(octave.info)Representing Images
32.3 Representing Images
========================
In general Octave supports four different kinds of images, grayscale
images, RGB images, binary images, and indexed images. A grayscale
image is represented with an M-by-N matrix in which each element
corresponds to the intensity of a pixel. An RGB image is represented
with an M-by-N-by-3 array where each 3-vector corresponds to the red,
green, and blue intensities of each pixel.
The actual meaning of the value of a pixel in a grayscale or RGB
image depends on the class of the matrix. If the matrix is of class
‘double’ pixel intensities are between 0 and 1, if it is of class
‘uint8’ intensities are between 0 and 255, and if it is of class
‘uint16’ intensities are between 0 and 65535.
A binary image is an M-by-N matrix of class ‘logical’. A pixel in a
binary image is black if it is ‘false’ and white if it is ‘true’.
An indexed image consists of an M-by-N matrix of integers and a
C-by-3 color map. Each integer corresponds to an index in the color
map, and each row in the color map corresponds to an RGB color. The
color map must be of class ‘double’ with values between 0 and 1.
-- : im2double (IMG)
-- : im2double (IMG, "indexed")
Convert image to double precision.
The conversion of IMG to double precision, is dependent on the type
of input image. The following input classes are supported:
‘uint8, uint16, and int16’
The range of values from the class is scaled to the interval
[0 1].
‘logical’
True and false values are assigned a value of 0 and 1
respectively.
‘single’
Values are cast to double.
‘double’
Returns the same image.
If IMG is an indexed image, then the second argument should be the
string "indexed". If so, then IMG must either be of floating point
class, or unsigned integer class and it will simply be cast to
double. If it is an integer class, a +1 offset is applied.
See also: Note: double.
-- : iscolormap (CMAP)
Return true if CMAP is a colormap.
A colormap is a real matrix, of class single or double, with 3
columns. Each row represents a single color. The 3 columns
contain red, green, and blue intensities respectively.
All values in a colormap should be in the [0 1] range but this is
not enforced. Each function must decide what to do for values
outside this range.
See also: Note: colormap, Note: rgbplot.
-- : IMG = gray2ind (I)
-- : IMG = gray2ind (I, N)
-- : IMG = gray2ind (BW)
-- : IMG = gray2ind (BW, N)
-- : [IMG, MAP] = gray2ind (...)
Convert a grayscale or binary intensity image to an indexed image.
The indexed image will consist of N different intensity values. If
not given N defaults to 64 for grayscale images or 2 for binary
black and white images.
The output IMG is of class uint8 if N is less than or equal to 256;
Otherwise the return class is uint16.
See also: Note: ind2gray, Note: rgb2ind.
-- : I = ind2gray (X, MAP)
Convert a color indexed image to a grayscale intensity image.
The image X must be an indexed image which will be converted using
the colormap MAP. If MAP does not contain enough colors for the
image, pixels in X outside the range are mapped to the last color
in the map before conversion to grayscale.
The output I is of the same class as the input X and may be one of
‘uint8’, ‘uint16’, ‘single’, or ‘double’.
Implementation Note: There are several ways of converting colors to
grayscale intensities. This functions uses the luminance value
obtained from ‘rgb2gray’ which is ‘I = 0.299*R + 0.587*G +
0.114*B’. Other possibilities include the value component from
‘rgb2hsv’ or using a single color channel from ‘ind2rgb’.
See also: Note: gray2ind, Note: ind2rgb.
-- : [X, MAP] = rgb2ind (RGB)
-- : [X, MAP] = rgb2ind (R, G, B)
Convert an image in red-green-blue (RGB) color space to an indexed
image.
The input image RGB can be specified as a single matrix of size
MxNx3, or as three separate variables, R, G, and B, its three color
channels, red, green, and blue.
It outputs an indexed image X and a colormap MAP to interpret an
image exactly the same as the input. No dithering or other form of
color quantization is performed. The output class of the indexed
image X can be uint8, uint16 or double, whichever is required to
specify the number of unique colors in the image (which will be
equal to the number of rows in MAP) in order.
Multi-dimensional indexed images (of size MxNx3xK) are also
supported, both via a single input (RGB) or its three color
channels as separate variables.
See also: Note: ind2rgb, Note: rgb2hsv,
Note: rgb2gray.
-- : RGB = ind2rgb (X, MAP)
-- : [R, G, B] = ind2rgb (X, MAP)
Convert an indexed image to red, green, and blue color components.
The image X must be an indexed image which will be converted using
the colormap MAP. If MAP does not contain enough colors for the
image, pixels in X outside the range are mapped to the last color
in the map.
