On Wed, 3 Jan 2007 17:10:54 +0200, "Aurimas Juška" <[EMAIL PROTECTED]> wrote:
> This is a summary of Bug 352262 – 32bit bitmaps (*.bmp) appear blank (
> http://bugzilla.gnome.org/show_bug.cgi?id=352262 ) which as Sven said
> is important to be settled before releasing 2.4 version.
> The main question: should those bytes be ignored (as it is done by
> many applications) or should some workaround be used (in case we want
> to support 32bit bitmaps with alpha channel)?

I did a quick web search to see how some applications dealt with this
problem and I found a number of links to photoshop plug-ins (including
one GPL program [1]) and standalone programs for converting or editing
Windows .BMP or .ICO files.  It should be noted that the support for
the alpha channel is an extension added in Windows XP (and made
partially obsolete by the usage of PNG in Windows Vista) so it is
unlikely to be available or even mentioned in programs, books or
official or unofficial specifications written before 2001.

As mentioned on many web sites, there does not seem to be any complete
and official specification of the BMP file format available anywhere.
Even the documentation from Microsoft (MSDN Library) does not mention
the usage of the alpha channel although it is used in Windows XP.

I have not tried these programs supporting alpha channels in BMP but
from their descriptions I could identify the following strategies when
loading 32-bit BMP or ICO files:
1) Always ask the user if the alpha channel should be used or not
2) Use the alpha channel if and only if it contains at least one
   non-zero value.
3) Use the alpha channel if the file name ends in .ICO, ignore it if
   it is .BMP.

The second strategy seems like a good compromise and would probably
work in most cases (i.e., match the user's expectations).


[1] http://www.telegraphics.com.au/svn/icoformat/trunk/dist/README.html
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