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39.17.2 Image Descriptors

An image descriptor is a list which specifies the underlying data for an image, and how to display it. It is typically used as the value of a display overlay or text property (see Other Display Specs); but See Showing Images, for convenient helper functions to insert images into buffers.

Each image descriptor has the form (image . props), where props is a property list of alternating keyword symbols and values, including at least the pair :type type that specifies the image type.

The following is a list of properties that are meaningful for all image types (there are also properties which are meaningful only for certain image types, as documented in the following subsections):

:type type​

The image type. See Image Formats. Every image descriptor must include this property.

:file file​

This says to load the image from file file. If file is not an absolute file name, it is expanded relative to the images subdirectory of data-directory, and failing that, relative to the directories listed by x-bitmap-file-path (see Face Attributes).

:data data​

This specifies the raw image data. Each image descriptor must have either :data or :file, but not both.

For most image types, the value of a :data property should be a string containing the image data. Some image types do not support :data; for some others, :data alone is not enough, so you need to use other image properties along with :data. See the following subsections for details.

:margin margin​

This 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​

This 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​

This 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.

:width width, :height height​

The :width and :height keywords are used for scaling the image. If only one of them is specified, the other one will be calculated so as to preserve the aspect ratio. If both are specified, aspect ratio may not be preserved.

:max-width max-width, :max-height max-height​

The :max-width and :max-height keywords are used for scaling if the size of the image exceeds these values. If :width is set, it will have precedence over max-width, and if :height is set, it will have precedence over max-height, but you can otherwise mix these keywords as you wish.

If both :max-width and :height are specified, but :width is not, preserving the aspect ratio might require that width exceeds :max-width. If this happens, scaling will use a smaller value for the height so as to preserve the aspect ratio while not exceeding :max-width. Similarly when both :max-height and :width are specified, but :height is not. For example, if you have a 200x100 image and specify that :width should be 400 and :max-height should be 150, you’ll end up with an image that is 300x150: Preserving the aspect ratio and not exceeding the “max" setting. This combination of parameters is a useful way of saying “display this image as large as possible, but no larger than the available display area".

:scale scale​

This should be a number, where values higher than 1 means to increase the size, and lower means to decrease the size, by multiplying both the width and height. For instance, a value of 0.25 will make the image a quarter size of what it originally was. If the scaling makes the image larger than specified by :max-width or :max-height, the resulting size will not exceed those two values. If both :scale and :height/:width are specified, the height/width will be adjusted by the specified scaling factor.

:rotation angle​

Specifies a rotation angle in degrees. Only multiples of 90 degrees are supported, unless the image type is imagemagick. Positive values rotate clockwise, negative values counter-clockwise. Rotation is performed after scaling and cropping.

:index frame​

See Multi-Frame Images.

:conversion algorithm​

This 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
   0  0 -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.

function image-mask-p spec \&optional frame​

This function returns t if image spec has a mask bitmap. frame is the frame on which the image will be displayed. frame nil or omitted means to use the selected frame (see Input Focus).

function image-transforms-p \&optional frame​

This function returns non-nil if frame supports image scaling and rotation. frame nil or omitted means to use the selected frame (see Input Focus). The returned list includes symbols that indicate which image transform operations are supported:

scale​

Image scaling is supported by frame via the :scale, :width, :height, :max-width, and :max-height properties.

rotate90​

Image rotation is supported by frame if the rotation angle is an integral multiple of 90 degrees.

If image transforms are not supported, :rotation, :crop, :width, :height, :scale, :max-width and :max-height will only be usable through ImageMagick, if available (see ImageMagick Images).