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Re: Image mode

From: Chong Yidong
Subject: Re: Image mode
Date: Mon, 05 Feb 2007 18:50:14 -0500
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.0.93 (gnu/linux)

"Juanma Barranquero" <address@hidden> writes:

> On 2/5/07, Chong Yidong <address@hidden> wrote:
>> So: can anyone provide an alternative?
> I think the first step would be to summarize the existing
> alternatives.

OK, here is my stab at this.

(First, let us recall that when choosing a major mode,
magic-mode-alist has precedence over auto-mode-alist.  The former
performs "autodetection", i.e. selecting the mode using the file
contents.  The latter performs what we will refer to as "filename
detection", i.e. selecting the mode using the filename.)

Here are three schemes.

Scheme 1: Image autodetection, with two classes of image types
   and user prompting

 - Handle image autodetection in two steps.
 - First, autodetect binary images (jpeg, png, gif, tiff).  If
   autodetected, check for a non-image mode in auto-mode-alist.  If no
   match is found, go ahead and open in image-mode.  If a match for a
   non-image mode is found, ask the user whether to open in
   image-mode, or in the mode specified in auto-mode-alist.
 - Second, autodetect text-based images (xpm, postscript, xbm).
   If autodetected, check for a non-image mode in auto-mode-alist.  If
   no match is found, open in image-mode (this should not happen).  If
   a match is found, open in the non-image major mode and
 - If autodetection fails, perform filename detection.

 - The (existing) option image-type-auto-detectable determines which
   image files can be autodetected.

 - Advantages: Seems to handle all known problematic cases.
   Text-based images are opened in foo-mode with image-minor-mode;
   images with funky file-names are opened in image-mode if no other
   suitable mode is found, or the user is prompted if there is an

 - Disadvantages: The behavior is complicated.  Also, prompting the
   user may not be straightforward for other uses of
   find-file-nonselect (are there any specific examples?).  Also,
   setting image-minor-mode is naughty: it violates the standard that
   Emacs does not automatically turn on minor-modes on for the user.

Scheme 2: Image autodetection with deference to auto-mode-alist for
   binary image files

 - As above, except that is a binary image is autodetected, and a
   match for a non-image mode is found, fall back on the non-image
   mode instead of prompting the user.

 - Advantages: Principle of least surprise.  A Jpeg file saved as
   foo.txt is handled as a text file; one saved as foo.JPG or foo.bar
   is handled as an image (since Emacs sees no other choice).
   Avoids possible difficulties with user prompting.

 - Disadvantages: As above, setting image-minor-mode is naughty.  If
   foo.c really IS a jpeg file, it might (?) be more correct to
   display it as a jpeg file; this doesn't do that.

Scheme 3: No image autodetection

 - Images are not autodetected.  Only auto-mode-alist is used.
 - In particular, XPM and XBM images are opened in C mode (see above).
 - The commonly-encountered .JPG extension should be recognized as an
   jpeg file, either by making auto-mode-case-fold default to t, or
   adding .JPG to auto-mode-alist.

 - Advantages: The behavior is simple.  As Stefan notes, few or no
   other programs auto-detect images as images using their content,
   and it may be safer to avoid surprising the user.

 - Disadvantages: Richard apparently dislikes auto-mode-case-fold
   (this may not be a theoretical disadvantage, but it is a practical
   one).  On the other hand, if we add .JPG to auto-mode-alist, we
   might as well add .PNG, .GIF, etc---which is ugly (though it may be
   the practical thing to do).

Personally, I think scheme 2 may be a good compromise.

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