Re: minor fix for GdkPixbuf

[Havoc Pennington <hp redhat com>]

Havoc Pennington <hp redhat com> writes: 
> Owen was suggesting that we actually put the grab in gdk_image_get()
> itself though - if the grab doesn't seem to matter for speed that's
> probably feasible.
> 

This is a bit more complicated than I thought - 
  - you can't do the grab/clip-to-screen in pixbuf_from_drawable() 
    because you don't know there if you're getting from the 
    window or the backing store
  - you can't do the grab/clip-to-screen in _gdk_x11_get_image() 
    because the caller does not know the position of the returned
    (smaller-than-expected) image in source drawable coordinates
  - ditto for gdk_drawable_get_image()

I believe we really want to clip in gdk_x11_get_image(). I can add a
gruesome private hack to get the location of the returned image in
source drawable coordinates back to pixbuf_get_from_drawable(), by way
of gdk_drawable_get_image().

However anyone else using gdk_drawable_get_image() also needs this
information, it really should not be private.

I'm considering two possible solutions.

Solution #1 is to just change gdk_drawable_get_image() since it's a
new function anyway, maybe something like:

 GdkImage*
 gdk_drawable_get_image (GdkDrawable *drawable,
                         gint         x,
                         gint         y,
                         gint         width,
                         gint         height,
                         GdkRectangle *retrieved_area);

Or is that terrible? You can pass NULL for retrieved_area, and
get_image is a fairly low-level thing anyway.

What do we do with gdk_image_get(), free the image and return NULL if
gotten_image->width != requested_width, etc.?

Solution #2 I'm considering is to change _gdk_x11_get_image() to first
create the image with XCreateImage(), then use XGetSubImage() to copy
in the clipped-to-screen area of the window. So we always return an
image of the requested size, but leave parts of it undefined if they
are offscreen.

I like this solution best, because it's the same behavior as for
gdk_pixbuf_get_from_drawable(), and is likely what people usually
want. Also it avoids any possible breakage for people using
gdk_image_get() in legacy code.

Havoc