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• Perspective transformations

  9 juin 2010, par Mikko Koppanen — Imagick, PHP stuff

  Finally (after a long break) I managed to force myself to update the PHP documentation and this time it was distortImage code example. Things have been hectic lately but that does not quite explain the 6 months(?) break between this and the previous post. As a matter of a fact there is no excuse for such a long silence so I will try to update this blog a bit more often from now on.

  Back in the day I used to blog the examples and update the documentation if I remembered but I am trying to fix this bad habit. Most of the latest examples have been updated in to the manual. In the case of the two last examples I updated the documentation first and then blogged on the subject.

  I took some time to actually understand the perspective transformations properly using the excellent ImageMagick examples (mainly created by Anthony Thyssen) as a reference. The basic idea of perspective distortion seems simple : to distort the control points to new locations. Grabbing the syntax for Imagick was easy, an array of control point pairs in the form of :

  php

  < view plain text >

  1. array(source_x, source_y, dest_x, dest_y ... )

  The following example uses the built-in checkerboard pattern to demonstrate perspective distortion :

  php

  < view plain text >

  1. < ?php
  2. /* Create new object */
  3. $im = new Imagick() ;
  4.  
  5. /* Create new checkerboard pattern */
  6. $im->newPseudoImage(100, 100, "pattern:checkerboard") ;
  7.  
  8. /* Set the image format to png */
  9. $im->setImageFormat(’png’) ;
  10.  
  11. /* Fill background area with transparent */
  12. $im->setImageVirtualPixelMethod(Imagick: :VIRTUALPIXELMETHOD_TRANSPARENT) ;
  13.  
  14. /* Activate matte */
  15. $im->setImageMatte(true) ;
  16.  
  17. /* Control points for the distortion */
  18. $controlPoints = array( 10, 10,
  19.             10, 5,
  20.  
  21.             10, $im->getImageHeight() - 20,
  22.             10, $im->getImageHeight() - 5,
  23.  
  24.             $im->getImageWidth() - 10, 10,
  25.             $im->getImageWidth() - 10, 20,
  26.  
  27.             $im->getImageWidth() - 10, $im->getImageHeight() - 10,
  28.             $im->getImageWidth() - 10, $im->getImageHeight() - 30) ;
  29.  
  30. /* Perform the distortion */ 
  31. $im->distortImage(Imagick: :DISTORTION_PERSPECTIVE, $controlPoints, true) ;
  32.  
  33. /* Ouput the image */ 
  34. header("Content-Type : image/png") ;
  35. echo $im ;
  36.  ?>

  Here is the source image :
  

  And the result :
  

• Multiprocess FATE Revisited

  26 juin 2010, par Multimedia Mike — FATE Server, Python

  I thought I had brainstormed a simple, elegant, multithreaded, deadlock-free refactoring for FATE in a previous post. However, I sort of glossed over the test ordering logic which I had not yet prototyped. The grim, possibly deadlock-afflicted reality is that the main thread needs to be notified as tests are completed. So, the main thread sends test specs through a queue to be executed by n tester threads and those threads send results to a results aggregator thread. Additionally, the results aggregator will need to send completed test IDs back to the main thread.

  
  
  

  But when I step back and look at the graph, I can’t rationalize why there should be a separate results aggregator thread. That was added to cut down on deadlock possibilities since the main thread and the tester threads would not be waiting for data from each other. Now that I’ve come to terms with the fact that the main and the testers need to exchange data in realtime, I think I can safely eliminate the result thread. Adding more threads is not the best way to guard against race conditions and deadlocks. Ask xine.

  
  
  

  I’m still hung up on the deadlock issue. I have these queues through which the threads communicate. At issue is the fact that they can cause a thread to block when inserting an item if the queue is "full". How full is full ? Immaterial ; seeking to answer such a question is not how you guard against race conditions. Rather, it seems to me that one side should be doing non-blocking queue operations.

  This is how I’m planning to revise the logic in the main thread :

  test_set = set of all tests to execute
  tests_pending = test_set
  tests_blocked = empty set
  tests_queue = multi-consumer queue to send test specs to tester threads
  results_queue = multi-producer queue through which tester threads send results
  while there are tests in tests_pending :
    pop a test from test_set
    if test depends on any tests that appear in tests_pending :
      add test to tests_blocked
    else :
      add test to tests_queue in a non-blocking manner
      if tests_queue is full, add test to tests_blocked
    while there are results in the results_queue :
  
      get a result from result_queue in non-blocking manner
  
      remove the corresponding test from tests_pending
  if tests_blocked is non-empty :
  
    sleep for 1 second
  
    test_set = tests_blocked
  
    tests_blocked = empty set
  
  else :
  
    insert n shutdown signals, one from each thread
  go to the top of the loop and repeat until there are no more tests
  while there are results in the results_queue :
  
    get a result from result_queue in a blocking manner
  

  Not mentioned in the pseudocode (so it doesn’t get too verbose) is logic to check whether the retrieved test result is actually an end-of-thread signal. These are accounted and the whole test process is done when one is received for each thread.

  On the tester thread side, it’s safe for them to do blocking test queue retrievals and blocking result queue insertions. The reason for the 1-second delay before resetting tests_blocked and looping again is because I want to guard against the situation where tests A and B are to be run, A depends of B running first, and while B is running (and happens to be a long encoding test), the main thread is spinning about, obsessively testing whether it’s time to insert A into the tests queue.

