38.16.2 Image Descriptors

An image description is a list of the form (image . props), where props is a property list containing alternating keyword symbols (symbols whose names start with a colon) and their values. You can use any Lisp object as a property, but the only properties that have any special meaning are certain symbols, all of them keywords.

Every image descriptor must contain the property :type type to specify the format of the image. The value of type should be an image type symbol; for example, xpm for an image in XPM format.

Here is a list of other properties that are meaningful for all image types:

:file file

The :file property says to load the image from file file. If file is not an absolute file name, it is expanded in data-directory.

:data data

The :data property says the actual contents of the image. Each image must use either :data or :file, but not both. For most image types, the value of the :data property should be a string containing the image data; we recommend using a unibyte string.

Before using :data, look for further information in the section below describing the specific image format. For some image types, :data may not be supported; for some, it allows other data types; for some, :data alone is not enough, so you need to use other image properties along with :data.

:margin margin

The :margin property specifies how many pixels to add as an extra margin around the image. The value, margin, must be a non-negative number, or a pair (x . y) of such numbers. If it is a pair, x specifies how many pixels to add horizontally, and y specifies how many pixels to add vertically. If :margin is not specified, the default is zero.

:ascent ascent

The :ascent property specifies the amount of the image's height to use for its ascent—that is, the part above the baseline. The value, ascent, must be a number in the range 0 to 100, or the symbol center.

If ascent is a number, that percentage of the image's height is used for its ascent.

If ascent is center, the image is vertically centered around a centerline which would be the vertical centerline of text drawn at the position of the image, in the manner specified by the text properties and overlays that apply to the image.

If this property is omitted, it defaults to 50.

:relief relief

The :relief property, if non-nil, adds a shadow rectangle around the image. The value, relief, specifies the width of the shadow lines, in pixels. If relief is negative, shadows are drawn so that the image appears as a pressed button; otherwise, it appears as an unpressed button.

:conversion algorithm

The :conversion property, if non-nil, specifies a conversion algorithm that should be applied to the image before it is displayed; the value, algorithm, specifies which algorithm.

laplace

emboss

Specifies the Laplace edge detection algorithm, which blurs out small differences in color while highlighting larger differences. People sometimes consider this useful for displaying the image for a “disabled” button.

(edge-detection :matrix matrix :color-adjust adjust)

Specifies a general edge-detection algorithm. matrix must be either a nine-element list or a nine-element vector of numbers. A pixel at position x/y in the transformed image is computed from original pixels around that position. matrix specifies, for each pixel in the neighborhood of x/y, a factor with which that pixel will influence the transformed pixel; element 0 specifies the factor for the pixel at x-1/y-1, element 1 the factor for the pixel at x/y-1 etc., as shown below:

                 (x-1/y-1  x/y-1  x+1/y-1
                  x-1/y    x/y    x+1/y
                  x-1/y+1  x/y+1  x+1/y+1)

The resulting pixel is computed from the color intensity of the color resulting from summing up the RGB values of surrounding pixels, multiplied by the specified factors, and dividing that sum by the sum of the factors' absolute values.

Laplace edge-detection currently uses a matrix of

                 (1  0  0
                  0  0  0
                  9  9 -1)

Emboss edge-detection uses a matrix of

                 ( 2 -1  0
                  -1  0  1
                   0  1 -2)

disabled

Specifies transforming the image so that it looks “disabled.”

:mask mask

If mask is heuristic or (heuristic bg), build a clipping mask for the image, so that the background of a frame is visible behind the image. If bg is not specified, or if bg is t, determine the background color of the image by looking at the four corners of the image, assuming the most frequently occurring color from the corners is the background color of the image. Otherwise, bg must be a list (red green blue) specifying the color to assume for the background of the image.

If mask is nil, remove a mask from the image, if it has one. Images in some formats include a mask which can be removed by specifying :mask nil.

:pointer shape

This specifies the pointer shape when the mouse pointer is over this image. See Pointer Shape, for available pointer shapes.

:map map

This associates an image map of hot spots with this image.

An image map is an alist where each element has the format (area id plist). An area is specified as either a rectangle, a circle, or a polygon.

A rectangle is a cons (rect . ((x0 . y0) . (x1 . y1))) which specifies the pixel coordinates of the upper left and bottom right corners of the rectangle area.

A circle is a cons (circle . ((x0 . y0) . r)) which specifies the center and the radius of the circle; r may be a float or integer.

A polygon is a cons (poly . [x0 y0 x1 y1 ...]) where each pair in the vector describes one corner in the polygon.

When the mouse pointer lies on a hot-spot area of an image, the plist of that hot-spot is consulted; if it contains a help-echo property, that defines a tool-tip for the hot-spot, and if it contains a pointer property, that defines the shape of the mouse cursor when it is on the hot-spot. See Pointer Shape, for available pointer shapes.

When you click the mouse when the mouse pointer is over a hot-spot, an event is composed by combining the id of the hot-spot with the mouse event; for instance, [area4 mouse-1] if the hot-spot's id is area4.