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FORMATION OF AN IMAGE on the screen:
GENERAL PRINCIPLES
(Compilation)
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(C) Tertius Gaudens
1 Vision
1.1 Psychophysiological characteristics
It is generally accepted that about 90% of the information
a person receives using the visual apparatus. And, before
turning to these the most "general principles" should consider
it some torye features. It should be noted that all of the
following, taken from different sources, the data obtained
experiment (so they can not be exact), and, above all in
relation to the TV image.
Perception is divided into the following stages:
- Detection;
- Distinction;
- Identification;
- Decoding (interpretation).
This process is characterized by the following notions.
Objectivity - the brightness, color, size, shape of the
image perceived as related to no one subject.
Integrity - the image is perceived as a whole (ie, it is
impossible to make the best of any of its individual setting).
Structure - the object is perceived as a structure, a set
of components. Basic information about the object is the
contour and kritcheskie point "circuit in which there is an
abrupt change of direction lines. Contours formed by straight
lines, are recognized faster than curvilinear.
Constancy - the subject is perceived regardless of the
conditions of its observation (eg, a lump of coal in the sun
shines much stronger than a sheet of white bumugi, but we do
not confuse-it).
Selectivity - the image is perceived as an object and
background. Object is allocated on the basis of of his color,
shape, size, placement, and - from my own experience. If
characteristics of the object and background are similar
(especially - the size and configuration), the value of the
subjective factor dramatically increases. Known textbook
example: when image and the background have the same area, but
poperemen- perceived as a background image is one thing and
then another.
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Apertseptsiya - the perception from my own experience.
(The object, slightly different from the previous one, it is
perceived as the previous one)
Adaptation - the reaction rate on the eye image and its
change. It depends on the level of image brightness, it
background environment, ie light images. Eye
adapts to changes in brightness in daylight for several seconds
in the dark - in minutes.
An interesting effect of "local adaptation":
tsvetorazlichitelnaya ability of the eye drops at PROLONGED
examining non-ferrous parts. At the same time, the sensitivity
of the eye increases if the background is tsvetnnost and
brightness close to chrominance components under consideration.
(Look at the three neighboring colored line of text: Do not
seem to you at a moment that the middle row has a slightly
different color and brightness?)
1.2 Field of view
The total field of vision in one eye is 140 ... 160 deg.,
two - more than 200 degrees.
Field of view consists of:
- In the central 4 deg.;
- Clear to 30 ... 35 deg. (Recognition without
discrimination fine detail);
- Peripheral in 75 ... 90 deg. (Identification without
recognition).
The difference in terms of visual angle horizontally and
vertically, leads to the fact that when you move the observed
object, its velocity in the vertical appears to be larger in
1,3 times higher than horizontally.
The minimum angular resolution of the eye varies from 0,5
... 2 'in ideal conditions, up to 3 ... 10' in time constraints
and other unfavorable factors.
1.3 The ability to scan
Lack of sharpness of peripheral vision is compensated by
the mobility of the eyes - search (detection) and the Gnostic
(Distinguishing details and recognition).
The process of visual search, detection and raspozavaniya
image within the angle of view is scanned through the eyes of
certain points of fixation. Duration of search depends on
angular size of the object and the search field. It is believed
that:
- Maximum points of fixation is concentrated in the
central and bottom right, and the minimum - in the bottom left;
- Medium angular distance between the fixation points was
8.6 degrees.;
- With the angle of view decreases the fixation time,
the distance between the fixation points increases and the
efficiency of search decreases dramatically, because
quantitative increases of fixation points outside the screen.
- Search is in the direction from left to right, top to
bottom. "
1.4 The perception of brightness
The human eye perceives the brightness of the following
ranges (Cd / sq.m):
-6 -4
- 3 * 10 ... 10 - the night;
-4
- 10 ... 3 - Twilight;
4
- 3 ... 3 * 10 - day.
