What format is for Android camera with raw picture

问题:

I am trying to use data from Android picture. I do not like JPEG format, since eventually I will use gray scale data. YUV format is fine with me, since the first half part is gray-scale.

from the Android development tutorial,

public final void takePicture (Camera.ShutterCallback shutter, Camera.PictureCallback raw, Camera.PictureCallback postview, Camera.PictureCallback jpeg)

Added in API level 5

Triggers an asynchronous image capture. The camera service will initiate a series of callbacks to the application as the image capture progresses. The shutter callback occurs after the image is captured. This can be used to trigger a sound to let the user know that image has been captured. The raw callback occurs when the raw image data is available (NOTE: the data will be null if there is no raw image callback buffer available or the raw image callback buffer is not large enough to hold the raw image). The postview callback occurs when a scaled, fully processed postview image is available (NOTE: not all hardware supports this). The jpeg callback occurs when the compressed image is available. If the application does not need a particular callback, a null can be passed instead of a callback method.

It talks about "the raw image data". However, I find nowhere information about the format for the raw image data? Do you have any idea about that?

I want to get the gray-scale data of the picture taken by the photo, and the data are located in the phone memory, so it would not cost time to write/read from image files, or convert between different image formats. Or maybe I have to sacrifice some to get it??

回答1:

After some search, I think I found the answer: From the Android tutorial:

"The raw callback occurs when the raw image data is available (NOTE: the data will be null if there is no raw image callback buffer available or the raw image callback buffer is not large enough to hold the raw image)."

See this link (2011/05/10) Android: Raw image callback supported devices Not all devices support raw pictureCallback.

https://groups.google.com/forum/?fromgroups=#!topic/android-developers/ZRkeoCD2uyc (2009) The employee Dave Sparks at Google said:

"The original intent was to return an uncompressed RGB565 frame, but this proved to be impractical. " "I am inclined to deprecate that API entirely and replace it with hooks for native signal processing. "

Many people report the similar problem. See: http://code.google.com/p/android/issues/detail?id=10910

Since many image processing processes are based on gray scale images, I am looking forward gray scale raw data in the memory produced for each picture by the Android.

回答2:

You may have some luck with getSupportedPictureFormats(). If it lists some YUV format, you can use setPictureFormat() and the desired resolution, and ciunterintuitively you will get the uncompressed high quality image in JpegPreview callback, from which grayscale (a.k.a. luminance) can be easily extracted.

Most devices will only list JPEG as a valid choice. That's because they perform compression in hardware, on the camera side. Note that the data transfer from camera to application RAM is often the bottleneck; if you can use stagefright hw JPEG decoder, you will actually get the result faster.

回答3:

The biggest problem with using the raw callback is that many developers have trouble with getting anything returned on many phones.

If you are satisfied with just the YUV array, your camera preview SurfaceView can implement PreviewCallback and you can add the onPreviewFrame method to your class. This function will allow you direct access to the YUV array for every frame. You can fetch it when you choose.

EDIT: I should specify that I was assuming you were building a custom camera application in which you extended SurfaceView for a custom camera preview surface. In order to follow my advice you will need to build a custom camera. If you are trying to do things quickly though I suggest building a new bitmap out of the JPEG data where you implement the greyscale yourself.