term% cat index.txt LAYER(2) System Calls Manual LAYER(2)
NAME
lalloc, lfree, ltofront, ltoback, lcstring - graphics layers
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <libg.h>
#include <layer.h>
Layer* lalloc(Cover *c, Rectangle r)
void lfree(Layer *l)
void ltofront(Layer *l)
void ltoback(Layer *l)
void lcstring(Bitmap *b, int height, uchar *widths, uchar *msg,
int n)
DESCRIPTION
The layer library extends the functionality of the bitmap graphics li‐
brary (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 over‐
loaded 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 imple‐
ment their operations. These functions operate on type Bitmap* but be‐
cause 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. Be‐
fore 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 per‐
mits) on the covering Bitmap and is cleared to all zeros.
Lfree frees the Layer l and restores the contents of its covering Bit‐
map.
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)
LAYER(2)