[9525] in The GTK GIMP ToolKit mailing list archive
[gtk-list] Re: Transparency in Pixmaps
daemon@ATHENA.MIT.EDU (Brandon Long)
Thu Oct 22 18:45:49 1998
Date: Thu, 22 Oct 1998 15:45:29 -0700
From: Brandon Long <blong@fiction.net>
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In-Reply-To: <Pine.LNX.3.96.981022125749.14530D-100000@genie.localnet>; from David Cunningham on Thu, Oct 22, 1998 at 01:01:06PM +0100
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On 10/22/98 David Cunningham uttered the following other thing:
> Hi,
>
> I have a background pixmap onto which i want to draw another pixmap which
> has transparency. All my attempts so far have failed to get the
> transparency to work, as it keeps removing the background where it
> shouldnt.
> Can anyone show me how its done, or point me to an example?
Here is an example I worked through earlier on the list.
Brandon
--
Brandon Long "Faith strikes me as intellectual laziness."
MD6 Crash Test Dummy -- Robert A. Heinlein
Intel Corporation
I'm too low on the totem pole to speak for Intel.
http://www.fiction.net/blong/
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From: Brandon Long <blong@fiction.net>
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In-Reply-To: <13857.10471.214612.248142@afroblue.zedat.fu-berlin.de>; from Stephan Krings on Mon, Oct 12, 1998 at 12:02:48AM +0200
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Subject: [gtk-list] Re: Clipping-Masks in Gdk
X-Rcpt-To: blong@fiction.net
On 10/12/98 Stephan Krings uttered the following other thing:
>
> Hello out there,
>
> could anyone please explain me while the code appended does not
> work. Forgive me if this is a stupid question, but although I'm
> beginning to feel comfortable with Gtk and Gdk, I don't know much
> about the underlying Xlib-calls and -mechanisms (maybe I need to
> invest in one of those expensive Xlib-books), and I'd guess this is
> the problem.
>
> I hope my intention is clear, when reading the code: I want to create
> an offscreen clipping-mask for a window or pixmap. Unfortunatly I'm
> not sure, how to create the GdkBitmap in memory (it works, when
> loading the bitmap from a file with gdk_bitmap_create_from_data for
> example).
>
> Oh yes, the program crashes with an abort-signal and BadMatch error
> message.
>
> So, how do I create this magic GdkBitmap or mask in memory?
Ok, you've made a couple mistakes here. I'll try to point them out:
> ---------------------------------------------------------------------------
> #include <gtk/gtk.h>
>
> int main (int argc, char **argv) {
>
> GtkWidget *window;
> GtkWidget *drawarea;
>
> GdkPixmap *mask;
> GdkGC *gc;
> GdkColor red;
> GdkColor white;
>
> gtk_init(&argc, &argv);
>
> drawarea = gtk_drawing_area_new();
> window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
> gtk_container_add(GTK_CONTAINER (window), drawarea);
> gtk_widget_show(GTK_WIDGET (drawarea));
> gtk_widget_show(GTK_WIDGET (window));
>
> gdk_color_parse("red", &red);
> gdk_color_alloc(gtk_widget_get_colormap(drawarea), &red);
>
> gdk_color_parse("white", &white);
> gdk_color_alloc(gtk_widget_get_colormap(drawarea), &white);
>
> gc = gdk_gc_new(drawarea->window);
> gdk_gc_copy(gc, drawarea->style->black_gc);
>
> gdk_gc_set_foreground(gc, &white);
>
> mask = gdk_pixmap_new(drawarea->window,
> drawarea->allocation.width,
> drawarea->allocation.height,
> -1);
A bitmask has 1 bit per pixel, and you probably aren't running X in
black & white, so the default (-1) isn't what you want. Just take away
the -.
Additionally, the GC is dependent on the pixel depth, so you should
create your GC for the mask using the mask instead of the drawing area
window, so add this:
gc = gdk_gc_new(mask);
You also can't copy from the drawing area GC, as that is for a different
depth.
Move this down here:
gdk_gc_set_foreground(gc, &white);
> gdk_draw_rectangle(mask, gc, TRUE, 10, 10, 20, 20);
Ok, the pixmap is completely uninitialized, so you should really draw a
rectangle to the full area:
gdk_draw_rectangle(mask, gc, TRUE, 0, 0, drawarea->allocation.width,
drawarea->allocation.height);
Additionally, white is the color used for "not" masked, so you should
use black for the actual area you want to blank:
GdkColor black;
gdk_color_parse("black", &black);
gdk_color_alloc(gtk_widget_get_colormap(drawarea), &black);
gdk_gc_set_foreground(gc, &black);
gdk_draw_rectangle(mask, gc, TRUE, 10, 10, 20, 20);
Now, your GC is for the wrong window, you need a different one:
gdk_gc_unref (gc);
gc = gdk_gc_new (drawarea->window);
gdk_gc_copy (gc, drawarea->style->black_gc);
> gdk_gc_set_foreground(gc, &red);
>
> gdk_gc_set_clip_mask(gc, mask);
> gdk_gc_set_clip_origin(gc, 0, 0);
>
> gdk_draw_rectangle(drawarea->window, gc, TRUE, 0, 0,
> drawarea->allocation.width,
> drawarea->allocation.height);
>
> gtk_main();
>
> return 0;
> }
And that should do it.
Something that might help in the future, gdk has a bunch of flags you
can pass on the command line, including --sync. --sync will enable
synchronous X communication, meaning that the program will wait after
every X call for a response. If you run under gdb, your program will
abort when the X error occurs, and you can use a bt to see what the
calling function was that caused the error, which might help lead you
to what is going wrong.
Brandon
--
Brandon Long "The most merciful thing in the world is the inability
MD6 Crash Test Dummy of the human mind to correlate all its contents."
Intel Corporation -- H. P. Lovecraft
I'm too low on the totem pole to speak for Intel.
http://www.fiction.net/blong/
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