Figure Overlays

The OVERLAY element is used to overlay images on top of a base figure. Figure overlays provide for more effective use of caching as small changes to a figure in a subsequent document incur only the penalty of downloading the overlays and not the larger base figure, as the latter is already in the cache. The overlay can be offset from the top left corner of the base image.

Permitted Attributes

SRC

Specifies the overlay image as a URI. This attribute may appear together with the MD attribute.

MD

Specifies a message digest or cryptographic checksum for the associated graphic specified by the SRC attribute. It is used when you want to be sure that a linked object is indeed the same one that the author intended, and hasn't been modified in any way. For instance, MD="md5:jV2OfH+nnXHU8bnkPAad/mSQlTDZ", which specifies an MD5 checksum encoded as a base64 character string. The MD attribute is generally allowed for all elements which support URI based links.

UNITS

Specifies the choice of units for width and height: units=pixels (the default) specifies pixels, while units=en specifies en units (a typographical unit equal to half the point size).

X

The X offset from the top left corner of the base image. X increases to the right, and is given in pixels or en units (according to the value of the UNITS attribute).

Y

The Y offset from the top left corner of the base image. Y increases downwards, and is given in pixels or en units (according to the value of the UNITS attribute).

WIDTH

Specifies the desired width in pixels or en units (according to the value of the UNITS attribute). User agents may scale the figure image to match this width.

HEIGHT

Specifies the desired height in pixels or en units (according to the value of the UNITS attribute). User agents may scale the figure image to match this height.

IMAGEMAP

Specifies a URI for processing image clicks and drags.