SAOimage color


color under the X window system on color workstations

Most color workstations use an 8-bit color map, making it possible to
choose a palette of 256 colors and to map each pixel on the screen to
one of the 256 possible colors.  Each of the 256 colors in the palette
can be any color specified by its red, green, and blue intensities. By
contrast, IBM PC's commonly offer palettes of 4 or 16 colors selected
from a restricted group of colors.  At the other extreme, 24 bit
displays allow each pixel to be mapped directly to virtually any color,
defined by its red, green, and blue intensities (using 8 bits for each).

X11 uses a color reservation system by which each application reserves a
portion of the palette for its own use.  Color reservation prevents an
application from changing the colors of other applications present on the
screen.  Typically, SAOimage will be able to reserve as many as 240 color
entries in the palette.  However, if you (or your window manager) have
tailored your X environment by specifying a variety of unusual colors for
terminal windows, the clock, and other commonly used applications, there
will be fewer unreserved colors in the palette.  Conversely, if you are
running an SAOimage which has reserved all of the available colors, you
may be unable to bring up a new application which expects to be able to
grab new colors of its own.  A color in the palette is often referred to
as a color cell (referring to its reservation) or color level (referring
to its index between 0 and 255).

SAOimage uses color cells for more than just rendering the image.  The
adjustable cursor and the region cursors must also be referenced to colors
in the color palette.  When the cursor is drawn, pixels in the display are
set to the value of the cursor color, overwriting the original image data.
If no special care was taken, the image would gradually be erased as the
cursor was moved about the display.  To avoid this problem, SAOimage uses
either of two strategies for tracking the cursor while it is in motion.

With one strategy, while the cursor is moving, or being adjusted, the
bits in the displayed image are simply reversed to represent the cursor,
and flipped back when the cursor moves away.  This doesn't draw the cursor
in its correct color, but, in most cases, it produces a visible cursor.
When the cursor adjustment ends (the mouse button is released), the
entire image is redrawn with the cursor in its new position.  This same
strategy is used to track the cursor on halftone displays.

The other strategy actually reserves half of the colors just for drawing
the cursor.  In this alternative strategy, each image color level has a
corresponding level in the palette which has the cursor color.  The image
color levels differ from those of the cursor by one bit, which can be set
and unset to represent either the cursor or the image.  Drawing the cursor
simply involves manipulating the distinguishing bit in display memory.  In
this mode, cursor can be tracked smoothly in its correct color without
erasing the image data.  This system is analogous to reserving "planes" of
display memory to be treated as overlay planes and image planes (X does
not as yet recognize true hardware overlay planes).  The advantage of the
"plane" reservation strategy is that cursor tracking is visually flawless.
The disadvantage is that fewer than half of the color levels are available
for rendering the image.  Fortunately, it is easy to switch between the
two modes (using the "ovlay" button), so one can choose whichever coloring
strategy is most appropriate for the current activity.


Color mapping

In order to maximize the use of the available colors, SAOimage offers
several facilities for assigning and altering colors in the palette.  The
The association of a palette level with an image data value is handled by
scaling and is explained in a separate section.  There are basically three
ways of assigning colors in the palette: true-color, gray-scale, and
pseudocolor.  SAOimage supports the latter two.  SAOimage also supports
half-toning on non-color workstations and by selecting the "mono" button
in the color submenu (see the halftone section).

In true color, each image data value has associated with it an actual
color.  True color mapping tries to associate colors, as near as possible
to the true color, with each pixel in the image display.  This is
difficult where there are few colors in the palette.  There is no support
in SAOimage for true-color mapping.

In gray scale, all pixels have the same color, but differ in intensity.
Basically, the colors range from black to white, with shades of gray in
between.  It could also be done with some other color such as shades of
red.  The lowest data values appear black while the highest appear white
(or visa-versa).  The image appears as a black-and-white photograph might
render it.

In pseudo-color, any color can be assigned to any level, but all pixels
with the same value will have the same color.  Typically, one might use
an analogy with heat, mapping the low values as shades of blue, the middle
values in shades of red and the highest values as yellow or white.  The
idea is to use the colors to highlight differences among the data values.
Depending on the levels which best distinguish the detail you wish to
study, the shifts from blue to red and red to yellow can be placed at
higher or lower image data values and closer together or farther apart.
The changes in color can be made gradual or sharp.

Color maps may simply be a list of colors for each level, or may be
created by specifying a few colors and levels and interpolating to
assign colors for the in-between levels.  SAOimage uses the latter.
If one were to graph the color map, having intensity of color on one
axis and palette level on the other, the graph would have fixed points
with ramps or steps between them.  The simplest gray scale has no
intensity for any color at one end of the palette and full intensity
for all colors at the other end of the palette, with a straight line
representing the interpolated colors in between.  The color graph is
in fact physically drawn by SAOimage in a separate window, and can be
directly manipulated.

