Re: pango_layout_set_width() versus measured widths

[Behdad Esfahbod <behdad behdad org>]

Hi Ben,

This is a bug in Pango.  When breaking lines, pango is still considering the 
width of the space character.  It's only after line breaking that pango 
changes the width of the space to zero.  I'm working on fixing that now.

Cheers,
behdad

On 12/22/2009 11:13 PM, Ben Pfaff wrote:
> In working with Pango I found an oddity that I do not understand.
> It may be a bug, or it might just be my misunderstanding.
>
> As part of a table layout procedure for printing, my code wishes
> to find out the width of the longest word in a Pango layout, so
> it sets the width of the layout to 1, turns on word wrapping, and
> measures the width of the layout, something like this:
>
>          pango_layout_set_width (layout, 1);
>          pango_layout_set_wrap (layout, PANGO_WRAP_WORD);
>          pango_layout_get_size (layout,&width, NULL);
>
> Later on, the width of the table column might have been adjusted
> upward or downward for other reasons, so it renders it with
> PANGO_WRAP_WORD_CHAR to prevent overflowing the column
> boundary.  I was surprised to discover that, when the layout
> width is set to the same value returned by
> pango_layout_get_size(), this caused some words to wrap anyhow.
>
> I'm enclosing a simple example program.  You can invoke it to
> measure the size of rendering "A string of text" with a width of
> 1 and word-wrapping, as above, e.g. showing input, output, and
> an ASCII rendering of the produced foo.pdf:
>
>          # ./a.out "A string of text" 1 word
>          size: width=47032
>          extents: ink width=45416, logical width=47032
>          pixel extents: ink width=45, logical width=46
>
>          +------+
>          |A     |
>          |string|
>          |of    |
>          |text  |
>          +------+
>
> Then giving the exact width reported by this run, plus word-char,
> yields:
>
>          # ./a.out "A string of text" 47032 word-char
>          size: width=36632
>          extents: ink width=35112, logical width=36632
>          pixel extents: ink width=35, logical width=36
>
>          +-----+
>          |A    |
>          |strin|
>          |g of |
>          |text |
>          +-----+
>
> To get word-char to produce the results that I expect, I have to
> add 5208, which is the reported width of " ":
>
>          # ./a.out "A string of text" 52240 word-char
>          size: width=47032
>          extents: ink width=45416, logical width=47032
>          pixel extents: ink width=45, logical width=46
>
>          +------+
>          |A     |
>          |string|
>          |of    |
>          |text  |
>          +------+
>
> Any guidance would be appreciated.  I can always add the width of
> a space myself, but I don't understand why it is necessary.
>
> Here is my test program:
>
> #include<pango/pangocairo.h>
> #include<cairo/cairo-pdf.h>
> #include<cairo/cairo.h>
> #include<stdlib.h>
> #include<string.h>
>
> int
> main (int argc, char **argv)
> {
>    cairo_t *cr;
>    char *filename;
>    cairo_status_t status;
>    cairo_surface_t *surface;
>    PangoLayout *layout;
>    PangoFontDescription *desc;
>    PangoRectangle ink, logical;
>    int width;
>
>    if (argc != 4)
>      {
>        g_printerr ("Usage: pangocairo STRING WIDTH WRAP_MODE\n");
>        return 1;
>      }
>
>    /* Create surface and clear to all-white. */
>    surface = cairo_pdf_surface_create ("foo.pdf", 200, 200);
>    cr = cairo_create (surface);
>    cairo_set_source_rgb (cr, 1.0, 1.0, 1.0);
>    cairo_paint (cr);
>
>    /* Create Pango layout. */
>    layout = pango_cairo_create_layout (cr);
>    desc = pango_font_description_from_string ("Sans 12");
>    pango_layout_set_font_description (layout, desc);
>    pango_font_description_free (desc);
>    pango_layout_set_text (layout, argv[1], -1);
>
>    /* Set Pango options according to command line. */
>    pango_layout_set_width (layout, atoi (argv[2]));
>    if (!strcmp (argv[3], "word"))
>      pango_layout_set_wrap (layout, PANGO_WRAP_WORD);
>    else if (!strcmp (argv[3], "char"))
>      pango_layout_set_wrap (layout, PANGO_WRAP_CHAR);
>    else if (!strcmp (argv[3], "word-char"))
>      pango_layout_set_wrap (layout, PANGO_WRAP_WORD_CHAR);
>    else
>      {
>        g_printerr ("WRAP_MODE must be 'word' or 'char' or 'word-char'");
>        return 1;
>      }
>
>    /* Draw layout. */
>    cairo_set_source_rgb (cr, 0.0, 0.0, 0.0);
>    cairo_move_to (cr, 0, 0);
>    pango_cairo_show_layout (cr, layout);
>
>    /* Print various sizes. */
>    pango_layout_get_size (layout,&width, NULL);
>    printf ("size: width=%d\n", width);
>
>    pango_layout_get_extents (layout,&ink,&logical);
>    printf ("extents: ink width=%d, logical width=%d\n",
>            ink.width, logical.width);
>
>    pango_layout_get_pixel_extents (layout,&ink,&logical);
>    printf ("pixel extents: ink width=%d, logical width=%d\n",
>            ink.width, logical.width);
>
>    /* Clean up. */
>    g_object_unref (layout);
>
>    cairo_destroy (cr);
>
>    cairo_surface_finish (surface);
>    status = cairo_surface_status (surface);
>    cairo_surface_destroy (surface);
>
>    if (status != CAIRO_STATUS_SUCCESS)
>      {
>        g_printerr ("Could not save pdf to '%s'\n", filename);
>        return 1;
>      }
>
>    return 0;
> }
> --
> Ben Pfaff
> http://benpfaff.org
>
> _______________________________________________
> gtk-list mailing list
> gtk-list gnome org
> http://mail.gnome.org/mailman/listinfo/gtk-list