[KimDaBa] Re: Some comments on plugins and a feature request

[Renchi Raju]

On Tue, 20 Jul 2004, Robert Krawitz wrote:

>    Date: Mon, 19 Jul 2004 22:25:50 -0500 (CDT)
>    From: Renchi Raju <renchi at pooh.tam.uiuc.edu>
>
>    now coming to your suggestion: it doesn't make sense to have
>    dialogs for jpeg lossless operations - rotation, black-white
>    conversion, flipping; as it should be quite obvious to user whats
>    going to happen to the image, no need for a preview or stuff like
>    that. these operations are meant to be quick access operations
>    (very frequently used by digital camera owners) and having a dialog
>    will slow things down for them.
>
> BW conversion in particular is not obvious at all.  What are the
> luminance constants used for each channel, and what will the effect
> be?

from the jpegtran manpage:

-grayscale
             Force grayscale output.

This  option discards the chrominance channels if the input image is YCbCr (ie, a
standard color JPEG), resulting in a grayscale JPEG file.  The luminance  channel
is  preserved  exactly,  so this is a better method of reducing to grayscale than
decompression, conversion, and recompression.  This switch is particularly  handy
for fixing a monochrome picture that was mistakenly encoded as a color JPEG.  (In
such a case, the space savings from getting rid of the near-empty chroma channels
won't  be large; but the decoding time for a grayscale JPEG is substantially less
than that for a color JPEG.)

renchi