Number of gradations of brightness perceived by the eye,
is q = 90 ... 200. Hence the number of bits of quantization
Video should be
m = log q> 7
2
1.5 Perception of color
Chroma relative spectral sensitivity of the eye
determines the comfort and resolution of perception. It is
believed that the human eye can perceive 3 ... 17 Thousands of
colors and brightness of the light - 3 ... 10 million
Ho if it is not about "aesthetic perception", and about
identification, the number of gradations is dramatically
reduced. For example, for mono playback are identi- Only 5 ...
8 halftones. With the application of paint (so-called
"Pseudocolor coding") of the image of his informative sequence
increases. Accordingly, increased accuracy and efficiency of
its perception.
At the same time, excessive "coloring" the image gets very
complicated color structure, the uniqueness interpretation of
which is deteriorating. In addition, disruption of the
integrity of the image contributes to violation of the
monochrome brightness and continuity in 3-dimensional color
prostanstve.
Therefore, if the coloring is used for convenience
operator, the number of grades should be the minimum necessary.
Most in effective pseudocolor coding is considered as m 4 0 <7
04.
Tsvetorazreshayuschaya ability of the eye several times
smaller than its luminance resolution.
Three-view exists only in the spatial frequency df <0,5
MHz (large objects). As reduce the size of non-ferrous parts of
the image the eye ceases to distinguish between the first
yellow and blue parts then - red and green. Later yellow and
bluish-green color begin to be perceived as white, blue and red
- as black.
The sharp decline sposbnosti to tsvetorazlicheniyu occurs
when angular size of parts in 15 '. On such "critical"
frequencies may be a "flashing colors" - some "translucent
through each other "colors.
This effect can be used to simulate the "Delete"
object - the degree of color change may signal
remoteness.
If 0,5 1,5 MHz (according to other sources -
if df> 4 MHz), the image is seen only in luminance gradation.
Therefore permissible to not pass on information about the
color of small details: the NTSC color df = 1.5 MHz, and
chrominance signals are transmitted through the line. In the
PAL chroma df + 4.44 MHz (approximately), and chrominance
signals are transmitted in each row.
Thus, on TV, we see no color, and "colored"
image.
1.6 Recognized color
It is believed that accurately identifies (other than
white and black) in the following colors:
- Purple;
- Blue;
- Blue-green;
- Bluish-green;
- Green;
- Yellow;
- Orange;
- Orange-red;
- Red.
Compare these data with the test TV table. With
single-digit RGB-encoding, supplemented by a three-digit code
of brightness on the monitor reproduced 8 colors:
The color code Brightness
Colour R G B Y2 Y1 Y0
White 1 1 1 1 1 1
yellow 1 1 0 1 1 0
Blue 0 1 1 1 0 1
Green 0 1 0 1 0 0
Magenta 1 0 1 0 1 1
Red 1 0 0 0 1 0
blue 0 1 1 0 0 1
Black 0 0 0 0 0 0
2 Image
2.1 Image coding
One should distinguish between two types of image:
- Against a spectator - one that should be as close to
real;
- Charge against the operator - one that should contain
minimum identifikaionnyh signs to the maximum acceleration of
recognition and decision.
Common methods of image coding are:
- Significant and coordinate (most advanced);
- Color and brightness (have limitations due to
subjective factor perception);
- By flashing images.
2.2 Aspect ratio
Playback Format F is chosen based on the characteristics
of and conditions for the optimum ratio scan eye horizontally
and vertically. Given the limited eye scan and vertical scan
them for convenience Horizontally, it is generally accepted
ratio r <2:1. In Standard TV sweeps F = 4:3, and the optimal
angle monitor screen is 25 ... 30 deg., which corresponds to
viewing distance of 4 ... 5 the screen height. Under different
conditions observations are possible, and other values F:
The image format format type
Numeric Value
parties value
5: 4 1.25 Optimal screen oscilloscope
4: 3 TV 1.33
- 1,37 European cinema
1 / 2
2: 1 1.41 Format ESKD
5: 3 Shield 1.67 TPCH
- 1,73 European wide
16: 9 1.78 Ekran HDTV
- 1,85 U.S. Wide
2: 1 2 Optimum cinematic
Compare these data with the formats of IBM-compatible systems:
Type of standard decomposition Format:
of relations. Num. means.
