cvFindContours & cvApproxPoly Performance Problem?

[wordin0]

cvFindContours & cvApproxPoly Performance Problem?

[GoogleCodeExporter]

This is most likely caused by the "auto-synch" feature in JNA, details here 
under "Large Structure": 
https://jna.dev.java.net/javadoc/overview-summary.html#performance

Unfortunately, functions like cvFindContours() and cvApproxPoly() create CvSeq 
objects and the programmer does not get a chance to setAutoSynch(false) before 
they get read... I'll think about what could be done to keep things clean. You 
may however modify the source code of cxcore.java, as a quick fix, by adding 
setAutoSynch(false) to constructors of the CvSeq class.

Samuel

Original comment by [email protected] on 16 Jul 2010 at 3:01

• Changed state: Accepted

[GoogleCodeExporter]

To answer some of the other questions, the number of parameters to the 
cvContourArea() method changes with the version of OpenCV, so you need to 
import one of the subclasses based on the version of OpenCV you wish to use.

The problem with blobslib is that it's written in C++.. It's quite hard to use 
C++ interfaces from other languages. Someone would need to produce an interface 
in C first.

Original comment by [email protected] on 16 Jul 2010 at 3:10

[GoogleCodeExporter]

Great,
Thanks, I will try your suggestion.

Additionally, I was wondering why cvContourArea was commented out?

Original comment by [email protected] on 16 Jul 2010 at 3:18

[GoogleCodeExporter]

Sorry, did not see your last reply before I posted my last comment.
Makes sense regarding parameter numbers changing.

I'll experiment with the setAutoSynch too. I'd assume that all the fields would 
be null until forcefully read into - not sure how to do that, but I'll look.
Thanks for pointing me in the right direction.

Greg.

Original comment by [email protected] on 16 Jul 2010 at 8:29

[GoogleCodeExporter]

I modified CvSeq slightly in this test package to include a static autoSynch 
flag:
    http://www.ok.ctrl.titech.ac.jp/~saudet/javacv.jar
With that, something like the following should disable the default auto-synch 
behavior of any newly constructed CvSeq objects:
    CvSeq.autoSynch = false;
Once "auto-synch" is disabled, you need to call the readField() method to read 
the native values of the fields in the Structure. The values of the Java fields 
will not be correct.. 

If that works, I plan to add the flag to all Structures in the next release... 
Please let me know if it works, thanks

Original comment by [email protected] on 12 Aug 2010 at 3:06

• Changed state: Started

[GoogleCodeExporter]

I am having the same problem. Blob detection is a pain using javaCV. I have 
turned to opencv for processing and it works quite good, but it would be nice 
to see a fix or improvement on the javaCV side.

that jar file did not work for me. Did not notice a difference.

Original comment by [email protected] on 25 Oct 2010 at 12:07

[GoogleCodeExporter]

You have to put this line somewhere before your blob detection:
    CvSeq.autoSynch = false;
If you did put this line and it did not improve the performance, then please 
let me know. Otherwise, I am considering this problem fixed, thank you.

Original comment by [email protected] on 25 Oct 2010 at 2:03

• Changed state: Fixed