NB. : in the lecture on Diplays devices, we insisted on
the particulate aspect of light (photon emission). But light is dual and today
we will discuss about the waving side of light.
If you want to see all the colors, use a rainbow or a
prism.
Color Perception
Color Spaces
Gamma
Alpha
Hearn & Baker Ch 15
Slide 3 : Elements of Color
Elements of Color
Slide 4 : Visible Spectrum
Visible Spectrum
We perceive electromagnetic energy having wavelengths in the range 400-700
nm as visible light.
What we perceive as "light", or different colours,
is a narrow frequency band within the electromagnetic spectrum.
Slide 5 : The Eye
The Eye
The photosensitive part of the eye is called the retina.
The retina is largely composed of two types of cells, called rodsand
cones. Only the cones are responsible for color perception.
Slide 6 : The Fovea
The Fovea
Cones are most densely packed within a region of the eye called the
fovea.
There are three types of cones, referred to as S, M, and L. They are
roughly equivalent to blue, green, and red sensors, respectively. Their
peak sensitivities are located at approximately 430nm, 560nm, and 610nm
for the "average" observer.
Slide 7 : The Fovea
The Fovea
Colorblindness results from a deficiency of one or more cone type.
Electron Micrograph of Human Visual Receptors
Slide 8 : Color Perception
Color Perception
Different spectra can result in a perceptually identical sensations called
metamers
Color perception results from the simultaneous stimulation of 3 cone types
(trichromat)
Our perception of color is also affected by surround effects and adaptation
In order to define the perceptual 3D space in a "standard" way, a
set of experiments can (and have been) carried by having observers try and match
color of a given wavelength, lambda, by mixing three other pure wavelengths,
such as R=700nm, G=546nm, and B=436nm in the following example.
Note that the phosphours of color TVs and other CRTs do not emit pure red,
green, or blue light of a single wavelength, as is the case for this experiment.
The scheme above can tell us what mix of R,G,Bis needed to reproduce the perceptual
equivalent of any wavelength. A problem exists, however, because sometimes the
red light needs to be added to the target before a match can be achieved. This
is shown on the graph by having its intensity, R, take on a negative value.
Slide 10 : CIE Color Space
CIE Color Space
In order to achieve a representation which uses only positive mixing coefficients,
the CIE ("Commission Internationale d'Eclairage") defined three new
hypothetical light sources, x, y, and z, which yield positive matching curves:
If we are given a spectrum and wish to find the corresponding X, Y, and Z
quantities, we can do so by integrating the product of the spectral power and
each of the three matching curves over all wavelengths. The weights X,Y,Z form
the three-dimensional CIE XYZ space, as shown below.
Slide 11 : CIE Chromaticity Diagram
CIE Chromaticity Diagram
Often it is convenient to work in a 2D color space.
This is commonly done by projecting the 3D color space onto the plane
X+Y+Z=1, yielding a CIE chromaticity diagram.
The projection is defined as:
Giving one of the following chromaticity diagram.
Slide 12 : A few definitions
A few definitions...
Spectrophotometer A device to measure the spectral energy
distribution. It can therefore also provide the CIE xyz tristimulus
values.
Illuminant C A standard for white light that approximates
sunlight. It is defined by a color temperature of 6774 K.
Complementary colors Colors which can be mixed together to yield
white light. For example, colors on segment CD are complementary to
the colors on segment CB.
Dominant wavelength The spectral color which can be mixed with
white light in order to reproduce the desired color. color B in the
above figure is the dominant wavelength for color A.
Non-spectral colors Colors not having a dominant wavelength.
For example, color E in the above figure.
Perceptually uniform color space A color space in which the distance between
two colors is always proportional to the perceived distance. The CIE
XYZ color space and the CIE chromaticity diagram are not perceptually
uniform, as the following figure illustrates. The CIE LUV color space
is designed with perceptual uniformity in mind.
Slide 13 : Color Gamuts
Color Gamuts
The chromaticity diagram can be used to compare the "gamuts" of
various possible output devices (i.e., monitors and printers).
