Re: GTK+2: gdk_draw_arc() -- changing arc colour

[Olexiy Avramchenko <ath beast stu cn ua>]

Chris Martin wrote:

> In the example shown in GTK API/Miscellaneous/GtkDrawingArea it seems
> impossible to change the colour the arc is drawn in.
>
> Has anybody a solution to this?
>
> Two examples of code tried -- the first is the "simple" method, the
> second a more devious one: seeing that gdk_gc_new() called
> gdk_gc_new_with_values() it seemed possible that supplying it with
> values might work...
It will not work, cause you are not specifying the pixel's value. 
gdk_color_parse
will fill the *red*,*green*,*blue* members of the GdkColor structure, 
not the
*pixel* field. You should call *gdk_colormap_alloc_color()* for this (and
*gdk_colormap_free_colors()* for freeing the color).

> Example 1:
>
> gboolean
> expose_event_callback(GtkWidget * widget, GdkEventExpose * event,
>                       gpointer data)
> {
>  GdkGC *gc;
>  GdkColor colour;
>
>  memset(&colour, 0, sizeof(colour));
>  if (gdk_color_parse(colour_str, &colour)) {
>    g_print("gdk_color_parse(%s) succeeds.\n", colour_str);
>    g_print("colour: pixel %lu, red 0x%04x, green 0x%04x, blue 0x%04x.\n",
>             colour.pixel, colour.red, colour.green, colour.blue);
>    g_print("colour: pixel %lu, red %u, green %u, blue %u.\n",
>             colour.pixel, colour.red>>8, colour.green>>8, colour.blue>>8);
>  } else
>    g_print("gdk_color_parse(%s) fails.\n", colour_str);
>
>  gc = gdk_gc_new(widget->window);
       gdk_colormap_color_alloc(gtk_widget_get_colormap(widget), 
&colour, FALSE,FALSE);

>  gdk_gc_set_foreground(gc, &colour);
>
>  gdk_draw_arc(widget->window, gc, TRUE,
>                0, 0, widget->allocation.width, widget->allocation.height,
>                0, 64 * 360);
>  return TRUE;
> }
>
> Example 2:
>
> gboolean
> expose_event_callback(GtkWidget * widget, GdkEventExpose * event,
>                       gpointer data)
> {
>  GdkGC *gc;
>  GdkGCValues values;
>  GdkColor colour;
>
>  gdk_gc_get_values(widget->style->fg_gc[GTK_WIDGET_STATE(widget)], &values);
>
>  memset(&colour, 0, sizeof(colour));
>  if (gdk_color_parse(colour_str, &colour)) {
>    g_print("gdk_color_parse(%s) succeeds.\n", colour_str);
>    g_print("colour: pixel %lu, red 0x%04x, green 0x%04x, blue 0x%04x.\n",
>             colour.pixel, colour.red, colour.green, colour.blue);
>    g_print("colour: pixel %lu, red %u, green %u, blue %u.\n",
>             colour.pixel, colour.red>>8, colour.green>>8, colour.blue>>8);
>  } else
>    fprintf(stderr, "gdk_color_parse(%s) fails.\n", colour_str);
       gdk_colormap_color_alloc(gtk_widget_get_colormap(widget), 
&colour, FALSE,FALSE);

>  values.foreground = colour;
>
>  gc = gdk_gc_new_with_values(widget->window, &values, GDK_GC_FOREGROUND);
>
>  gdk_draw_arc(widget->window, gc, TRUE,
>                0, 0, widget->allocation.width, widget->allocation.height,
>                0, 64 * 360);
>
>  return TRUE;
> }
>
> In both cases the filled arc is drawn in black -- the output shows
> that the colour was parsed correctly and the GdkColor was set
> correctly:
>
> Example 1:
>
> $ ./try-colour1
> gdk_color_parse(LightGrey) succeeds.
> colour: pixel 0, red 0xd3d3, green 0xd3d3, blue 0xd3d3.
> colour: pixel 0, red 211, green 211, blue 211.
Pixel is zero - you'll probably see the black color in the screen (in 
TrueColor mode at least)

> Example 2:
> $ ./try-colour2
> gdk_color_parse(LightGrey) succeeds.
> colour: pixel 0, red 0xd3d3, green 0xd3d3, blue 0xd3d3.
> colour: pixel 0, red 211, green 211, blue 211.
The same situation

Olexiy