The output may be a single RGB image (MxNx3 matrix where M and N
are the original image X dimensions, one for each of the red, green
and blue channels). Alternatively, the individual red, green, and
blue color matrices of size MxN may be returned.
Multi-dimensional indexed images (of size MxNx1xK) are also
supported.
See also: Note: rgb2ind, Note: ind2gray,
Note: hsv2rgb.
-- : FRAME = getframe ()
-- : FRAME = getframe (HAX)
-- : FRAME = getframe (HFIG)
-- : FRAME = getframe (..., RECT)
Capture a figure or axes as a movie frame structure.
Without an argument, capture the current axes excluding ticklabels,
title, and x/y/zlabels. The returned structure FRAME has a field
‘cdata’, which contains the actual image data in the form of an
NxMx3 (RGB) uint8 matrix, and a field ‘colormap’ which is provided
for MATLAB compatibility but is always empty.
If the first argument HAX is an axes handle, then capture this
axes, rather than the current axes returned by ‘gca’.
If the first argument HFIG is a figure handle then the entire
corresponding figure canvas is captured.
Finally, if a second argument RECT is provided it must be a
four-element vector ([left bottom width height]) defining the
region inside the figure to be captured. Regardless of the figure
"units" property, RECT must be defined in *pixels*.
See also: Note: im2frame, *note frame2im:
XREFframe2im.
-- : [X, MAP] = frame2im (FRAME)
Convert movie frame to indexed image.
A movie frame is simply a struct with the fields "cdata" and
"colormap".
Support for N-dimensional images or movies is given when FRAME is a
struct array. In such cases, X will be a MxNx1xK or MxNx3xK for
indexed and RGB movies respectively, with each frame concatenated
along the 4th dimension.
See also: Note: im2frame, *note getframe:
XREFgetframe.
-- : im2frame (RGB)
-- : im2frame (X, MAP)
Convert image to movie frame.
A movie frame is simply a struct with the fields "cdata" and
"colormap".
Support for N-dimensional images is given when each image
projection, matrix sizes of MxN and MxNx3 for RGB images, is
concatenated along the fourth dimension. In such cases, the
returned value is a struct array.
See also: Note: frame2im.
-- : CMAP = colormap ()
-- : CMAP = colormap (MAP)
-- : CMAP = colormap ("default")
-- : CMAP = colormap (MAP_NAME)
-- : CMAP = colormap (HAX, ...)
-- : colormap MAP_NAME
Query or set the current colormap.
With no input arguments, ‘colormap’ returns the current color map.
‘colormap (MAP)’ sets the current colormap to MAP. The colormap
should be an N row by 3 column matrix. The columns contain red,
green, and blue intensities respectively. All entries must be
between 0 and 1 inclusive. The new colormap is returned.
‘colormap ("default")’ restores the default colormap (the ‘viridis’
map with 64 entries). The default colormap is returned.
The map may also be specified by a string, MAP_NAME, which is the
name of a function that returns a colormap.
If the first argument HAX is an axes handle, then the colormap for
those axes is queried or set.
For convenience, it is also possible to use this function with the
command form, ‘colormap MAP_NAME’.
The list of built-in colormaps is:
Map Description
--------------------------------------------------------------------------
viridis default
jet colormap traversing blue, cyan, green, yellow, red.
cubehelix colormap traversing black, blue, green, red, white with
increasing intensity.
hsv cyclic colormap traversing Hue, Saturation, Value space.
rainbow colormap traversing red, yellow, blue, green, violet.
————- ———————————————————————————————
hot colormap traversing black, red, orange, yellow, white.
cool colormap traversing cyan, purple, magenta.
spring colormap traversing magenta to yellow.
summer colormap traversing green to yellow.
autumn colormap traversing red, orange, yellow.
winter colormap traversing blue to green.
————- ———————————————————————————————
gray colormap traversing black to white in shades of gray.
bone colormap traversing black, gray-blue, white.
copper colormap traversing black to light copper.
pink colormap traversing black, gray-pink, white.
ocean colormap traversing black, dark-blue, white.
————- ———————————————————————————————
colorcube equally spaced colors in RGB color space.
flag cyclic 4-color map of red, white, blue, black.
lines cyclic colormap with colors from axes "ColorOrder"
property.
prism cyclic 6-color map of red, orange, yellow, green, blue,
violet.
————- ———————————————————————————————
white all white colormap (no colors).
