NAME¶

Tk::Photo - Full-color images

SYNOPSIS¶

    $widget->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 (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.

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 => 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 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 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 ?,option value(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:

$image->data(?option value(s), ...?)

Returns image data in the form of a string. The following options may be specified:

$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=> x1 y1 ?x2 y2??)

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:

$image->read(filename ?,option value(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:

$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->write(filename ?,option value(s), ...?)

Writes image data from $image to a file named filename. The following options may be specified:

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.

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.

SEE ALSO¶

Tk::Bitmap Tk::Image Tk::Pixmap

KEYWORDS¶

photo, image, color