Shape Attribute of Pango Increases Line Height

[Koga Osamu <osak 63 gmail com>]

Hello,

I am trying to render messages from SNS with Pango and Cairo. Normal texts are rendered fine, but when I use shape attributes of Pango to render custom emoji of Slack and/or Mastodon, the line that contains the shape attribute is extended by some pixels even if the shape is smaller than original line height.

How do I get rid of extra height? I wonder if I'm missing something or it's actually a bug of Pango.

I'm pasting a source code to reproduce this issue at end of this mail and attaching a png file generated by this program.

Library versions are the latest available version with Ubuntu 18.04 LTS.

% pkg-config --modversion pango
1.40.14
% pkg-config --modversion cairo
1.15.10

Thanks,

Osamu

#include <math.h>
#include <pango/pangocairo.h>

#define FONT "Sans Bold 11"

void render_shape(cairo_t *cr, PangoAttrShape *attr, gboolean do_path, gpointer data) {
    PangoRectangle rect = attr->ink_rect;
    cairo_translate(cr, rect.x / PANGO_SCALE, rect.y / PANGO_SCALE);
    cairo_rectangle(cr, 0, 0, rect.width / PANGO_SCALE, rect.height / PANGO_SCALE);
    cairo_set_source_rgb(cr, 1, 0, 0);
    cairo_fill(cr);
}

void draw_text (cairo_t *cr) {
    PangoLayout *layout;
    PangoFontDescription *desc;

    layout = pango_cairo_create_layout (cr);

    // Setup base text
    pango_layout_set_text (layout, "Text\nText\nText", -1);
    pango_layout_set_wrap(layout, PANGO_WRAP_CHAR);
    pango_layout_set_width(layout, 100 * PANGO_SCALE);
    desc = pango_font_description_from_string (FONT);
    pango_layout_set_font_description (layout, desc);
    pango_font_description_free (desc);

    // Setup emoji
    PangoRectangle ink_rect = {
            .x = 0,
            .y = 0,
            .width = 11 * PANGO_SCALE,
            .height = 11 * PANGO_SCALE
    };
    PangoRectangle logical_rect = {
            .x = 0,
            .y = 0,
            .width = 11 * PANGO_SCALE,
            .height = 11 * PANGO_SCALE
    };
    PangoAttribute *emoji = pango_attr_shape_new(&ink_rect, &logical_rect);
    emoji->start_index = 0;
    emoji->end_index = 1;

    // Highlight the first line to show its height
    PangoAttribute *first_line_bg = pango_attr_background_new(0, 0xffff, 0xffff);
    first_line_bg->start_index = 1;
    first_line_bg->end_index = 4;

    // Highlight the second line to show its height
    PangoAttribute *second_line_bg = pango_attr_background_new(0xffff, 0xffff, 0);
    second_line_bg->start_index = 5;
    second_line_bg->end_index = 9;

    // Setup attributes
    PangoAttrList *attr_list = pango_attr_list_new();
    pango_attr_list_insert(attr_list, emoji);
    pango_attr_list_insert(attr_list, first_line_bg);
    pango_attr_list_insert(attr_list, second_line_bg);
    pango_layout_set_attributes(layout, attr_list);
    pango_cairo_context_set_shape_renderer(pango_layout_get_context(layout), render_shape, NULL, NULL);

    // Render
    cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
    cairo_move_to(cr, 0.0, 0.0);
    pango_cairo_show_layout(cr, layout);

    // Cleanup
    g_object_unref (layout);
    pango_attr_list_unref(attr_list);
}

int main (int argc, char **argv) {
    cairo_t *cr;
    char *filename;
    cairo_status_t status;
    cairo_surface_t *surface;

    if (argc != 2) {
        g_printerr ("Usage: cairosimple OUTPUT_FILENAME\n");
        return 1;
    }

    filename = argv[1];

    surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 100, 100);
    cr = cairo_create (surface);

    cairo_set_source_rgb (cr, 1.0, 1.0, 1.0);
    cairo_paint (cr);
    draw_text (cr);
    cairo_destroy (cr);

    status = cairo_surface_write_to_png (surface, filename);
    cairo_surface_destroy (surface);

    if (status != CAIRO_STATUS_SUCCESS) {
        g_printerr ("Could not save png to '%s'\n", filename);
        return 1;
    }

    return 0;
}

Attachment: a.png
Description: PNG image