NAME

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 (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 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 transparency set subcommand.

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 string should contain binary data or, for some formats, base64-encoded data (this is currently guaranteed to be supported for GIF images). 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 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 ...?

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 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:

imageName data ?option value(s) ...?

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

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 ?option value(s) ...?

Sets pixels in imageName 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 imageName (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:

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:

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 transparency subcommand ?arg arg ...?

Allows examination and manipulation of the transparency information in the photo image. Several subcommands are available:

imageName write filename ?option value(s) ...?

Writes image data from imageName 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 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 does not, 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.

EXAMPLE

Load an image from a file and tile it to the size of a window, which is useful for producing a tiled background:

# These lines should be called once
image create photo untiled -file "theFile.ppm"
image create photo tiled
# These lines should be called whenever .someWidget changes
# size; a <Configure> binding is useful here
set width  [winfo width .someWidget]
set height [winfo height .someWidget]
tiled copy untiled -to 0 0 $width $height -shrink

SEE ALSO

image(n)

KEYWORDS

photo, image, color