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Orbbec tutorial: Sync frames from two streams and process depth & color simultaneously

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Igor Murzov
2020-11-23 14:05:55 +03:00
parent ef0eed8d3c
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doc/tutorials/videoio/orbbec-astra/orbbec_astra.markdown
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@@ -9,8 +9,8 @@ Using Orbbec Astra 3D cameras {#tutorial_orbbec_astra}
This tutorial is devoted to the Astra Series of Orbbec 3D cameras (https://orbbec3d.com/product-astra-pro/).
That cameras have a depth sensor in addition to a common color sensor. The depth sensors can be read using
the OpenNI interface with @ref cv::VideoCapture class. The video stream is provided through the regular camera
interface.
the open source OpenNI API with @ref cv::VideoCapture class. The video stream is provided through the regular
camera interface.
### Installation Instructions
@@ -70,15 +70,20 @@ In order to use a depth sensor with OpenCV you should do the following steps:
### Code
To get both depth and color frames, two @ref cv::VideoCapture objects should be created:
The Astra Pro camera has two sensors -- a depth sensor and a color sensor. The depth sensors
can be read using the OpenNI interface with @ref cv::VideoCapture class. The video stream is
not available through OpenNI API and is only provided through the regular camera interface.
So, to get both depth and color frames, two @ref cv::VideoCapture objects should be created:
@snippetlineno samples/cpp/tutorial_code/videoio/orbbec_astra/orbbec_astra.cpp Open streams
The first object will use the regular Video4Linux2 interface to access the color sensor. The second one
The first object will use the Video4Linux2 interface to access the color sensor. The second one
is using OpenNI2 API to retrieve depth data.
Before using the created VideoCapture objects you may want to setup stream parameters by setting
objects' properties. The most important parameters are frame width, frame height and fps:
Before using the created VideoCapture objects you may want to set up stream parameters by setting
objects' properties. The most important parameters are frame width, frame height and fps.
For this example, we’ll configure width and height of both streams to VGA resolution as that’s
the maximum resolution available for both sensors and we’d like both stream parameters to be the same:
@snippetlineno samples/cpp/tutorial_code/videoio/orbbec_astra/orbbec_astra.cpp Setup streams
@@ -113,8 +118,9 @@ After the VideoCapture objects are set up you can start reading frames from them
to avoid one stream blocking while another stream is being read. VideoCapture is not a
thread-safe class, so you need to be careful to avoid any possible deadlocks or data races.
Example implementation that gets frames from each sensor in a new thread and stores them
in a list along with their timestamps:
As there are two video sources that should be read simultaneously, it’s necessary to create two
threads to avoid blocking. Example implementation that gets frames from each sensor in a new thread
and stores them in a list along with their timestamps:
@snippetlineno samples/cpp/tutorial_code/videoio/orbbec_astra/orbbec_astra.cpp Read streams
@@ -130,17 +136,24 @@ VideoCapture can retrieve the following data:
-# data given from the color sensor is a regular BGR image (CV_8UC3).
When new data is available a reading thread notifies the main thread. A frame is stored in the
ordered list -- the first frame is the latest one:
When new data are available a reading thread notifies the main thread using a condition variable.
A frame is stored in the ordered list -- the first frame is the latest one. As depth and color frames
are read from independent sources two video streams may become out of sync even when both streams
are set up for the same frame rate. A post-synchronization procedure can be applied to the streams
to combine depth and color frames into pairs. The sample code below demonstrates this procedure:
@snippetlineno samples/cpp/tutorial_code/videoio/orbbec_astra/orbbec_astra.cpp Show color frame
@snippetlineno samples/cpp/tutorial_code/videoio/orbbec_astra/orbbec_astra.cpp Pair frames
Depth frames can be picked the same way from the `depthFrames` list.
In the code snippet above the execution is blocked until there are some frames in both frame lists.
When there are new frames, their timestamps are being checked -- if they differ more than a half of
the frame period then one of the frames is dropped. If timestamps are close enough, then two frames
are paired. Now, we have two frames: one containing color information and another one -- depth information.
In the example above retrieved frames are simply shown with cv::imshow function, but you can insert
any other processing code here.
After that, you'll have two frames: one containing color information and another one -- depth
information. In the sample images below you can see the color frame and the depth frame showing
the same scene. Looking at the color frame it's hard to distinguish plant leaves from leaves painted
on a wall, but the depth data makes it easy.
In the sample images below you can see the color frame and the depth frame representing the same scene.
Looking at the color frame it's hard to distinguish plant leaves from leaves painted on a wall,
but the depth data makes it easy.
![Color frame](images/astra_color.jpg)
![Depth frame](images/astra_depth.png)