Tk::Photo - Full-color images

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.

Creating Photos

Photos are created using the Photo method. Photo supports the following options:

-data => string
Specifies the contents of the image as a string. The string can contain base64 encoded data or
binary data. 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 => namename 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 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.

Credits

       The  photo  image  type was designed and implemented by Paul Mackerras, based on his earlier photo widget
       and some suggestions from John Ousterhout.

Description

       A photo is an image whose pixels can display any color or be transparent.  A photo image is stored
       internally in full color (32 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, XBM, XPM, BMP, JPEG, PNG 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 or where it has been set transparent by the
       transparencySet subcommand.

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 XBM, XPM, BMP, JPEG, PNG and PPM/PGM 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 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. Note that not all image handlers may support
writing transparency data to a file, even where the target image format does.

Image Methods

When a photo image is created, Tk also creates a new object. This object supports the configure and cget
methods described in Tk::options which can be used to enquire and modify the options described above.

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 addition methods are available for photo images:

$image->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.

$image->copy(sourceImage ?,optionvalue(s)...?)
Copies a region from the image called $sourceImage (which must be a photo image) to the image called
$image, possibly with pixel zooming and/or subsampling. If no options are specified, this method
copies the whole of $sourceImage into $image, starting at coordinates (0,0) in $image. 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 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.

-compositingrule => rule
Specifies how transparent pixels in the source image are combined with the destination image.
When a compositing rule of overlay is set, the old contents of the destination image are
visible, as if the source image were printed on a piece of transparent film and placed over
the top of the destination. When a compositing rule of set is set, the old contents of the
destination image are discarded and the source image is used as-is. The default compositing
rule is overlay.

$image->data(?optionvalue(s),...?)
Returns image data in the form of a string. The following options may be specified:

-background => color
If the color is specified, the data will not contain any transparency information. In all
transparent pixels the color will be replaced by the specified color.

-format => format-name
Specifies the name of the image file format handler to be used. Specifically, this method
searches for the first handler whose name matches a initial substring of format-name and
which has the capability to read this image data. If this option is not given, this method
uses the first handler that has the capability to read the image data.

-from => x1,y1,?,x2,y2?
Specifies a rectangular region of $image to be returned. If only x1 and y1 are specified,
the region extends from (x1,y1) to the bottom-right corner of $image. If all four
coordinates are given, they specify diagonally opposite corners of the rectangular region,
including x1,y1 and excluding x2,y2. The default, if this option is not given, is the whole
image.

-grayscale
If this options is specified, the data will not contain color information. All pixel data
will be transformed into grayscale.

$image->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.

$image->put(data ?,-format=>format-name? ?,-to=> x1y1?x2y2??)
Sets pixels in $image to the data specified in data. This command first searches the list of image
file format handlers for a handler that can interpret the data in data, and then reads the image
encoded within into $image (the destination image). If data does not match any known format, an
attempt to interpret it as a (top-to-bottom) list of scan-lines is made, with each scan-line being a
(left-to-right) list of pixel colors (see Tk_GetColor for a description of valid colors.) Every
scan-line must be of the same length. Note that when data is a single color name, you are
instructing Tk to fill a rectangular region with that color. The following options may be specified:

-format =>format-name
Specifies the format of the image data in data. 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.

-to =>x,y ?,x2,y2?
Specifies the coordinates of the top-left corner (x1,y1) of the region of $image into which data
from filename are to be read. The default is (0,0). If x2,y2 is given and data is not large
enough to cover the rectangle specified by this option, the image data extracted will be tiled so
it covers the entire destination rectangle. Note that if data specifies a single color value,
then a region extending to the bottom-right corner represented by (x2,y2) will be filled with
that color.

$image->read(filename ?,optionvalue(s),...?)
Reads image data from the file named filename into the image. This method 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 $image (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 is specified, the size of $image 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 $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.

-to => x,y
Specifies the coordinates of the top-left corner of the region of $image into which data from
filename are to be read. The default is (0,0).

$image->redither
The dithering algorithm used in displaying photo images propagates quantization errors from one pixel
to its neighbors. If the image data for $image 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 method can
be used to recalculate the dithered image in each window where the image is displayed.

$image->transparency(subcommand,?arg,arg...?);
Allows examination and manipulation of the transparency information in the photo image. Several
subcommands are available:

$image->transparencyGet(x,y);
Returns a boolean indicating if the pixel at (x,y) is transparent.

$image->transparencySet(x,y,boolean);
Makes the pixel at (x,y) transparent if boolean is true, and makes that pixel opaque otherwise.

$image->write(filename ?,optionvalue(s),...?)
Writes image data from $image to a file named filename. The following options may be specified:

-background => color
If the color is specified, the data will not contain any transparency information. In all
transparent pixels the color will be replaced by the specified color.

-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 $image 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 $image. 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.