Note that a color printer cannot reproduce all the colors visible
on a color monitor.
Slide 14 : The RGB Color Cube
The RGB Color Cube
The additive color model used for computer graphics is represented by the
RGB color cube, where R, G, and B represent the colors produced by red, green
and blue phosphours, respectively.
The color cube sits within the CIE XYZ color space as follows.
Slide 15 : Color Printing
Color Printing
Green paper is green because it reflects green and absorbs other wavelengths.
The following table summarizes the properties of the four primary types of printing
ink.
dye color
absorbs
reflects
cyan
red
blue and green
magenta
green
blue and red
yellow
blue
red and green
black
all
none
To produce blue, one would mix cyan and magenta inks, as they
both reflect blue while each absorbing one of green and red. Unfortunately,
inks also interact in non-linear ways. This makes the process of converting
a given monitor color to an equivalent printer color a challenging problem.
Black ink is used to ensure that a high quality black can always be printed,
and is often referred to as to K. Printers thus use a CMYK color model.
Slide 16 : Color Conversion
Color Conversion
To convert from one color gamut to another is a simple procedure.
Each phosphour color can be represented by a combination of the CIE XYZ primaries,
yielding the following transformation from RGB to CIE XYZ:
The transformation
yields the color on monitor 2 which is equivalent to a given color on monitor
1. Conversion to-and-from printer gamuts is difficult. A first approximation
is as follows:
C = 1 - R
M = 1 - G
Y = 1 - B
The fourth color, K, can be used to replace equal amounts
of CMY:
K = min(C,M,Y) C' = C - K
M' = M - K
Y' = Y - K
Slide 17 : RGB-Based Color Spaces
RGB-Based Color Spaces
The RGB space is a three-dimensional
color space whose components are the red, green, and blue intensities that make
up a given color. For example, scanners read the amounts of red, green, and
blue light that are reflected from or transmitted through an image and then
convert those amounts into digital values. Information displayed on a color
monitor begins with digital values that are converted to analog signals for
display on the monitor. The analog signals are transmitted to the phosphors
on the face of the monitor, causing them to glow at various intensities of red,
green, and blue (the combination of which makes up the required hue, saturation,
and brightness of the desired colors).
RGB-based color spaces are the most commonly used color
spaces in computer graphics, primarily because they are directly supported by
most color displays and scanners. RGB color spaces are device dependent and
additive. The groups of color spaces within the RGB base family include
RGB spaces
HSV and HLS spaces
RGB Spaces
Anycolor expressed in RGB space is some mixture of
three primary colors: red, green, and blue. Most RGB-based color spaces can
be visualized as a cube, as in the following figure, with corners of black,
the three primaries (red, green, and blue), the three secondaries (cyan, magenta,
and yellow), and white.
RGB color space (Red corner is hidden from view)
sRGB Color Space
The sRGB color space is based
on the ITU-R BT.709 standard. It specifies a gamma of 2.2 and a white point
of 6500 degrees K. You can read more about sRGB space at the International
Color Consortium site at http://www.color.org
. This space gives a complimentary solution to the current strategies of color
management systems, by offering an alternate, device-independent color definition
that is easier to handle for device manufacturers and the consumer market.
sRGB color space can be used if no other RGB profile is specified or available.
High end users with good quality reproduction
devices may find that the sRGB space, which limits colors to the sRGB gamut,
is too restrictive for their required color quality.
HSV and HLS Color Spaces
HSV space
and HLS space are transformations of RGB space that can
describe colors in terms more natural to an artist. The name
HSV stands for hue, saturation, and
value . ( HSB space , or hue
, saturation , and brightness
, is synonymous with HSV space.) HLS stands for hue, lightness,
and saturation. The two spaces can be thought of as
being single and double cones, as shown in next figure .
The components in HLS space
are analogous, but not completely identical, to the components in HSV space:
The hue component in both
color spaces is an angular measurement, analogous to position around a color
wheel. A hue value of 0 indicates the color red; the color green is at a
value corresponding to 120°, and the color blue is at a value corresponding
to 240°. Horizontal planes through the cones in next figure are hexagons;
the primaries and secondaries (red, yellow, green, cyan, blue, and magenta)
occur at the vertices of the hexagons.
