photo - Full-color images
SYNOPSIS
image create photo ?name? ?options?
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 the raw PPM
file format is supported; 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 from a file.
- -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
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
- 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 decimation. 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 source
region (after decimation 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.
- -decimate 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
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 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 a handler for raw PPM files, which
is 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
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