Re: ffi helper demo: cairo text extents

[Matt Wette]

> On Sep 13, 2017, at 3:04 PM, Matt Wette <address@hidden> wrote:
> 
> 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/
> 
> Some notes:
> 1) The FFI Helper uses the bytestructures package to deal with C structs.
>   The bs-ref macro defined below is used to save typing.
> 2) The FFI Helper uses Guile structs to wrap types:
>    scheme@(guile-user)> extents
>    $1 = #<cairo_text_extents_t 0x1079aaa70>
>    scheme@(guile-user)> te
>    $2 = #<bytestructure 0x108709930>
>    scheme@(guile-user)> 
> 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;