The saturation component
in both color spaces describes color intensity. A saturation value of 0
(in the middle of a hexagon) means that the color is "colorless"
(gray); a saturation value at the maximum (at the outer edge of a hexagon)
means that the color is at maximum "colorfulness" for that hue
angle and brightness.
HSV (or HSB) color space and
HLS color space
The value component in
HSV describes the brightness. In both color spaces, a value of 0 represents
the absence of light, or black. In HSV space, a maximum value means that
the color is at its brightest. In HLS space, a maximum value for lightness
means that the color is white, regardless of the current values of the hue
and saturation components.
are three variants of what is effectively the same colour space.
They all are effectively the RGB space twisted so that
the neutral diagonal becomes the lightness axis, the saturation the distance
from the central lightness axis and the hue the position around the center.
The only difference between these models is the measurement of saturation, or
the strength of the colour.
In HSV, created in 1978 by Alvey Ray Smith, saturation
had a maximum on 0 when lightness was zero and 1 when lightness was 1. This
is somewhat odd and anti-intuitive when the strength of the colour of white
is considered. HLS is much more intuitive. The saturation peaks at a lightness
of 0.5 and has a maximum of zero at black and white. The HSI colour space is
in fact identical to HLS under a different name.
Figure: The RGB derived colour spaces (a) HSV, (b) HLS and
(c) the most commonly used HSV space.
The most commonly used one, confusingly also called HSV,
is a combination and variation on HSV and HLS, appearing to be a simple cylinder.
This was done to have a uniform saturation limit. There are the same anomalies
at white and black with saturation as with the original HSV. Whenever HSV is
referred to in this report it will be to this colour space.
HSV is almost identical to RGB, slightly warped for symmetry's
and simplicity's sake. It has made no attempts whatsoever towards perceptual
uniformity but has achieved more intuitive dimensions.
When we "compute" colors we generally assume that they are
linear quanities. Unfortunately, most display devices are nonlinear.
The most common correction method is called gamma correction.
Usually, this gamma correction curves is got from lookup
tables
Slide 20 : CGSD - Gamma Correction Explained
CGSD - Gamma Correction ExplainedCGSD - Gamma Correction Explained</u></font></h4>
<HR>
<h1>What is Gamma Correction?</h1>
<P class="lecturetexte"> In the field of Computer Graphics, one often hears the
phrase "gamma correction." What is this strange sounding thing and why does
it matter to you?
<P class="lecturetexte"> Gamma correction matters if you have any interest in
displaying an image accurately on a computer screen. Gamma correction controls
the overall brightness of an image. Images which are not properly corrected
can look either bleached out, or too dark. Trying to reproduce colors accurately
also requires some knowledge of gamma. Varying the amount of gamma correction
changes not only the brightness, but also the ratios of red to green to blue.
(<a href=gamma_colorspace.en.html>Example of this color phenomenon</a>).Gamma
correction also plays a big role in <a href=http://www.cgsd.com/papers/gamma.web.html>making
images for the WWW.</a>
<h2>Computer Monitor</h2>
<P class="lecturetexte"> To explain gamma correction we will begin with where
you are looking - your computer monitor.
<P class="lecturetexte"> Almost every computer monitor, from whatever manufacturer,
has one thing in common. They all have a intensity to voltage response curve
which is roughly a 2.5 power function. Don't be afraid, this just means that
if you send your computer monitor a message that a certain pixel should have
intensity equal to x, it will actually display a pixel which has intensity equal
to x ^ 2.5 Because the range of voltages sent to the monitor is between 0 and
1, this means that the intensity value displayed will be less than what you
wanted it to be. (0.5 ^ 2.5 = 0.177 for example) Monitors, then, are said to
have a gamma of 2.5
<blockquote>
<pre>
<IMG SRC="Image/scal-100.gif" align=middle width="64" height="64"> Sample Input to Monitor <IMG SRC="Image/grph-100.gif" align=middle width="64" height="66"> Graph of Input<br>
<IMG SRC="Image/scal-040.gif" align=middle width="64" height="64"> Output from Monitor <IMG SRC="Image/grph-250.gif" ALIGN=middle width="64" height="67"> Graph of Output <b> L = V ^ 2.5</b>
</pre>
<P>
<i>Note: The grayscale images will not look very good on 8-bit color computers.
