photo(n) Tk (4.0) photo(n)
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NAME
photo - Full-color images
SYNOPSIS
image create photo ?name? ?options?
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DESCRIPTION
A photo is an image whose pixels can display any color or be
transparent. A photo image is stored internally in full
color (24 bits per pixel), and is displayed using dithering
if necessary. Image data for a photo image can be obtained
from a file or a string, or it can be supplied from C code
through a procedural interface. At present, only GIF and
PPM/PGM formats are supported, but an interface exists to
allow additional image file formats to be added easily. A
photo image is transparent in regions where no image data
has been supplied.
CREATING PHOTOS
Like all images, photos are created using the image create
command. Photos support the following options:
-data string
Specifies the contents of the image as a string. The
format of the string must be one of those for which
there is an image file format handler that will accept
string data. If both the -data and -file options are
specified, the -file option takes precedence.
-format format-name
Specifies the name of the file format for the data
specified with the -data or -file option.
-file name
name gives the name of a file that is to be read to
supply data for the photo image. The file format must
be one of those for which there is an image file format
handler that can read data.
-gamma value
Specifies that the colors allocated for displaying this
image in a window should be corrected for a non-linear
display with the specified gamma exponent value. (The
intensity produced by most CRT displays is a power
function of the input value, to a good approximation;
gamma is the exponent and is typically around 2). The
value specified must be greater than zero. The default
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value is one (no correction). In general, values
greater than one will make the image lighter, and
values less than one will make it darker.
-height number
Specifies the height of the image, in pixels. This
option is useful primarily in situations where the user
wishes to build up the contents of the image piece by
piece. A value of zero (the default) allows the image
to expand or shrink vertically to fit the data stored
in it.
-palette palette-spec
Specifies the resolution of the color cube to be
allocated for displaying this image, and thus the
number of colors used from the colormaps of the windows
where it is displayed. The palette-spec string may be
either a single decimal number, specifying the number
of shades of gray to use, or three decimal numbers
separated by slashes (/), specifying the number of
shades of red, green and blue to use, respectively. If
the first form (a single number) is used, the image
will be displayed in monochrome (i.e., grayscale).
-width number
Specifies the width of the image, in pixels. This
option is useful primarily in situations where the user
wishes to build up the contents of the image piece by
piece. A value of zero (the default) allows the image
to expand or shrink horizontally to fit the data stored
in it.
IMAGE COMMAND
When a photo image is created, Tk also creates a new command
whose name is the same as the image. This command may be
used to invoke various operations on the image. It has the
following general form:
imageName option ?arg arg ...?
Option and the args determine the exact behavior of the
command.
Those options that write data to the image generally expand
the size of the image, if necessary, to accommodate the data
written to the image, unless the user has specified non-zero
values for the -width and/or -height configuration options,
in which case the width and/or height, respectively, of the
image will not be changed.
The following commands are possible for photo images:
imageName blank
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Blank the image; that is, set the entire image to have
no data, so it will be displayed as transparent, and
the background of whatever window it is displayed in
will show through.
imageName cget option
Returns the current value of the configuration option
given by option. Option may have any of the values
accepted by the image create photo command.
imageName configure ?option? ?value option value ...?
Query or modify the configuration options for the
image. If no option is specified, returns a list
describing all of the available options for imageName
(see Tk_ConfigureInfo for information on the format of
this list). If option is specified with no value, then
the command returns a list describing the one named
option (this list will be identical to the
corresponding sublist of the value returned if no
option is specified). If one or more option-value
pairs are specified, then the command modifies the
given option(s) to have the given value(s); in this
case the command returns an empty string. Option may
have any of the values accepted by the image create
photo command.
imageName copy sourceImage ?option value(s) ...?
Copies a region from the image called sourceImage
(which must be a photo image) to the image called
imageName, possibly with pixel zooming and/or
subsampling. If no options are specified, this command
copies the whole of sourceImage into imageName,
starting at coordinates (0,0) in imageName. The
following options may be specified:
-from x1 y1 x2 y2
Specifies a rectangular sub-region of the source
image to be copied. (x1,y1) and (x2,y2) specify
diagonally opposite corners of the rectangle. If
x2 and y2 are not specified, the default value is
the bottom-right corner of the source image. The
pixels copied will include the left and top edges
of the specified rectangle but not the bottom or
right edges. If the -from option is not given,
the default is the whole source image.
-to x1 y1 x2 y2
Specifies a rectangular sub-region of the
destination image to be affected. (x1,y1) and
(x2,y2) specify diagonally opposite corners of the
rectangle. If x2 and y2 are not specified, the
default value is (x1,y1) plus the size of the
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source region (after subsampling and zooming, if
specified). If x2 and y2 are specified, the
source region will be replicated if necessary to
fill the destination region in a tiled fashion.
-shrink
Specifies that the size of the destination image
should be reduced, if necessary, so that the
region being copied into is at the bottom-right
corner of the image. This option will not affect
the width or height of the image if the user has
specified a non-zero value for the -width or
-height configuration option, respectively.
-zoom x y
Specifies that the source region should be
magnified by a factor of x in the X direction and
y in the Y direction. If y is not given, the
default value is the same as x. With this option,
each pixel in the source image will be expanded
into a block of x x y pixels in the destination
image, all the same color. x and y must be
greater than 0.
-subsample x y
Specifies that the source image should be reduced
in size by using only every xth pixel in the X
direction and yth pixel in the Y direction.
