Re: the problem of transparency

[<learfox furry ao net>]

> Dear every professional Linux engineer:
>     We are developing a project with gtk of Linux but It is very hard for us
> to deal transparency problem.
>     The following is  about the detail of the problem.
>     We want loading two JPG files, one of both is background picture ,
> and the other is transparecy picture over the background  picture . Can you
> tell us the method of realizing it with gtk of imlib.
>     We are using the following method to deal it but failed
>
>  GdkImlibImage *im;
>  GdkImColor color;
>  int r ,g ,b;
>  gdk_imlib_get_image_shape(im,&color);
>  r = color.r
>  g = color.g
>  b = color.b
>  color.r = 255;
>  color.g = 255;
>  color.b = 255;
> gdk_imlib_set_image_shape(im,&color);
>
> the picture of im can not be changed to be transparency.
> We feel very upset and please tell us how to slove the problem.

I can't recall if imlib returns a bitmap, but a bitmap is the key to your
problem.

Once you have a bitmap, create a GdkBitmap!  Then create a GtkDrawingArea.

What you'll do is first `clear' the GtkDrawingArea by simply drawing your
background image from the GdkPixmap.

        gdk_draw_pixmap(...);

Then (and this will require the use of a GC, create a new GdkGC) draw the
foreground image with transparency.

        gdk_gc_set_clip_mask(gc_fg, mask);
        gdk_gc_set_clip_origin(gc_fg, x, y);

        gdk_draw_pixmap(...);

The coordinates passed to gdk_gc_set_clip_origin() must match those you
specified for the foreground pixmap in gdk_draw_pixmap().

So to review:

Create a GtkDrawingArea widget to draw on to, next:

You'll need a GdkPixmap of the background image, a GdkPixmap and GdkBitmap
(the mask) of the foreground image.  Also a special GC that you keep track
of when drawing the foreground image (for transparency).

Xlib/Gdk *DOES* support transparency, it's just ignorance that some people
think it dosen't.

Note, you can draw the background image with the regular GC.

--
Sincerely,                  ,"-_                         \|/
-Capt. Taura M.             ,   O=__                    --X--
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                  http://furry.ao.net/~learfox/