Also the gamma correction for these images is 1.0 so they were actually designed
to be viewed on a system such as a Sun or a PC with no hardware correction. They
may appear brighter on other systems. The important thing here is the
relative difference that you see.</i>
</blockquote>
<h2>Gamma correction of the input</h2>
<p class="lecturetexte">To correct this annoying little problem, the input signal
to the monitor (the voltage) must be "gamma corrected". </p>
<P class="lecturetexte"> The solution, fortunately, is a simple one. Since we
know the relationship between the voltage sent to the monitor and the intensity
which it produces, we can correct the signal before it gets to the monitor.
The signal is adjusted so that it is essentially the complement of the curve
shown above. There are other considerations as well when one speaks of a "correct"
image. These include the ambient light in a room where the computer is, the
brightness and contrast settings on the monitor, and finally personal taste.
<blockquote>
<pre>
<IMG SRC="Image/scal-100.gif" align=middle width="64" height="64"> Sample Input <IMG SRC="Image/grph-100.gif" align=middle width="64" height="66"> Graph of Input<br>
<IMG SRC="Image/scal-250.gif" align=middle width="64" height="64"> Gamma Corrected Input <IMG SRC="Image/grph-125.gif" ALIGN=middle width="64" height="66"> Graph of Correction <b> L' = L ^ (1/2.5)</b><br>
<IMG SRC="Image/scal-100.gif" align=middle width="64" height="64"> Monitor Output <IMG SRC="Image/grph-100.gif" align=middle width="64" height="66"> Graph of Output<br>
</pre>
</blockquote>
<P>
If gamma correction is done properly for the computer system, then the output
should accurately reflect the image input.
<P class="lecturetexte"> Note that the task of gamma correction is accomplished
by raising the input value to the 1/2.5 power. This is referred to as a gamma
correction of 2.5. because we are correcting the input for a monitor whose gamma
is 2.5
<P>
<h2>Real World Application</h2>
In the real world, it isn't quite this simple, especially when an image needs to
look good on different systems, or platforms.
<P class="lecturetexte"> As we mentioned above, most monitors work in about the
same way with respect to gamma correction. Most computers, or more specifically,
most computer systems, do not work in exactly the same way, however.
<P class="lecturetexte"> By computer systems we mean everything from the software
that is running (like Netscape) to the graphics cards installed, to the standard
hardware on the motherboard. Different computers do different things and many
"systems" have different configurations of all of the above things.
<P class="lecturetexte"> Macintoshes, for example, have partial gamma correction
built-in to their hardware. Silicon Graphics computers also have built-in gamma
correction, but it is different from the Macintosh. Suns and PCs have no standard
built-in gamma correction but some graphics cards installed in these computers
may provide this functionality.
<P>
<h2 class="lecturetexte">System Gamma</h2>
<span class="lecturetexte">The idea of system gamma, is the gamma correction that
should be applied in the software to reproduce an accurate image on the monitor
for an uncorrected image on a particular computer "system." </span>
<P> <span class="lecturetexte"><b>Macintosh</b> </span>
<blockquote><span class="lecturetexte"> The Macintosh has built-in gamma correction
of 1.4. This means that after the software sends the signal to the framebuffer,
there are internal hardware corrections which will further process the signal,
specifically by gamma correcting it another 1.4 - That is, the signal is raised
to the 1/1.4. Therefore, to get full correction, the software itself should
first adjust the signal by raising it to the 1/1.8 power. (2.5/1.4 = 1.8) Thus
the system gamma on a Macintosh is 1.8. </span>
<P class="lecturetexte"> Note some graphics cards in Macintoshes may have their
own software to change the standard gamma and Adobe Photoshop 3.0 is now released
with a gamma control panel from Knoll software which allows the user to change
the system gamma of their Macintosh. The 1.8 standard is still accepted as
the universal Mac System Gamma, but users should be aware that a Mac can be
set differently. The Knoll software control panel for the Mac rewrites the
look up table, (LUT) with a value of g/2.5 where g is the gamma the user selects.
