How to filter OpenCV error messages in Java

Question

The issue of this question has already been discussed e.g. for

C++
Python

ErrorCallback cv::redirectError     (   ErrorCallback   errCallback,
        void *      userdata = 0,
        void **     prevUserdata = 0 
    )

How can this be made to use to e.g. filter out annoying messages?

An example is

[mjpeg @ 0x7fe5a696ea00] unable to decode APP fields: Invalid data found when processing input

from a Logitech USB Webcam mjpeg stream which is created on every single frame and is superflous and not needed.

There is also a loglevel available. Unfortunately the import org.opencv.utils only contains "Converters" but no logging as of OpenCV 3.4.8

How could the loglevel be set from Java?

enum    LogLevel {
  LOG_LEVEL_SILENT = 0,
  LOG_LEVEL_FATAL = 1,
  LOG_LEVEL_ERROR = 2,
  LOG_LEVEL_WARNING = 3,
  LOG_LEVEL_INFO = 4,
  LOG_LEVEL_DEBUG = 5,
  LOG_LEVEL_VERBOSE = 6
}

Would Redirect System.out and System.err to slf4j help?

IIRC, the message you provide as an example is not from OpenCV, it's from ffmpeg, which just writes it to standard error stream. — Dan Mašek, Jan 25 '20 at 15:48
@DanMašek thx it would see https://stackoverflow.com/a/35215447/1497139 points in the right direction then. But how can this be achieved in the OpenCV scenario where ffmpeg is embedded as a library? — Wolfgang Fahl, Jan 26 '20 at 07:35

score 0 · Accepted Answer · answered Jan 27 '20 at 10:13

How can this be made to use to e.g. filter out annoying messages? How could the loglevel be set from Java?

At this time (2020-01) it can't. Even if the API would be accessible from Java the bug https://github.com/opencv/opencv/issues/12780 would prevent it.

Would [Redirect System.out and System.err to slf4j][5] help?

No - see Junit test case below. The result is:

11:05:56.407 [main] DEBUG u.o.l.s.c.SysOutOverSLF4JInitialiser - Your logging framework class ch.qos.logback.classic.Logger should not need access to the standard println methods on the console, so you should not need to register a logging system package.
11:05:56.417 [main] INFO  u.o.l.s.context.SysOutOverSLF4J - Replaced standard System.out and System.err PrintStreams with SLF4JPrintStreams
11:05:56.420 [main] INFO  u.o.l.s.context.SysOutOverSLF4J - Redirected System.out and System.err to SLF4J for this context
11:05:56.421 [main] ERROR org.rcdukes.roi.TestROI - testing stderr via slf4j
[mjpeg @ 0x7f958b1b0400] unable to decode APP fields: Invalid data found when processing input
[mjpeg @ 0x7f958b027a00] unable to decode APP fields: Invalid data found when processing input

where the decode APP field part is still showing up via some stderr magic.

  @Test
  public void testLogStderr() throws Exception {
    NativeLibrary.logStdErr();
    System.err.println("testing stderr via slf4j");
    NativeLibrary.load();
    VideoCapture capture = new VideoCapture();
    // Dorf Appenzell
    //String url="http://213.193.89.202/axis-cgi/mjpg/video.cgi";
    // Logitech Cam on test car
    // url="http://picarford:8080/?action=stream";
    File imgRoot = new File(testPath);
    File testStream=new File(imgRoot,"logitech_test_stream.mjpg");
    assertTrue(testStream.canRead());
    capture.open(testStream.getPath());
    Mat image=new Mat();
    capture.read(image);
    assertEquals(640,image.width());
    assertEquals(480,image.height());
    capture.release();
  }

Funny side fact

According to the ffmpeg documentation Log coloring can be disabled setting the environment variable AV_LOG_FORCE_NOCOLOR or NO_COLOR, or can be forced setting the environment variable AV_LOG_FORCE_COLOR. The use of the environment variable NO_COLOR is deprecated and will be dropped in a future FFmpeg version

But there seems to be no option to change the Logging level from an environment variable ...

How to filter OpenCV error messages in Java

1 Answers1