Q:

Parallel image detection and camera preview OpenCV Android

I'm using OpenCV to detect an image. Here is my problem: my function detect_image(mRgba) needs some time to perform operations and give some results. While function is computing camera preview is frozen because it only shows image when code reaches return inputFrame.rgba() I would like to know how to make those operation parallel, function will be computing in a background while camera preview is working with normal speed.    
public Mat onCameraFrame(CvCameraViewFrame inputFrame) {
mRgba = inputFrame.rgba();
detect_image(mRgba);
return inputFrame.rgba();
}

A:

To just get a taste at parallelization, the simple approach would be to just use an AsyncTask to process your images:
AsyncTask reference page
A more friendly introduction can be found here:
http://android-developers.blogspot.co.il/2010/07/multithreading-for-performance.html
while this:
http://developer.att.com/developer/forward.jsp?passedItemId=11900176
is a nice all-around introduction to multi-threading on Android.
If you want to just get started, a simple algorithm should work like this:

from within your "onCameraFrame" method check if you have an AsyncThread for processing the image which is already running
if the answer is "yes", just show mRgba in the preview window and return
if the answer is "no" start a new AsyncThread and let it run "detectImage" on mRgba, making sure that the results are saved in the onPostExecute method.

With this algorithm, if your system can detect 4 images per second while taking a preview at 60fps (for example), you will be able to get a smooth video with a new result about each 20-30 frames on a single processor device, under the realistic assumption that detect_image is CPU intensive while the camera preview/display are I/O intensive.

   Capture:     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x....
   Processing:  1.......1.......1.......1.....1.......1....
                time ------------------------------------>

Starting with HoneyComb, a more refined approach would be to account for the number of cores in your CPU (multicore phones/tablets are becoming increasingly common) and start N AsyncTask in parallel (one for each core), feeding a different preview image to each one (maybe using a thread pool...). 
If you separate each thread by a fixed delay (about the duration of detectImage/N ), you should get a constant stream of results with a frequency that should be a multiple of the single threaded version.

   Capture:     x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x.x....
   Processing:  1.2.3.4.1.2.3.4.1.2.3.4.1.2.3.4.1.2.3.4....
                time ------------------------------------>

Hope this helps