Thus selecting 1.8 will rewrite the LUT with 1.8/2.5 = 1/1.4 - the default
setting. (The values in the LUT are 1/1.4 and this is called a 1.4 correction)
</blockquote>
<span class="lecturetexte"><b>SGI</b> </span>
<blockquote><span class="lecturetexte"> The SGI is similar to the Macintosh but
instead of a hardware correction of 1.4 the SGI applies gamma correction of
1.7. Thus the system gamma for an SGI is 2.5/1.7 or roughly 1.5. Sometimes you
may see that an SGI has a system gamma of 1.4. This calculation is made on the
assumption that monitors have a response curve closer to a 2.4 power function.
</span>
<P class="lecturetexte"> SGI's also come with a utility to rewrite the internal
hardware correction. These values are stored in a look up table, (LUT) and
can be altered. The default is 1/1.7 as mentioned above. (The values in the
table are 1/1.7, so we call this a 1.7 correction) Sometimes the value in
the LUT may be referred to as the SGI system gamma. This is not the definition
used on other platforms. Unlike the Mac gamma control panel, the SGI gamma
utility will rewrite the LUT with the actual value set by the user.
</blockquote>
<span class="lecturetexte"><b>Suns and PCs</b> </span>
<blockquote class="lecturetexte"> Suns and PCs have no standard hardware correction
(although certain graphics cards for these platforms may) and therefore their
system gammas are roughly 2.5. <a href=http://www-swiss.ai.mit.edu/~philg/photo/fixing-gamma.html>
(More about Sun's SX hardware and gamma)</a> </blockquote>
<P class="lecturetexte"> Common graphics software such as Adobe Photoshop allows
the user to set the gamma correction value they want. (In Photoshop it is found
in Monitor Setup under Preferences under the File Menu.)<br>
<br>
<p class="lecturetexte">Extract from : CGSD - Gamma Correction Explained : <a href="http://www.cgsd.com/papers/gamma_intro.html">http://www.cgsd.com/papers/gamma_intro.html</a>
</p>
<P>
<HR>
<H4><font color="#000000"><u>Slide 21 : CGSD - Gamma Correction ExplainedCGSD - Gamma Correction Explained</u></font></h4>
<HR>
<h1><b>Cumulative</b> effect of each of the gamma steps on different plateforms
<hr>
</h1>
<p class="lecturenotes">Extract from : CGSD - Gamma Correction Explained : <a href="http://www.cgsd.com/papers/gamma_diagram.html">http://www.cgsd.com/papers/gamma_diagram.html</a></p>
<P class="lecturetexte"> <b>Note: This diagram will not look very good in 8-bit
color and your browser must support tables for the correct layout.</b><br>
<P class="lecturetexte"> This diagram shows the <b>cumulative</b> effect of each
of the gamma steps
<P>
<p> </p><table border cellpadding="5">
<TR>
<th> </th>
<th align=center colspan=2>Input Signal </th>
<th align=center colspan=2>Software Correction Required </th>
<th align=center colspan=2>Built In Hardware Correction </th>
<th align=center colspan=2>Output Signal </th>
</TR>
<tr>
<th align=left >Sun & PC<br>
no<br>
Correction<br>
</th>
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64"> </td>
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66"> </td>
<td align=center >. </td>
<td align=center >. </td>
<td align=center >. </td>
<td align=center >. </td>
<td align=center ><IMG SRC="Image/scal-040.gif" width="64" height="64"> </td>
<td align=center ><IMG SRC="Image/grph-250.gif" width="64" height="67"> </td>
</tr>
<tr>
<th align=left >Sun & PC<br>
full<br>
Software<br>
Correction<br>
</tH>
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64"> </td>
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66"> </td>
<td align=center ><IMG SRC="Image/scal-250.gif" width="64" height="64"> </td>
<td align=center ><IMG SRC="Image/grph-125.gif" width="64" height="66"> </td>
<td align=center >. </td>
<td align=center >. </td>
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64"> </td>
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66"> </td>
</tr>
<tr>
<th > Macintosh<br>
Hardware<br>
Correction<br>
Only<br>
</th>
<td align=center><IMG SRC="Image/scal-100.gif" width="64" height="64"> </td>
<td align=center><IMG SRC="Image/grph-100.gif" width="64" height="66"> </td>
<td align=center>. </td>
<td align=center>. </td>
<td align=center><IMG SRC="Image/scal-140.gif" width="64" height="64"> </td>
