) algorithm). This means that the files
    are smaller,
  • but pixels cant be accessed directly. The
    data must
  • be decompressed first.
  • 2. Pixels are not grey or color values. They are
    indexes into a
  • table of colours. The index is 8 bits.
  • Called a colour map.

59
Image Files
Colour map a table of colours indexed by an
integer. 0 1 2
3 4 5
Pixel colours are given as indices into this
table 0 0 1 1 0 0 1 0 4 4 5 would be
60
Image Files
The GIF file has all pixels, compressed, and
must also Contain the colour map being used.
There are 256 colours specified in this map,
indexed 0 .. 255 Two gifs need not have the same
colour map. Indeed, they might not even have any
colors in common. One special colour in a GIF
image can be identified as transparent or
background, meaning that is has no
colour. Whatever is underneath that colour will
be seen through background pixels.

61
Image Files
Left a GIF with a background colour specified
(blue) Centre the small image pasted over a
larger one. Right How it would look with no
transparency.
62
Image Files
JPEG Joint Photographic Experts group Is a
file format, highly compressed using a lossy
algorithm. The User can specify the trade off
between size and quality. GIF
JPEG
63
Image Files
Next class tools.
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