Re: Drawing lines

[Olexiy Avramchenko <ath beast stu cn ua>]

Bryan Brown wrote:

> Taking a cue from a previous poster, I'd like to draw a line in a drawing
> area between two points, starting where the user presses a mouse button
> and ending where the button is released.
>
> The code fragment in
>
>  http://www.radix.net/~bbrown/temp/draw_lines.c
>
> almost does this.  It reports the coordinates where the button is pressed
> and released just fine.  The problem is, when I uncomment-out the line
> that calls gdk_draw_line I get a segmentation fault.
>
> So, what's missing in the code fragment?
You have a bug there:
1. here's what you do
gdk_draw_line((GdkDrawable*)drawing_area, ...);    /* *BUG* drawing_area 
is GtkWidget, not GdkDrawable */
2. here's what you have to do:
gdk_draw_line(drawing_area->window, ...);    /* all's ok: 
drawing_area->window is valid drawable */

> In addition, how can I get the color of each pixel in the line to be the
> XOR of the pixel's original color?  I want to do that so I can eventually
> redraw the line and recover the original underlying image.
GdkGC has a field 'function'. You can change it via 
gdk_gc_set_function(), so
gdk_gc_set_function(gc, GDK_XOR) is what you need.
gtk-doc/gdk/gdk-graphics-contexts.html#GDK-GC-SET-FUNCTION

Olexiy