Xkb provides a number of convenience functions to help use a keyboard geometry. These include functions to return the bounding box of a shape's top surface and to update the bounding box of a shape row or section.
A shape is made up of a number of outlines. Each outline is a polygon made up of a number of points. The bounding box of a shape is a rectangle that contains all the outlines of that shape.
XkbComputeShapeTop returns a BoundsRec that contains two x and y coordinates. These coordinates describe the corners of a rectangle that contains the outline that describes the top surface of the shape. The top surface is defined to be the approximating outline if the approx field of shape is not NULL. If approx is NULL, the top surface is defined as the last outline in the shape's array of outlines. XkbComputeShapeTop returns False if shape is NULL or if there are no outlines for the shape; otherwise, it returns True.
typedef struct _XkbBounds {
short x1,y1; /* upper left corner of the bounds, in mm/10 */
short x2,y2; /* lower right corner of the bounds, in mm/10 */
} XkbBoundsRec, *XkbBoundsPtr;