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I am reading the code about Android Camera2 APIs from here: https://github.com/googlesamples/android-Camera2Basic

And it is confusing in this lines: https://github.com/googlesamples/android-Camera2Basic/blob/master/Application/src/main/java/com/example/android/camera2basic/Camera2BasicFragment.java#L570-L574

that the previewRequest builder only add surface, which is the TextureView to show, as target. But the following line actually add both as the targets. As I understand, this should not fire the "OnImageAvailable" Lisenter during preview, no? So why this add the imagereader's surface here?

I tried to removed this imagereader's surface here but got error when I really want to capture an image.....

SOOO CONFUSING!!!

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  • ImageReader is used to get captured image data to save in a file. And the surface of ImageReader must be added to CaptureSession before you use CaptureRequest.Builder to capture image, according to doc Commented Aug 20, 2015 at 2:53

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You need to declare all output Surfaces that image data might be sent to at the time you create a CameraCaptureSession. This is just the way the framework is designed.

Whenever you create a CaptureRequest, you add a (list of) target output Surface(s). This is where the image data from the captured frame will go- it may be a Surface associated with a TextureView for displaying, or with an ImageReader for saving, or with an Allocation for processing, etc. (A Surface is really just a buffer which can take the data output by the camera. The type of object that buffer is associated with determines how you can access/work with the data.)

You don't have to send the data from each frame to all registered Surfaces, but it has to be sent to a subset of them. You can't add a Surface as a target to a CaptureRequest if it wasn't registered with the CameraCaptureSession when it was created. Well, you can, but passing it to the session will cause a crash, so don't.

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Hi Sumner, thanks for the explanation. In my understanding, there are two different capture sessions, one for preview and one for real capture. I am confused because I should only register the image reader's surface to the real capture session, no?
CameraCaptureSessions are large and costly and rarely created. Typically, you decide what output surfaces you want to send to for your various purposes (display, saving, etc), and then create a single Session with each of these registered to it, so that you can use them whenever you want to by issuing CaptureRequests to that section. You should notice in camera2basic that they only create one CameraCaptureSession.
A CameraCaptureSession is like an configuration for the entire camera pipeline, and the Surfaces registered with it are output pipes- hardwired into the infrastructure that gets created. You don't want to have to build multiple infrastructures, you just set up one for multiple outputs.
@Sumber Thanks! And I am kind of interested to use Allocation to store an image buffer, I did some search and it seems confusing, would you please give me a pointer to an example where I can use Allocation properly? (And maybe how to proper get bitmap from this?) THANKS!!!
I usually actually only use display outputs and ImageReaders for saving various formats to disk, so then I can work on them in other environments, so I am unfamiliar with using Allocations and RenderScript for on-device image processing of bitmaps... sorry! There must be tutorials out there for that aspect of it, though!
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