<td align=center><IMG SRC="Image/grph-114.gif" width="64" height="67"> </td>
<td align=center><IMG SRC="Image/scal-055.gif" width="64" height="64"> </td>
<td align=center><IMG SRC="Image/grph-180.gif" width="64" height="67">
</tr>
<tr>
<th >Macintosh<br>
Hardware<br>
and<br>
Software<br>
Correction<br>
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66">
<td align=center ><IMG SRC="Image/scal-180.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-118.gif" width="64" height="66">
<td align=center ><IMG SRC="Image/scal-250.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-125.gif" width="64" height="66">
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66">
</tr>
<tr>
<th > SGI<br>
Hardware<br>
Correction<br>
Only<br>
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66">
<td align=center >.
<td align=center >.
<td align=center ><IMG SRC="Image/scal-170.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-117.gif" width="64" height="66">
<td align=center ><IMG SRC="Image/scal-058.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-140.gif" width="64" height="67">
</tr>
<tr>
<th >SGI<br>
Hardware<br>
and<br>
Software<br>
Correction<br>
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66">
<td align=center ><IMG SRC="Image/scal-140.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-114.gif" width="64" height="67">
<td align=center ><IMG SRC="Image/scal-250.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-125.gif" width="64" height="66">
<td align=center ><IMG SRC="Image/scal-100.gif" width="64" height="64">
<td align=center ><IMG SRC="Image/grph-100.gif" width="64" height="66">
</tr>
</table>
<P>
<P class="lecturetexte">The table above shows the effects of various gamma corrections
for a sample input. The sample input is a linear greyscale which was first made
in Lab color space in Adobe Photoshop. The input is changed by software gamma
correction, hardware gamma correction and, at the far right, by the the CRT
display. This document assumes that the CRT has a luminance to input voltage
response curve which matches a 2.5 power function.
<P class="lecturetexte"> The Software Correction Required column shows what level
of gamma correction should be applied in software on the various platforms to
achieve the correct output signal. In this case, the correct output means that
it exactly matches the input signal - the linear greyscale.
<P class="lecturetexte"> The Built-In Hardware Correction column refers to the
gamma correction done by the hardware on each platform. It is stated that Suns
and PC's (Intel Standard) have no built-in hardware correction. This is the
standard for these platforms, but certain graphics cards on each platform may
perform some gamma correction.
<P class="lecturetexte"> The Output Signal column refers to the luminance to input
voltage response curve as described above.
<P class="lecturetexte"> <i>Note: The greyscale images shown above could themselves
require gamma correction for display on your monitor. They were design to be
viewed on a system that does no gamma correction in its hardware (such as a
Sun or a PC) If you are viewing these images on a Macintosh or an SGI or have
some other system (including Sun and PC) configured differently, they will appear
a little too light (bleached out). Their purpose is mostly to illustrate what
happens in the various steps relative to each other, and not as absolute representations
of what you will on each system when viewed from your system. The only solution
to this would be for us to put up many different copies corrected for different
gamma settings and ask you which one you want to view.</i>
<P>
<P><br>
<P>
<HR>
<H4><font color="#000000"><u>Slide 22 : Gamma Correction and Color SpaceCGSD Gamma Correction and Color SpaceCGSD Gamma Correction and Color Space</u></font></h4>
<HR>
<h1>Gamma Correction and Color Space</h1>
<p class="lecturetexte"><b>Gamma</b> does not affect the representation of a color
in all color spaces equally and has no affect on color representation in many
color spaces. (A color is represented in a "color space" as a set of coordinates
(R,G,B = 255,255,255) for example. The are many different color spaces commonly
used. Each is a 3 or 4 dimensional coordinate system used to express color information.)
