[gentoo-user] Question about difference between emerge --update world and emerge vigra

[SOrCErEr <omentie@gmail.com>]

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Hello~

Yesterday, I did update my gentoo like this.

$ sudo emerge --update --deep --newuse world

As a result, one of the updated package tiff said like this.
 * Messages for package media-libs/tiff-3.8.2-r5:

 * JBIG support is intended for Hylafax fax compression, so we
 * really need more feedback in other areas (most testing has
 * been done with fax).  Be sure to recompile anything linked
 * against tiff if you rebuild it with jbig support.

So I checked what package linked with libtiff(Actually, there is no need to
do this. I just want to know).

$ revdep-rebuild -p --library /usr/lib/libtiff.so.3

Then, revdep-rebuild display like this.
[ebuild   R   ] media-libs/tiff-3.8.2-r5
[ebuild   R   ] media-gfx/imagemagick-6.4.3.5
[ebuild   R   ] media-libs/lcms-1.17
[ebuild     U ] media-libs/vigra-1.6.0 [1.5.0-r1]
[ebuild   R   ] net-print/cups-1.3.8-r1
[ebuild   R   ] x11-libs/gtk+-2.12.12
[ebuild   R   ] app-text/ghostscript-gpl-8.63
[ebuild   R   ] app-text/evince-2.22.2-r1
[ebuild   R   ] media-gfx/gthumb-2.10.9


As you can see, there is a package "vigra" with flag "U".
As I know, U is need to update, and if I do emerge --update world, it will
be update.

But there is no "vigra" in result of "emerge -p --update world"(with --deep
also).
Of course, it still shows flag "U" if I do "emerge -p vigra"

What is difference with emerge --update world and emerge vigra?
And how can I update all new package without exceptions?

If you know, please let me know.

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