Negative values will cause the image to be flipped
about the Y or X axes, respectively. If y is not
given, the default value is the same as x.
imageName get x y
Returns the color of the pixel at coordinates (x,y) in
the image as a list of three integers between 0 and
255, representing the red, green and blue components
respectively.
imageName put data ?-to x1 y1 x2 y2?
Sets pixels in imageName to the colors specified in
data. data is used to form a two-dimensional array of
pixels that are then copied into the imageName. data
is structured as a list of horizontal rows, from top to
bottom, each of which is a list of colors, listed from
left to right. Each color may be specified by name
(e.g., blue) or in hexadecimal form (e.g., #2376af).
The -to option can be used to specify the area of
imageName to be affected. If only x1 and y1 are given,
the area affected has its top-left corner at (x1,y1)
and is the same size as the array given in data. If
all four coordinates are given, they specify diagonally
opposite corners of the affected rectangle, and the
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array given in data will be replicated as necessary in
the X and Y directions to fill the rectangle.
imageName read filename ?option value(s) ...?
Reads image data from the file named filename into the
image. This command first searches the list of image
file format handlers for a handler that can interpret
the data in filename, and then reads the image in
filename into imageName (the destination image). The
following options may be specified:
-format format-name
Specifies the format of the image data in
filename. Specifically, only image file format
handlers whose names begin with format-name will
be used while searching for an image data format
handler to read the data.
-from x1 y1 x2 y2
Specifies a rectangular sub-region of the image
file data to be copied to the destination image.
If only x1 and y1 are specified, the region
extends from (x1,y1) to the bottom-right corner of
the image in the image file. If all four
coordinates are specified, they specify diagonally
opposite corners or the region. The default, if
this option is not specified, is the whole of the
image in the image file.
-shrink
If this option, the size of imageName will be
reduced, if necessary, so that the region into
which the image file data are read is at the
bottom-right corner of the imageName. This option
will not affect the width or height of the image
if the user has specified a non-zero value for the
-width or -height configuration option,
respectively.
-to x y
Specifies the coordinates of the top-left corner
of the region of imageName into which data from
filename are to be read. The default is (0,0).
imageName redither
The dithering algorithm used in displaying photo images
propagates quantization errors from one pixel to its
neighbors. If the image data for imageName is supplied
in pieces, the dithered image may not be exactly
correct. Normally the difference is not noticeable,
but if it is a problem, this command can be used to
recalculate the dithered image in each window where the
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image is displayed.
imageName write filename ?option value(s) ...?
Writes image data from imageName to a file named
filename. The following options may be specified:
-format format-name
Specifies the name of the image file format
handler to be used to write the data to the file.
Specifically, this subcommand searches for the
first handler whose name matches a initial
substring of format-name and which has the
capability to write an image file. If this option
is not given, this subcommand uses the first
handler that has the capability to write an image
file.
-from x1 y1 x2 y2
Specifies a rectangular region of imageName to be
written to the image file. If only x1 and y1 are
specified, the region extends from (x1,y1) to the
bottom-right corner of imageName. If all four
coordinates are given, they specify diagonally
opposite corners of the rectangular region. The
default, if this option is not given, is the whole
image.
IMAGE FORMATS
The photo image code is structured to allow handlers for
additional image file formats to be added easily. The photo
image code maintains a list of these handlers. Handlers are
added to the list by registering them with a call to
Tk_CreatePhotoImageFormat. The standard Tk distribution
comes with handlers for PPM/PGM and GIF formats, which are
automatically registered on initialization.
When reading an image file or processing string data
specified with the -data configuration option, the photo
image code invokes each handler in turn until one is found
that claims to be able to read the data in the file or
string. Usually this will find the correct handler, but if
it doesn't, the user may give a format name with the -format
option to specify which handler to use. In fact the photo
image code will try those handlers whose names begin with
the string specified for the -format option (the comparison
is case-insensitive). For example, if the user specifies
-format gif, then a handler named GIF87 or GIF89 may be
invoked, but a handler named JPEG may not (assuming that
such handlers had been registered).
When writing image data to a file, the processing of the
-format option is slightly different: the string value given
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for the -format option must begin with the complete name of
the requested handler, and may contain additional
information following that, which the handler can use, for
example, to specify which variant to use of the formats
supported by the handler.
COLOR ALLOCATION
When a photo image is displayed in a window, the photo image
code allocates colors to use to display the image and
dithers the image, if necessary, to display a reasonable
approximation to the image using the colors that are
available. The colors are allocated as a color cube, that
is, the number of colors allocated is the product of the
number of shades of red, green and blue.
Normally, the number of colors allocated is chosen based on
the depth of the window. For example, in an 8-bit
PseudoColor window, the photo image code will attempt to
allocate seven shades of red, seven shades of green and four
shades of blue, for a total of 198 colors. In a 1-bit
StaticGray (monochrome) window, it will allocate two colors,
black and white. In a 24-bit DirectColor or TrueColor
window, it will allocate 256 shades each of red, green and
blue. Fortunately, because of the way that pixel values can
be combined in DirectColor and TrueColor windows, this only
requires 256 colors to be allocated. If not all of the
colors can be allocated, the photo image code reduces the
number of shades of each primary color and tries again.
The user can exercise some control over the number of colors
that a photo image uses with the -palette configuration
option. If this option is used, it specifies the maximum
number of shades of each primary color to try to allocate.
It can also be used to force the image to be displayed in
shades of gray, even on a color display, by giving a single
number rather than three numbers separated by slashes.
CREDITS
The photo image type was designed and implemented by Paul
Mackerras, based on his earlier photo widget and some
suggestions from John Ousterhout.
KEYWORDS
photo, image, color
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