Re: Writing a PangoLayoutLine on a line in cairo

[Behdad Esfahbod <behdad behdad org>]

Also check pango/examples/cairotwisted.c for some interesting stuff you
can do with pangocairo.

behdad

On Sat, 2007-02-03 at 21:48 +0200, Dov Grobgeld wrote:
> I'm trying to understand how to get the baseline information from a
> single line PangoLayout or equivalently a PangoLayoutLine.
> 
> More concretely I would like to translate the following postscript
> program into Pango/Cairo:
> 
>   %!
>   1.0 0 0 setrgbcolor
>   100 100 moveto
>   150 0 rlineto stroke
>   100 100 moveto
>   /Helvetica findfont 36 scalefont setfont
>   0 setgray
>   (yd) show
>   showpage
> 
> This programs draws a red line at position 100 100 from the lower
> corner left of the page and then writes "yd" on it. Note that the
> descender of the y descends below the y=100 line.
> 
> In pango_cairo_show_layout() on the other hand, the layout logical
> upper left corner is aligned at the current cairo point when drawing,
> and I could not find any PangoLayout call that gets the distance from
> the top of the logical rectangle to the baseline position. Is this a
> bug, or did I miss something?
> 
> Here is some pangocairo code that shows this alignment. So the
> question is how can I get the distance from the top of the PangoLayout
> to the baseline?
> 
> Regards,
> Dov
> 
> //======================================================================
> // Example that shows the relation between cairo coordinates and
> // Pango coordinates.
> //
> // Dov Grobgeld <dov grobgeld gmail com>
> // Sat 2007-02-03 21:43
> //
> // This program is in the public domain
> //
> // Compile with:
> //
> //    gcc `pkg-config --cflags --libs pangocairo` -o pango-bbox  pango-bbox.c
> //----------------------------------------------------------------------
> #include <math.h>
> #include <pango/pangocairo.h>
> 
> #define WIDTH 800
> #define HEIGHT 300
> 
> static void
> draw_text (cairo_t *cr,
>            const char *text)
> {
>   PangoLayout *layout;
>   PangoFontDescription *desc;
>   PangoRectangle ink_rect, logical_rect;
>   int width, height;
>   double x, y;
> 
>   // Center coordinates on the middle of the region we are drawing
>   cairo_translate (cr, WIDTH/2, HEIGHT/2);
> 
>   // Create a PangoLayout, set the font and text
>   layout = pango_cairo_create_layout (cr);
> 
>   pango_layout_set_markup (layout, text, -1);
>   desc = pango_font_description_from_string ("Serif 70");
>   pango_layout_set_font_description (layout, desc);
>   pango_font_description_free (desc);
> 
>   pango_layout_get_extents(layout,
>                            &ink_rect,
>                            &logical_rect);
>   x = -((double)logical_rect.width / PANGO_SCALE) / 2;
>   y = -((double)logical_rect.height / PANGO_SCALE) / 2;
> 
>   // Draw a green point at the current x,y position
>   cairo_set_source_rgba (cr, 0.0, 1.0, 0.0, 0.5); // green
>   cairo_arc(cr,
>             x, y,
>             5, 0, 2*M_PI);
>   cairo_fill(cr);
> 
>   // Draw the logical rectangle
>   cairo_set_source_rgba (cr, 0.0, 0.0, 1.0, 0.5); // blue
>   cairo_rectangle(cr,
>                   x+logical_rect.x/PANGO_SCALE,
>                   y+logical_rect.y/PANGO_SCALE,
>                   logical_rect.width/PANGO_SCALE,
>                   logical_rect.height/PANGO_SCALE);
>   cairo_stroke(cr);
> 
>   // Draw the text
>   cairo_move_to (cr, x, y);
>   cairo_set_source_rgb (cr, 0.0, 0.0, 0.0); // black
>   pango_cairo_show_layout (cr, layout);
> 
>   // Question: How can I draw a line at the baseline position?
> 
>   // free the layout object
>   g_object_unref (layout);
> }
> 
> int main (int argc, char **argv)
> {
>   cairo_t *cr;
>   const char *filename = "pango-bbox.png";
>   cairo_surface_t *surface;
> 
>   surface = cairo_image_surface_create (CAIRO_FORMAT_RGB24, WIDTH, HEIGHT);
> 
>   cr = cairo_create (surface);
> 
>   cairo_set_source_rgb (cr, 1.0, 1.0, 1.0);
>   cairo_paint (cr);
>   draw_text (cr,
>              "yd"
>              );
>   cairo_show_page (cr);
>   cairo_destroy (cr);
> 
>   cairo_surface_write_to_png(surface, filename);
> 
>   cairo_surface_destroy (surface);
> 
>   return 0;
> }
-- 
behdad
http://behdad.org/

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