Category: Linux (general)

Creating multi-page PDF files with GIMP and `convert`

Occasionally I have to sign some document (old style, with a pen) and send it electronically. Sometimes those are multi-page documents. Since it is uncommon to send it back as multiple image files after scanning, and multi-page image formats are uncommon as well, I’d like to send them as PDF file. Before I discovered this method, I used to insert the scanned images into OpenOffice Writer, and then create the PDF with it. This works, but it is a bit cumbersome to tell OpenOffice Writer to maximise the images (eliminating page borders, etc.), especially when there are a lot of pages. It just doesn’t feel like a real solution.

So, here we go:

Prerequisites:

  • GIMP (I’m currently at version 2.6.8, but this will probably work with older versions as well)
  • GraphicsMagick (tested with 1.3.8) or ImageMagick (tested with 6.5.8.8)

Procedure:

  1. Get the scanned pages opened as layers of one image in GIMP. If they are available as files already, you can use File / Open as Layers….
  2. Make sure that the layers are ordered in the following way: Page 1 must be the bottom layer, the last page must be the top layer. You can reorder them via the “Layers” dialogue (activate it via the Windows / Dockable Dialogues menu if you don’t see it)
  3. Save As… and choose “MNG animation” or just add “.mng” to the filename. (In case you are wondering, MNG is the animated counterpart to PNG).
    A dialogue window saying “MNG plug-in can only handle layers as animation frames” will come up – choose “Save as Animation” here and press the Export button. In the next dialogue you don’t need to make any changes to the defaults, just press the Save button.
  4. Now, open a console window and simply enter
    convert document.mng document.pdf

That’s it – you now have your PDF file ready for sending!

Update (2010-02-08):
As chithanh pointed out in comment 1, there is another convenient way to accomplish the same. It does not involve GIMP, but instead requires pdftk to concatenate PDF files. Please see comment 2 for details.

Update (2010-03-01):
And yet another way (definitely the most straight-forward one, if you have the pages as single image files already) was pointed out by goffrie in comment 5.

An equaliser that works with MPD and ALSA

A few months ago I was looking for a way to lower the amplification of the lower frequencies while listening to music on extremely bass-heavy in-ear headphones. The only way to accomplish that, seemed to be using JACK. Since that seemed overkill to me, I gave up.

Today I found a solution that does not require JACK: Charles Eidsness’ ALSAEQUAL. This is how you get it working with MPD on Gentoo:

  1. `emerge alsaequal`
  2. Create an .asoundrc file in the home directory of the user that runs MPD on your system (see `grep "^user" /etc/mpd.conf`) with the following content:

    ctl.equal {
  type equal;
}

pcm.plugequal {
  type equal;
  slave.pcm "plug:dmix";
}

pcm.equal {
  # Or if you want the equalizer to be your
  # default soundcard uncomment the following
  # line and comment the above line.
# pcm.!default {
  type plug;
  slave.pcm plugequal;
}

    (copied from the ALSAEQUAL website – modified so that dmix is being used, which allows playing multiple audio sources simultaneously)

  3. Find the audio_output section in your /etc/mpd.conf where you currently configure MPD to use ALSA’s default device. Change it so that it looks like 
    audio_output {
  type    "alsa"
  name    "equal"
  device  "plug:plugequal"
}
  4. `/etc/init.d/alsasound restart && /etc/init.d/mpd restart`
  5. Play a track with MPD, then run `alsamixer -D equal`*. Modify any of the frequency band sliders and observe the effect.

Other applications will be unaffected by the equaliser, if you don’t change the .asoundrc according to the comment, and if you don’t configure them to use the ‘equal’ ALSA device.

*) Note that alsamixer needs to be run as the user that runs mpd. See Ian’s comment for details.

Update (2012-06-16): Having the .asoundrc in place causes Skype to crash after a few seconds (see also this Gentoo Forums post).

Fresh vs. rotten ext3

Did you ever hear sentences like “Linux/Unix filesystems are superior, to stuff like NTFS, let alone FAT32 – you don’t even need a defragmentation tool.”?

That statement may be technically correct, since fragmentation is really rare with ext3 – but what about spatial locality of reference – files that are often accessed at nearly the same time being spread over the whole disk, thus causing long access times due to head positioning?
There is (AFAIK) no way to (programmatically) optimise / sort a Linux filesystem in a way that for example all init scripts, binaries, libraries, etc. reside in nearby sectors on a harddisk. Those tools do exist in the Windows world (built into those 3rd party defragmenters).

The only way to do this optimisation is “by hand”: make a backup, mkfs and restore the backup – but that’s something you only do when you have a lot of time, or when I/O finally got painfully slow.

One factor that probably plays an important role, is, that I usually only have one partition (+swap) that contains everything – etc, usr, usr/portage, var, home, …
With such a setup, after only a few months of updating the system, downloading stuff from the net, copying pictures from the DSLR to the harddisk, updating the system again, etc. will lead to all kinds of files being well spread throughout the whole disk. Imagine some init scripts and binaries they call still being at the “beginning” of the disk, some others, due to updates, at the “end”. And that’s what makes I/O slow… see for yourself:

From loading the kernel to KDM being ready for login on a

(sorry for the poor video quality)

That is an ~80% performance decrease.

How I got the videos:

  1. Capture startup of my system stored on a months old ext3 partition
  2. Backup (with dar)
  3. mkfs.ext3
  4. Restore
  5. Capture startup of my system on a freshly made ext3 partition
  6. Trim the videos so that frame 1 is the first frame on which kernel output is visible and the last frame is the first frame on which KDM shows the input widgets

Hoping that this situation will improve with ext4 – I heard online defragmentation will be possible at some point, and that probably also makes “sorting” the filesystem possible.