See also: Note: viridis, Note: jet, Note:
cubehelix, Note: hsv, *note rainbow:
XREFrainbow, Note: hot, Note: cool, Note:
spring, Note: summer, *note autumn:
XREFautumn, Note: winter, Note: gray, Note:
bone, Note: copper, Note: pink,
Note: ocean, Note: colorcube, *note flag:
XREFflag, Note: lines, Note: prism, Note:
white.
-- : rgbplot (CMAP)
-- : rgbplot (CMAP, STYLE)
-- : H = rgbplot (...)
Plot the components of a colormap.
Two different STYLEs are available for displaying the CMAP:
profile (default)
Plot the RGB line profile of the colormap for each of the
channels (red, green and blue) with the plot lines colored
appropriately. Each line represents the intensity of an RGB
component across the colormap.
composite
Draw the colormap across the X-axis so that the actual index
colors are visible rather than the individual color
components.
The optional return value H is a graphics handle to the created
plot.
Run ‘demo rgbplot’ to see an example of ‘rgbplot’ and each style
option.
See also: Note: colormap.
-- : MAP = autumn ()
-- : MAP = autumn (N)
Create color colormap. This colormap ranges from red through
orange to yellow.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = bone ()
-- : MAP = bone (N)
Create color colormap. This colormap varies from black to white
with gray-blue shades.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = colorcube ()
-- : MAP = colorcube (N)
Create color colormap. This colormap is composed of as many
equally spaced colors (not grays) in the RGB color space as
possible.
If there are not a perfect number N of regularly spaced colors then
the remaining entries in the colormap are gradients of pure red,
green, blue, and gray.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = cool ()
-- : MAP = cool (N)
Create color colormap. The colormap varies from cyan to magenta.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = copper ()
-- : MAP = copper (N)
Create color colormap. This colormap varies from black to a light
copper tone.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = cubehelix ()
-- : MAP = cubehelix (N)
Create cubehelix colormap.
This colormap varies from black to white going though blue, green,
and red tones while maintaining a monotonically increasing
perception of intensity. This is achieved by traversing a color
cube from black to white through a helix, hence the name cubehelix,
while taking into account the perceived brightness of each channel
according to the NTSC specifications from 1953.
rgbplot (cubehelix (256))
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
Reference: Green, D. A., 2011, ‘"A colour scheme for the display of
astronomical intensity images"’, Bulletin of the Astronomical
Society of India, 39, 289.
See also: Note: colormap.
-- : MAP = flag ()
-- : MAP = flag (N)
Create color colormap. This colormap cycles through red, white,
blue, and black with each index change.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = gray ()
-- : MAP = gray (N)
Create gray colormap. This colormap varies from black to white
with shades of gray.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = hot ()
-- : MAP = hot (N)
Create color colormap. This colormap ranges from black through
dark red, red, orange, yellow, to white.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : hsv (N)
Create color colormap. This colormap begins with red, changes
through yellow, green, cyan, blue, and magenta, before returning to
red.
It is useful for displaying periodic functions. The map is
obtained by linearly varying the hue through all possible values
while keeping constant maximum saturation and value. The
equivalent code is ‘hsv2rgb ([(0:N-1)'/N, ones(N,2)])’.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = jet ()
-- : MAP = jet (N)
Create color colormap. This colormap ranges from dark blue through
blue, cyan, green, yellow, red, to dark red.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = lines ()
-- : MAP = lines (N)
Create color colormap. This colormap is composed of the list of
colors in the current axes "ColorOrder" property. The default is
blue, orange, yellow, purple, green, light blue, and dark red.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = ocean ()
-- : MAP = ocean (N)
Create color colormap. This colormap varies from black to white
with shades of blue.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = pink ()
-- : MAP = pink (N)
Create color colormap. This colormap varies from black to white
with shades of gray-pink.
This colormap gives a sepia tone when used on grayscale images.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = prism ()
-- : MAP = prism (N)
Create color colormap. This colormap cycles through red, orange,
yellow, green, blue and violet with each index change.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = rainbow ()
-- : MAP = rainbow (N)
Create color colormap. This colormap ranges from red through
orange, yellow, green, blue, to violet.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = spring ()
-- : MAP = spring (N)
Create color colormap. This colormap varies from magenta to
yellow.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = summer ()
-- : MAP = summer (N)
Create color colormap. This colormap varies from green to yellow.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = viridis ()
-- : MAP = viridis (N)
Create color colormap. This colormap ranges from dark
purplish-blue through blue, green, to yellow.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = white ()
-- : MAP = white (N)
Create color colormap. This colormap is completely white.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : MAP = winter ()
-- : MAP = winter (N)
Create color colormap. This colormap varies from blue to green.
The argument N must be a scalar. If unspecified, the length of the
current colormap, or 64, is used.
