DESCRIPTION

The layer library extends the functionality of the bitmap graphics library
(see
graphics(2)) to overlapping independent rectangular windows, or
layers, on a single bitmap, typically the screen.
The entry points
bitblt, point, segment, string, subfontstring, and
texture are overloaded in the layer library to apply these routines equally
to bitmaps and layers.
Other than
lcstring, which is rarely needed,
there are no special entry points for drawing on layers.


The data structures associated with the main type,
Layer, are defined in
<layer.h>:

typedef struct Layer Layer;

typedef struct Cover Cover;

typedef enum Lvis {

	Visible,

	Obscured,

	Invisible,

}Lvis;



struct Layer {

	Bitmap;		/* Bitmap.cache!=0 ==> layer */

	Layer	*next;	/* next layer from front to back */

	Cover	*cover;	/* layer etc. that derived this one */

	int	user;	/* a place for the user to stick stuff */

	Lvis	vis;	/* visibility state */

};



struct Cover {

	Layer	*layer;	/* layer on which these are painted */

	Layer	*front;	/* first sublayer */

	Bitmap	*ground;	/* background texture */

};

Layers and Bitmaps are distinguished by the cache element of their structures: cache is non-zero in a Layer. The layer library's versions of the graphics routines listed above use cache to decide how to implement their operations. These functions operate on type Bitmap* but because Bitmap is included in Layer, the C compiler will permit passing a Layer to these routines. The routines promote the type to Layer* if they see cache is non-zero. (Note that these actions apply only in the layer library; although cache is defined in Bitmaps, the standard graphics library does not support layers.)

Lalloc allocates a new Layer to occupy Rectangle r in a Bitmap. The argument Cover c connects the set of Layers to a covering Bitmap. Before the first call to lalloc, c should be allocated and initialized so c->layer is the Bitmap on which the Layers will be drawn, c->front is zero, c->ground is a background texture to fill the interstices between Layers, and c->layer is textured with c->ground It is legal for c->layer itself to be a Layer for recursive layering. The rectangle r may have arbitrary overlap, including none, with c->layer->r After calling lalloc, the new Layer is fully visible (as far as geometry permits) on the covering Bitmap and is cleared to all zeros.

Lfree frees the Layer l and restores the contents of its covering Bitmap.

Ltofront makes l fully visible within its covering Bitmap. Ltoback pushes l behind any other Layers on the same covering Bitmap. Neither function changes the x-y location of the Layer.

Lcstring is peculiar to programs, such as 8½(1), that multiplex client access to the display. It acts as a feed-through for the 's' message generated by string (see bit(3)). B is the bitmap (or layer) and height is the height of the font in which the string is to be drawn. Widths is an array of character widths, indexed by font cache position. Msg is a pointer to the string message; it contains the header and n cache indices.

SOURCE

/sys/src/liblayer

SEE ALSO

graphics(2), bitblt(2), cachechars(2), bit(3)