</p>
<P class="lecturetexte"> Below are two lists of color spaces. The colors listed
as <b>device dependent</b> are functions of the "system gamma" used. The colors
spaces listed as <b>device independent</b> are not a function of gamma at all.
<P>
<h3>Device Dependent Color Spaces</h3>
<ul>
<li> RGB
<li> CMY
<li> CMYK
<li> HSB
<li> HSV
</ul>
<h3>Device Independent Color Spaces</h3>
<ul>
<li> CIE XYZ
<li> CIE xyY
<li> Lab
<li> Luv
<li> Munsell
</ul>
<P>
<h2>Brightness</h2>
<p class="lecturetexte">The greatest effect of Gamma on the representations of
these colors is a change in overall brightness. </p>
<P class="lecturetexte"> To see this visually using Adobe Photoshop, change the
Gamma setting in the Monitor Setup (File Menu -> Preferences -> Monitor Setup).
An image in RGB space should become lighter as you raise the gamma setting and
darker as you decrease the gamma setting. An image in Lab space (Photoshop calls
this Lab Mode) should not be affected by gamma. Some earlier versions of Photoshop
may not support Lab color space.
<P>
<h2>Color or Hue</h2>
<p>A change in gamma can also effect hue of a particular color representation.
</p>
<p>For example, the color below, represented in RGB space has 80% Red, 20% Green
and 20% Blue, or 8:2:2 ratio.<br>
</p>
<center>
<IMG SRC="Image/lightred.jpg" width="80" height="80"><br>
</center>
However, if that color is not gamma corrected properly the ratios between red,
green and blue will change. The color below simulates the above color with gamma
correction of 1.0. The new ratios are 32:1:1
<center>
<p><IMG SRC="Image/darkred.jpg" width="80" height="80"></p>
<h4>Simulated gamma correction<br>
</h4>
</center>
<p class="lecturetexte">One say the gamma correction is simulated, because your
browser has no notion of gamma correction and therefore One can't tell it that
these two colors need different gamma correction. Even if your browser did no
about gamma, and know what to do with it, the JPEG and GIF formats do not store
gamma. The colors were made in Adobe Photoshop in Lab color space and converted
to RGB space using the gamma mentioned. </p>
<pre>
<IMG SRC="Image/darkred.jpg" width="80" height="80"> <IMG SRC="Image/lightred.jpg" width="80" height="80">
Gamma Correction = 1.0 Gamma Correction = 2.5
Source Output Source Output
R 80% R 57% R 80% R 80%
G 20% G ~0% G 20% G 20%
B 20% B ~0% B 20% B 20%
</pre>
<p><br>
<span class="lecturetexte">Extract from : CGSD - Gamma Correction Explained
: <a href="http://www.cgsd.com/papers/gamma_intro.html">http://www.cgsd.com/papers/gamma_intro.html</a>
</span> <br>
</p>
<P>
<HR>
<H4><font color="#000000"><u>Slide 23 : True-Color Frame Buffers</u></font></h4>
<HR>
<h1>True-Color Frame Buffers</h1>
<ul>
<li>
<h3>Each pixel requires at least 3 bytes. One byte for each primary color.
