ffi helper demo: cairo text extents

Matt Wette

2017-09-13 22:08:53 UTC

Post by Matt Wette
I have a small FFI Helper demo using cairo.
I used my ffi-helper, all written in Guile Scheme, to generate a (ffi cairo) module, in Scheme, from cairo.h.
Now compare the program below to the demo at
https://www.cairographics.org/samples/text_extents/
1) The FFI Helper uses the bytestructures package to deal with C structs.
The bs-ref macro defined below is used to save typing.
$1 = #<cairo_text_extents_t 0x1079aaa70>
$2 = #<bytestructure 0x108709930>
3) The procecure fh-object-val gets the object value (often a bytestructure) from the FFI-helper type.
4) You can use quoted symbols where enums are used in C code.
5) You can use strings in Scheme and the unwrapper within (ffi cairo) will apply string->pointer for you.
6) For reference, cairo.h contains
typedef struct {
double ascent;
double descent;
double height;
double max_x_advance;
double max_y_advance;
} cairo_font_extents_t;
7) Below is a full script. If saved as demo.scm, `guile demo.scm' will generate the expected .png file.
(use-modules (ffi cairo))
(use-modules (system ffi-help-rt))
(use-modules (bytestructures guile))
(define-syntax-rule (bs-ref bs ix ...)
(bytestructure-ref bs ix ...))
(define pngfile "te.png")
(define srf (cairo_image_surface_create 'CAIRO_FORMAT_ARGB32 600 400))
(define cr (cairo_create srf))
;; -------------------------------------------------------
;; Go look at
;; https://www.cairographics.org/samples/text_extents/
;; and compare to the following.
(define extents (make-cairo_text_extents_t))
(define utf8 "cairo")
(define x)
(define y)
(cairo_select_font_face cr "Sans"
'CAIRO_FONT_SLANT_NORMAL
'CAIRO_FONT_WEIGHT_NORMAL)
(cairo_set_font_size cr 100.0)
(cairo_text_extents cr utf8 (pointer-to extents))
(define te (fh-object-val extents))
(set! x 25.0)
(set! y 150.0)
(cairo_move_to cr x y)
(cairo_show_text cr utf8)
;; draw helping lines
(cairo_set_source_rgba cr 1 0.2 0.2 0.6)
(cairo_set_line_width cr 6.0)
(cairo_arc cr x y 10.0 0 (* 2 3.15))
(cairo_fill cr)
(cairo_move_to cr x y)
(cairo_rel_line_to cr 0 (- (bs-ref te 'height)))
(cairo_rel_line_to cr (bs-ref te 'width) 0)
(cairo_rel_line_to cr (bs-ref te 'x_bearing) (- (bs-ref te 'y_bearing)))
(cairo_stroke cr)
;; -------------------------------------------------------
(cairo_surface_write_to_png srf pngfile)
(cairo_destroy cr)
(cairo_surface_destroy srf)

I can't believe I goofed the struct. Here is the applicable struct from cairo.h:

typedef struct {
double x_bearing;
double y_bearing;
double width;
double height;
double x_advance;
double y_advance;
} cairo_text_extents_t;