On 12/31/21 2:21 PM, Albrecht Schlosser wrote:

I don't know how the coordinate systems are (or should be) translated, but if I change only your two drawing code lines to

cairo_move_to(cr, 0.0, 0.0); cairo_line_to(cr, 1.0, 1.0); // draw diagonal "\"

cairo_move_to(cr, 0.0, 1.0); cairo_line_to(cr, 1.0, 0.0); // draw diagonal "/"

the "green X" works for me.

   Thanks, I get that too.

   Though still not sure how to achieve FLTK drawing coordinates.

// Cairo rendering cb called during Fl_Cairo_Window::draw()
static void my_cairo_draw_cb(Fl_Cairo_Window *window, cairo_t *cr) {
    const double xmax = (window->w() - 1);
    const double ymax = (window->h() - 1);

    // Draw green "X"
    //     Draws an X to four corners of resizable window.
    //
    cairo_set_antialias(cr, CAIRO_ANTIALIAS_BEST);
    cairo_set_line_width(cr, 1.00);                               // line width for drawing XXX what coord space is this?!
    cairo_set_source_rgb(cr, 0.0, 1.0, 0.0);                      // green
    cairo_move_to(cr, 0.0, 0.0);  cairo_line_to(cr, xmax, ymax);  // draw diagonal "\"
    cairo_move_to(cr, 0.0, ymax); cairo_line_to(cr, xmax, 0.0);   // draw diagonal "/"
    cairo_stroke(cr);                                             // stroke the lines
}

int main(int argc, char **argv) {
  Fl_Cairo_Window window(300, 300, "Cairo Draw 'X'");
  window.color(FL_BLACK);
  window.resizable(&window);
  window.set_draw_cb(my_cairo_draw_cb);
  window.show(argc, argv);
  return Fl::run();
}