</h3>
</li>
<li>
<h3>Sometimes combined with a look-up table per primary </h3>
</li>
<li>
<h3>Each pixel can be one of 2^24 colors (= 16777216 =~ 16millions)</h3>
</li>
<li>
<h3>Worry about your Endians </h3>
</li>
</ul>
<p><IMG src="Image/TrueColor.gif" width="512" height="300"></p>
<h2> </h2>
<h2>Endians</h2>
<p class="lecturetexte">Computer memory is traditionally organized in bytes, which
are units of 8 consecutive bits. The string "Hello" would typically
occupy 6 consecutive byte addresses in memory, represented schematically as
follows: </p>
<pre class="lecturenotes">0x00329384 'H'
0x00329385 'e'
0x00329386 'l'
0x00329387 'l'
0x00329388 'o'
0x00329389 '0'</pre>
<p class="lecturetexte">The (hexadecimal) address of each memory location is shown
at the left, and the value in that location is shown at the right. Note that
the end of the string is indicated by a null character ('0') in some languages,
such as C and C++. Other languages may use different conventions.</p>
<h2>Suppose, we have a 2-byte integer variable.</h2>
<p class="lecturetexte">In C and C++, this could be indicated by the data type
"short" or "unsigned short", or in Java by "short".
For definiteness, suppose this integer has the value 12, which is "0x000c"
in hexadecimal. If this were stored at the same address as the 'H' in the word
"Hello", how would it look?</p>
<h3>One possibility would be </h3>
<pre class="javacode">0x00329384 '00'
0x00329385 '0c'</pre>
<h2>This is called "big-endian", </h2>
<p class="lecturetexte">because the most significant byte of the word is stored
first (at lower address). Most computer chips use this convention.</p>
<h2>The other possibility is, of course, </h2>
<pre class="javacode">0x00329384 '0c'
0x00329385 '00'</pre>
<h2>This is called "little-endian", </h2>
<p class="lecturetexte">for the obvious reason. Intel uses this convention, I
suppose just to be different.</p>
<h2>Who Cares?</h2>
<p class="lecturetexte">Suppose you read a binary file written on a big-endian
computer system using a little-endian system? You will get all the values wrong
unless you exchange the order of the bytes. In such a case, you care! </p>
<P>
<HR>
<H4><font color="#000000"><u>Slide 24 : Indexed-Color Frame Buffers</u></font></h4>
<HR>
<h1>Indexed-Color Frame Buffers</h1>
<ul>
<li>
<h3>Each pixel uses one byte </h3>
</li>
<li>
<h3>Each byte is an index into a color map </h3>
</li>
<li>
<h3>If the color map is not updated synchronously then Color-map flashing
may occcur. </h3>
</li>
<li>
<h3>Color-map Animations </h3>
</li>
<li>
<h3>Each pixel may be one of 2^24 colors, but only 256 color be displayed
at a time </h3>
</li>
</ul>
<IMG src="Image/ColMap.gif" width="512" height="300">
<P>
<HR>
<H4><font color="#000000"><u>Slide 25 : Indexed-Color Frame Buffers</u></font></h4>
<HR>
<h1>Indexed-Color Frame Buffers</h1>
<ul>
<li>
<h3>Each pixel uses one byte </h3>
</li>
<li>
<h3>Each byte is an index into a color map </h3>
</li>
<li>
<h3>If the color map is not updated synchronously then Color-map flashing
may occcur. </h3>
</li>
<li>
<h3>Color-map Animations </h3>
</li>
<li>
<h3>Each pixel may be one of 2^24 colors, but only 256 color be displayed
at a time </h3>
</li>
</ul>
<IMG src="Image/ColMap.gif" width="512" height="300">
<P>
<HR>
<H4><font color="#000000"><u>Slide 26 : Other Color Systems</u></font></h4>
<HR>
<h1>Other Color Systems</h1>
<h3>Several other color models also exist. Models such as HSV (hue, saturation,
value) and HLS (hue, luminosity, saturation) are designed for intuitive understanding.
Using these color models, the user of a paint program would quickly be able
to select a desired color. </h3>
<h2>Example: NTSC YIQ color space </h2>
<h2 align="center"><IMG src="Image/RGBTOYIQ.gif" width="300" height="190"></h2>
<P>
</BODY>
</HTML>
yields the color on monitor 2 which is equivalent to a given color on monitor
1. Conversion to-and-from printer gamuts is difficult. A first approximation
is as follows: 
