Real-time camera viewfinder filters in Native code
This article explains how to create real-time camera filters, using native code (C++).
Windows Phone 8
Note: This is an "internal" entry in the Windows Phone 8 Wiki Competition 2012Q4. The author is a Nokia / Microsoft employee.
In this article, we will have a quick look how one can create real-time filters for the camera viewfinder, using native code (C++). For sake of simplicity, the example will implement a simple gray filter. For that we will use the new Windows PRT camera API that are now available in the WP8 SDK. We will also use the possibility to write C++ code, another new functionality of the WP8 SDK. The end result will look like this:
The source code of the full project can be downloaded from the link in top right corner of this page.
Why Native filters?
Microsoft has published a very similar example to this project, where they convert the camera viewfinder images to grayscale, which works well in WP7 and WP8. Get it here. This works well, but the gray filter is quite simple, more complicated filters will require more computation, and the CPU limits are quickly reached when processing VGA frames (604x480) at 30 frame per seconds. The speed gain by going closer to the metal may be needed for complex algorithm. Also, you might already have your own image filters written in C++ for other platforms, that you can reuse without converting them to C#. Finally, as we will see in other wiki entries, the native side opens further optimization possibilities, like using DirectX or the ARM Neon instruction set.
Setting up the viewfinder
Our UI will be XAML based. The UI will control everything, while the C++ side is rather dumb, simply executing the filtering when it is asked to. Let's first create the projects for both the XAML and the C++ components:
- Start by creating a new project, of type Windows Phone App. You will find the template under the Visual C#/Windows Phone category. That will be our UI.
- Add a new project to your solution, of type Windows Phone Runtime Component. That template is under the Visual C++/Windows Phone category. That will be our image filter.
We then add a live camera stream to our UI. That is easily done by :
- In your XAML, define a rectangle that will be painted using a video brush:
<Grid x:Name="LayoutRoot" Background="Transparent">
<Rectangle Width="640" Height="480" Canvas.ZIndex="1">
<VideoBrush x:Name="viewfinderBrush" />
- In the page loaded event, Create a PhotoCaptureDevice, and set it as the source of the video brush:
Windows.Foundation.Size resolution = new Windows.Foundation.Size(640, 480);
m_camera = await PhotoCaptureDevice.OpenAsync(CameraSensorLocation.Back, resolution);
By now, with these 10 lines of codes, you should have an application with a functional camera ! Note that in the last step, we have used the Windows PRT class Windows.Phone.Media.Capture.PhotoCaptureDevice which is new to Windows Phone 8. In WP7, one would use the Silverlight/.NET class Microsoft.Devices.PhotoCamera().
Talking with the native(s)
So far, that was easy, but now we need to be a bit careful. We will handle quite huge amount of data. The viewfinder frames, 640x480 pixels refreshed at a rate of 30 frames per second, will go back and forth between managed code (our UI) and the native site (our C++ filter). That's a lot of pixels per seconds! We have to avoid any useless copy operations, as copies (memory accesses) will hurt our performance.
We will implement the following sequence diagram, for each new viewfinder frame coming from the camera:
To get the data of the frames coming from the camera, we will have to call to the platform functionality Windows::Phone::Media::Capture::ICameraCaptureDevice::GetPreviewBufferArgb. Let's look at its definition:
void GetPreviewBufferArgb(Platform::WriteOnlyArray<int, 1U>^ pixels)
That function fills an array that we provide with the camera data. That's a copy operation we can't avoid. The buffer is of type Platform::WriteOnlyArray, which according to MSDN documentation, is to be used when the caller passes an array for the method to fill . Makes sense. We will also use that buffer type to communicate between our managed component and our native component
The public interface of our native component will be:
public ref class WindowsPhoneRuntimeComponent sealed
void Initialize(Windows::Phone::Media::Capture::PhotoCaptureDevice^ captureDevice);
void NewViewfinderFrame( Platform::WriteOnlyArray<int,1U>^ frameData);
On the managed side, we allocate the buffer when the application is initialized, and reuse that buffer for every frames:
m_frameData = new int[(int)m_camera.PreviewResolution.Height * (int)m_camera.PreviewResolution.Width];
Every time the camera subsystem fires a PreviewFrameAvailable event we will call the native NewViewfinderFrame method who will take care of filling the buffer with camera data and filter that data.
When the UI side receives back the filtered camera data buffer, it is ready to be displayed. The UI side will do that with the following :
m_processingFrame = false;
Again, an unavoidable copy! This time the data is copied to a WriteableBitmap, m_wb. At the initialization phase, we defined m_wb as the source for the XAML Image component that displays our filtered viewfinder.
So, we end up copying two times the camera data, first from camera subsystem into our application, then from our application to the display subsystem.
That covers the big lines of the data handling of the application. I skipped the not-so-interesting code, get the source code of the full project from the link in top right corner of this page. The last thing to do is the filtering itself.
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