Re: How to determinate bit depth of a Gtk::Image?

[Daniel Elstner <daniel kitta googlemail com>]

Am Montag, den 27.11.2006, 17:43 +0100 schrieb Christophe Piault: 
> Hi all,
> 
> I'm trying to got the bit depth (typically 8, 16, 24 or 32) of a Gtk::Image.
> Gdk::Pixbuf::get_bits_per_sample() seems to be what I'm looking for
> but it always returns 8. Others get methods work perfectly well.	

With the current GdkPixbuf implementation get_bits_per_sample() will
always return 8.  Note that bits_per_sample is not the same as the "bit
depth".  That would be get_n_channels() * get_bits_per_sample().
However, at the moment GdkPixbuf only supports the RGB and RGBA
colorspace -- even grayscale images are expanded to RGB on load.

To cut a long story short, there is no such thing as a "bit depth of a
Gtk::Image".  Gtk::Image is a container that can hold several kinds of
image representations.

> I also tried with Gdk::Drawable::get_depth() after getting the pixmap
> with Gtk::Image::get_pixmap. Unfortunately get_pixmap works only
> for empty and pixmap image.

Gdk::Pixmap represents an entity of the windowing system.  In fact, on
most platforms out there the bit depth of a Gdk::Pixmap to be displayed
has to match the desktop's bit depth -- at least as far as I know.  Only
highend graphics hardware supports displaying multiple bit depths at the
same time.

So, Gdk::Pixmap is probably not what you want.  And Gdk::Pixbuf
abstracts the details so much that it's impossible to probe the internal
format of the original image, save for querying the existence of an
alpha channel.

--Daniel