EGA 640x350 64:35 1,83
HGC 720x348 60:29 2,01
VGA 640x400 8:5 1,6
720x400 9:5 1,8
640x480 4:3 1,33
SVGA 800x600 4:3 1,33
GDC 960x720 4:3 1,33
1024x768 4:3 1.33
WYSE 1280x800 8:5 1,6
1664x1200 104:75 1.39
This shows that F = 16:9 = 5,33:3 = 1.78 in good agreement
with the European (1,73) and the U.S. (1,85) cinematic format
as well as scans VGA (1,6; 1 8) and EGA (1,83). That is why it
is optimal for large screen HDTV.
02.03 frame rate and scan type
The signal frequency at which it seems to be continuous
nazyvantsya "critical flicker frequency. This figure is growing
exponentially with an increase in brightness and area of
images.
In 10 20 40 60
(Cd / m 1) 0
1 0 1 0 1 0 1 0 1
f (Hz) 20 25 30 32
For capturing the image of a moving object rather
ensure that its scanning frequency of 20 ... 25 fps. In the
film taken of the frame rate 24 fps.
In TV systems, the quality of visual images
depends on the frequency of the playback frame fb.
Fb (Hz) 17 25 30 40 50 55
% 50 86 94 98.9 99.5 99.9
With an average brightness critical flicker frequency
Image fb 4 0> 46 Hz. The same relationship exists
to scan an image, but in this case, the frequency
scanning (Fc) can be reduced by half. In the conventional TV
systems implementation f. = FB> 46 Hz Nes How many is difficult.
Therefore, considering the admissibility of fb = 2Fs, they use
Interlace. Frame decomposes, is at CZK 4 0poley
(CZK 4 0 - koeffitsieent interlaced). Paws at Kolka
increase CZK 4 0rezko increased interference, caused by nye
shift lines, the TV system is adopted 4h K 0 = 2. In this case,
even field of raster lines are reproduced 2i (i = 0,1,2 ,...),
in odd - 2i + 1.
- A string of even field
- A line of odd field
Displacement of even and odd lines to each other
achieved an odd number of scanning lines Z and
difference in polstroki between fields.
_ __ _ _
_ _ _ _ _
When playing an eye on the queue and even vosprinimant
odd-numbered lines which, due to the inertia of central vision,
form a single image. Thus, when interlaced frame consists of
two fields:
yc yc
2Fk = Fn = 2Fsk = fb,
and progressive (Kč 4 0 = 1) contains only one field:
ps ps
F. 4 0 = Fn = Fsk 4 0 = fb
ps yc
where f., f. 5 0 - frame rate, respectively, interlaced and
progressive scan;
Fn - frequency of the field.
On this basis, the European TV systems PAL and SECAM
CR pr
adopted F. 5 0 = 25 Hz and Fn 4 0 5 0 = F. = 50 Hz, and
American NTSC
CR
F. 5 0 = 29.97 (approximately 7,030) Hz and F 4n 7060
(Approximately) Hz.
(Because of the movies on TV are going faster than a movie).
When playing on large screens and displays the angular
size and brightness increase. This leads to
introduced in the monitoring process more inertial peripheral
GOVERNMENTAL zones of the eye, and begins to be observed
flicker, caused by the interlaced and insufficient for such
angles view f.. Therefore, in display systems use only
progressive scan, and F., increased compared with the TV. (In
quality systems F. 4 0povyshena doubled compared with the
progressive-scan TV, ie may be 100 or 120 Hz).
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