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 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. from which this is derived ∗/
intuser;/∗ a place for the user to stick stuff ∗/
Lvisvis;/∗ 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->cover 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->cover is textured with c->ground.  It is legal for c->cover itself to be a ­Layer for recursive layering.  The rectangle ­r may have arbitrary overlap, including none, with the c->cover->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. 

SEE ALSO

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

Plan 9  —  January 03, 1993