See also: Note: colormap.
-- : CMAP = contrast (X)
-- : CMAP = contrast (X, N)
Return a gray colormap that maximizes the contrast in an image.
The returned colormap will have N rows. If N is not defined then
the size of the current colormap is used.
See also: Note: colormap, *note brighten:
XREFbrighten.
The following three functions modify the existing colormap rather
than replace it.
-- : MAP_OUT = brighten (BETA)
-- : MAP_OUT = brighten (MAP, BETA)
-- : MAP_OUT = brighten (H, BETA)
-- : brighten (...)
Brighten or darken a colormap.
The argument BETA must be a scalar between -1 and 1, where a
negative value darkens and a positive value brightens the colormap.
If the MAP argument is omitted, the function is applied to the
current colormap.
The first argument can also be a valid graphics handle H, in which
case ‘brighten’ is applied to the colormap associated with this
handle.
If no output is specified then the result is written to the current
colormap.
See also: Note: colormap, *note contrast:
XREFcontrast.
-- : spinmap ()
-- : spinmap (T)
-- : spinmap (T, INC)
-- : spinmap ("inf")
Cycle the colormap for T seconds with a color increment of INC.
Both parameters are optional. The default cycle time is 5 seconds
and the default increment is 2. If the option "inf" is given then
cycle continuously until ‘Control-C’ is pressed.
When rotating, the original color 1 becomes color 2, color 2
becomes color 3, etc. A positive or negative increment is allowed
and a higher value of INC will cause faster cycling through the
colormap.
See also: Note: colormap.
-- : whitebg ()
-- : whitebg (COLOR)
-- : whitebg ("none")
-- : whitebg (HFIG)
-- : whitebg (HFIG, COLOR)
-- : whitebg (HFIG, "none")
Invert the colors in the current color scheme.
The root properties are also inverted such that all subsequent
plots will use the new color scheme.
If the optional argument COLOR is present then the background color
is set to COLOR rather than inverted. COLOR may be a string
representing one of the eight known colors or an RGB triplet. The
special string argument "none" restores the plot to the factory
default colors.
If the first argument HFIG is a figure handle or list of figure
handles, then operate on these figures rather than the current
figure returned by ‘gcf’. The root properties will not be changed
unless 0 is in the list of figures.
Programming Note: ‘whitebg’ operates by changing the color
properties of the children of the specified figures. Only objects
with a single color are affected. For example, a patch with a
single "FaceColor" will be changed, but a patch with shading
("interp") will not be modified. For inversion, the new color is
simply the inversion in RGB space: ‘CNEW = [1-R 1-G 1-B]’. When a
color is specified, the axes and figure are set to the new color,
and the color of child objects are then adjusted to have some
contrast (visibility) against the new background.
See also: Note: reset, Note: get, *note set:
XREFset.
The following functions can be used to manipulate colormaps.
-- : [Y, NEWMAP] = cmunique (X, MAP)
-- : [Y, NEWMAP] = cmunique (RGB)
-- : [Y, NEWMAP] = cmunique (I)
Convert an input image X to an ouput indexed image Y which uses the
smallest colormap possible NEWMAP.
When the input is an indexed image (X with colormap MAP) the output
is a colormap NEWMAP from which any repeated rows have been
eliminated. The output image, Y, is the original input image with
the indices adjusted to match the new, possibly smaller, colormap.
When the input is an RGB image (an MxNx3 array), the output
colormap will contain one entry for every unique color in the
original image. In the worst case the new map could have as many
rows as the number of pixels in the original image.
When the input is a grayscale image I, the output colormap will
contain one entry for every unique intensity value in the original
image. In the worst case the new map could have as many rows as
the number of pixels in the original image.
Implementation Details:
NEWMAP is always an Mx3 matrix, even if the input image is an
intensity grayscale image I (all three RGB planes are assigned the
same value).
The output image is of class uint8 if the size of the new colormap
is less than or equal to 256. Otherwise, the output image is of
class double.
See also: Note: rgb2ind, Note: gray2ind.
-- : [Y, NEWMAP] = cmpermute (X, MAP)
-- : [Y, NEWMAP] = cmpermute (X, MAP, INDEX)
Reorder colors in a colormap.
When called with only two arguments, ‘cmpermute’ randomly
rearranges the colormap MAP and returns a new colormap NEWMAP. It
also returns the indexed image Y which is the equivalent of the
original input image X when displayed using NEWMAP.
When called with an optional third argument the order of colors in
the new colormap is defined by INDEX.
*Caution:* INDEX should not have repeated elements or the function
will fail.
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