  It all sounds just crazy enough to work. In fact, I coded it up and it does work, sort of. The queue gets blocked pretty quickly. Instead of sleeping, I decided it’s better to perform the put operation using a 1-second timeout.

  Still, I’m paranoid about the precise operation of the IPC queue mechanism at work here. What happens if I try to stuff in a test spec that’s a bit too large ? Will the module take whatever I give it and serialize it through the queue as soon as it can ? I think an impromptu science project is in order.

  big-queue.py :

  PLAIN TEXT

  PYTHON :

  1. # !/usr/bin/python
  2.  
  3. import multiprocessing
  4. import Queue
  5.  
  6. def f(q) :
  7.   str = q.get()
  8.   print "reader function got a string of %d characters" % (len(str))
  9.  
  10. q = multiprocessing.Queue()
  11. p = multiprocessing.Process(target=f, args=(q,))
  12. p.start()
  13. try :
  14.   q.put_nowait(’a’ * 100000000)
  15. except Queue.Full :
  16.   print "queue full"

  $ ./big-queue.py
  reader function got a string of 100000000 characters
  

  Since 100 MB doesn’t even make it choke, FATE’s little test specs shouldn’t pose any difficulty.

• Monster Battery Power Revisited

  28 mai 2010, par Multimedia Mike — Python, Science Projects

  So I have this new fat netbook battery and I performed an experiment to determine how long it really lasts. In my last post on the matter, it was suggested that I should rely on the information that gnome-power-manager is giving me. However, I have rarely seen GPM report more than about 2 hours of charge ; even on a full battery, it only reports 3h25m when I profiled it as lasting over 5 hours in my typical use. So I started digging to understand how GPM gets its numbers and determine if, perhaps, it’s not getting accurate data from the system.

  I started poking around /proc for the data I wanted. You can learn a lot in /proc as long as you know the right question to ask. I had to remember what the power subsystem is called — ACPI — and this led me to /proc/acpi/battery/BAT0/state which has data such as :

  present :                 yes
  capacity state :          ok
  charging state :          charged
  present rate :            unknown
  remaining capacity :      100 mAh
  present voltage :         8326 mV
  

  "Remaining capacity" rated in mAh is a little odd ; I would later determine that this should actually be expressed as a percentage (i.e., 100% charge at the time of this reading). Examining the GPM source code, it seems to determine as a function of the current CPU load (queried via /proc/stat) and the battery state queried via a facility called devicekit. I couldn’t immediately find any source code to the latter but I was able to install a utility called ’devkit-power’. Mostly, it appears to rehash data already found in the above /proc file.

  Curiously, the file /proc/acpi/battery/BAT0/info, which displays essential information about the battery, reports the design capacity of my battery as only 4400 mAh which is true for the original battery ; the new monster battery is supposed to be 10400 mAh. I can imagine that all of these data points could be conspiring to under-report my remaining battery life.

  Science project : Repeat the previous power-related science project but also parse and track the remaining capacity and present voltage fields from the battery state proc file.

  Let’s skip straight to the results (which are consistent with my last set of results in terms of longevity) :

  
  
  

  So there is definitely something strange going on with the reporting— the 4400 mAh battery reports discharge at a linear rate while the 10400 mAh battery reports precipitous dropoff after 60%.

  Another curious item is that my script broke at first when there was 20% power remaining which, as you can imagine, is a really annoying time to discover such a bug. At that point, the "time to empty" reported by devkit-power jumped from 0 seconds to 20 hours (the first state change observed for that field).

  Here’s my script, this time elevated from Bash script to Python. It requires xdotool and devkit-power to be installed (both should be available in the package manager for a distro).
  

  PLAIN TEXT

  PYTHON :

  1. # !/usr/bin/python
  2.  
  3. import commands
  4. import random
  5. import sys
  6. import time
  7.  
  8. XDOTOOL = "/usr/bin/xdotool"
  9. BATTERY_STATE = "/proc/acpi/battery/BAT0/state"
  10. DEVKIT_POWER = "/usr/bin/devkit-power -i /org/freedesktop/DeviceKit/Power/devices/battery_BAT0"
  11.  
  12. print "count, unixtime, proc_remaining_capacity, proc_present_voltage, devkit_percentage, devkit_voltage"
  13.  
  14. count = 0
  15. while 1 :
  16.   commands.getstatusoutput("%s mousemove %d %d" % (XDOTOOL, random.randrange(0,800), random.randrange(0, 480)))
  17.   battery_state = open(BATTERY_STATE).read().splitlines()
  18.   for line in battery_state :
  19.     if line.startswith("remaining capacity :") :
  20.       proc_remaining_capacity = int(line.lstrip("remaining capacity : ").rstrip("mAh"))
  21.     elif line.startswith("present voltage :") :
  22.       proc_present_voltage = int(line.lstrip("present voltage : ").rstrip("mV"))
  23.   devkit_state = commands.getoutput(DEVKIT_POWER).splitlines()
  24.   for line in devkit_state :
  25.     line = line.strip()
  26.     if line.startswith("percentage :") :
  27.       devkit_percentage = int(line.lstrip("percentage :").rstrip(’\%’))
  28.     elif line.startswith("voltage :") :
  29.       devkit_voltage = float(line.lstrip("voltage :").rstrip(’V’)) * 1000
  30.   print "%d, %d, %d, %d, %d, %d" % (count, time.time(), proc_remaining_capacity, proc_present_voltage, devkit_percentage, devkit_voltage)
  31.   sys.stdout.flush()
  32.   time.sleep(60)
  33.   count += 1