bug#7716: 23.2.91; Pretest Windows binary doesn't support PNG images

[Eli Zaretskii <eliz@gnu.org>]

> From: "Drew Adams" <drew.adams@oracle.com>
> Cc: <7716@debbugs.gnu.org>, <sean.sieger@gmail.com>
> Date: Thu, 23 Dec 2010 09:36:55 -0800
> 
> > > (image-type-available-p 'png) => nil
> > 
> > Do you have one of the PNG DLLs installed that are mentioned in
> > image-library-alist?
> 
> No.

Then that's why you get nil.  In your case, you cannot expect to have
PNG images displayed as images.

> (BTW, that variable is apparently now obsolete.)

Not in Emacs 23, which is what this bug is about.

> Or rather, how would I know?  Where is the variable/function that tells me which
> image DLLS are installed?

There isn't one, I think.  You could use image-search-load-path to see
if Emacs finds something (e.g., map it over the corresponding part of
image-library-alist).  Not sure why would you need that, though.

> And shouldn't we be delivering such DLLs with Emacs?

There are licensing issues with some of them.  We distribute the XPM
library, which is the minimum required for displaying the tool-bar
icons in color.  It should be downloadable from the same place where
you get the precompiled binaries.

> And where are the instructions that explain where to get such DLLs and what to
> do with them?

See nt/INSTALL (search for "image").

> Does that "installing" require compiling etc. C sources, or just
> putting some DLL files in some directory?

The latter; see nt/INSTALL.

> What tools does a (Windows) user need to "install" the DLLs?

Nothing but bare hands.

> I tried `i image'(...), but saw nothing about what to do to make
> Emacs images-enabled.

That's because they are enabled by default, if Emacs can find the
necessary support libraries.  PBM is built into Emacs, and the XPM
library is available together with Emacs.