SAOimage has a basic gray scale and several pseudocolor maps available
in the "cmap" page of the "Color" submenu.  Once you have selected a
color map, you may choose to manipulate it, as described below.
Reselecting the same color map, or selecting a new color map from
"cmap" submenu, sets the selected color map, eliminating any
adjustments you may have made.


Color manipulation

The color graph window is normally not displayed.  It is summoned
(and hidden again) by clicking the mouse on the color bar next to the
display window.  In the graph, each color is graphed separately, with
little squares to represent the fixed points in the graph.  Where two
or more color lines overlap, the line (or box) appears black.  One may
create a new fixed point by positioning the mouse icon in the graph and
pressing a mouse button.  The three mouse buttons control red, green,
and blue, respectively from left to right.  By holding the mouse button
(or buttons) down, the fixed point can be dragged anywhere on the graph.
An existing fixed point can be grabbed for dragging by positioning the
mouse icon over it when pressing the button.

The graph can be adjusted, en mass, by moving the mouse in the main
display window with a mouse button depressed.  There are two different
kinds of adjustment, threshold/saturation and contrast/bias, and an
intensity adjustment called gamma.

With threshold/saturation, moving the mouse horizontally moves the lower
(threshold) end of the graph up or down, while moving the mouse vertically
moves the upper (saturation) end of the graph up and down relative to the
palette.   With contrast/bias, moving the mouse along the axis of the
color bar shifts the entire graph up or down (bias) relative to the
palette, while moving the mouse perpendicular to the color bar moves the
ends of the graph closer together or farther apart about a middle position
(contrast).  Threshold/saturation is easy to implement in software and
common in pseudocolor display systems. Contrast/bias corresponds more
closely to the kinds of adjustments familiar to photographers.  In both
cases, the middle of the display window is the default graph position
relative to the palette.

The intensities of the colors are normally given relative to voltage
applied to the color guns in the monitor.  Half intensity is half of
full voltage.  Unfortunately, this does not really correspond to the
sensitivity of the eye.  Double the voltage does not seem like doubling
the intensity.  Half voltage on a gray scale does not seem like a
middle gray.  The gray scale seems to favor the darker shades.  The
relationship between voltage and perception is generally thought to be
an exponential one and is represented by the symbol small gamma.
Changing the gamma produces a non-linear (exponential) adjustment in
contrast.  A gamma of between 2 and 2.2 is considered correct for a
typical monitor.  You can play with the gamma adjustment by selecting
the "power" mode in the "Color" submenu.  Moving the mouse horizontally
in the main display window with a mouse button down adjusts the gamma.
The gamma values for each color are printed beside the color graph.
Gamma of 1 (linear) is in the middle of the main window.  The intensity
adjustment is applied directly to the palette colors and does not affect
the points used to map the colors.  Gamma values below 1 may be useful
for sharpening the contrast before making a hard copy.  You can drag
beyond the main window for gamma values outside the normal range.

Normally, all adjustments are applied equally to each of the three colors.
However, the adjustments can be applied to any one or two of the colors
by holding down the control key.  Then the three mouse buttons control
red, green, and blue, respectively, as in the graph window.

The "invert" button in the "Color" submenu inverts the intensities
(minimum intensity becomes maximum intensity) without changing the graph
points.


Updating the graph and gamma display

Drawing the graph and printing the gamma values takes up computing time
and may slow the response to your movement of the mouse.  Therefore,
while the colors are always continuously updated, updating of the graph
occurs only when you finish (release the mouse button), unless you
specifically request it.  The "track" button in the "etc" submenu,
controls whether the color graph is updated continuously or only upon
completion of the manipulation.  Tracking may be temporarily activated
(or deactivated) by pressing a "shift" key on the keyboard or toggling
the "Caps" shiftlock key.


Saving color map entries and reading them back in (color only)

The current colormap can be written to a disk file, and a previously
saved colormap can be read from a disk file.  The format of the disk
file is ASCII and can be edited if the format is followed.  The file
can have comments on any line, starting with a '#' symbol.  The first
non-comment word in the file must be 'PSEUDOCOLOR'.  Each color's
table is defined separately.  Each color's table must begin with the
color name 'RED', 'GREEN', or 'BLUE'.  The color name may optionally
be followed by the word GAMMA, then followed by a gamma value for
that color.  The vertex points in the table are defined by pairs:
'(level, intensity)'.  The intensities range from 0 (minimum) to 1.0
(maximum).  The levels range from 0 (lowest level) to 1.0 (highest
level).  The points must be in ascending order by level.  All three
colors must be described, and each color must have at least 2 points.

When a color map is being manipulated, the effective levels of points
may be shifted or stretched above 1.0 or below 0.0.  These points
are preserved in the disk file, and when read in, they may be shifted
back into the visible range.  The novice user should remember that if
one starts with the 'A' colormap, shifts it, and then writes it out,
the stored colormap is the shifted map, not the original 'A' map.