Camera API1 和 API2 设置获取试试预览的回调数据的格式
名字有点拗口哈，本文想说的是在获得camera 实时预览的数据时，怎么设置需要的数据的格式。
API 1
设置预览格式和预览回调：
parameters.setPreviewFormat(ImageFormat.NV21);
mCamera.setParameters(parameters);
mCamera.setPreviewCallback(previewCallback);
默认NV21 格式，传入参数支持NV21 和YV12，也可以通过 getSupportedPreviewFormats 查询支持的格式。
* @param pixel_format the desired preview picture format, defined by
* one of the {@link android.graphics.ImageFormat} constants. (E.g.,
* <var>ImageFormat.NV21</var> (default), or
* <var>ImageFormat.YV12</var>)
*
* @see android.graphics.ImageFormat
* @see android.hardware.Camera.Parameters#getSupportedPreviewFormats
*/
public void setPreviewFormat(int pixel_format) {
String s = cameraFormatForPixelFormat(pixel_format);
if (s == null) {
throw new IllegalArgumentException(
"Invalid pixel_format=" + pixel_format);
}
set(KEY_PREVIEW_FORMAT, s);
}
如果getSupportedPreviewFormats 返回 [17, 842094169]，可以查到这两种正是NV21 和YV12 格式的十进制表示：
/**
* YCrCb format used for images, which uses the NV21 encoding format.
*
* <p>This is the default format
* for {@link android.hardware.Camera} preview images, when not otherwise set with
* {@link android.hardware.Camera.Parameters#setPreviewFormat(int)}.</p>
*
* <p>For the {@link android.hardware.camera2} API, the {@link #YUV_420_888} format is
* recommended for YUV output instead.</p>
*/
public static final int NV21 = 0x11;
/**
...
* @see android.hardware.Camera.Parameters#setPreviewCallback
* @see android.hardware.Camera.Parameters#setPreviewFormat
* @see android.hardware.Camera.Parameters#getSupportedPreviewFormats
* </p>
*/
public static final int YV12 = 0x32315659;
另外 pixel format 和camera format 的转换如下
private String cameraFormatForPixelFormat(int pixel_format) {
switch(pixel_format) {
case ImageFormat.NV16: return PIXEL_FORMAT_YUV422SP;
case ImageFormat.NV21: return PIXEL_FORMAT_YUV420SP;
case ImageFormat.YUY2: return PIXEL_FORMAT_YUV422I;
case ImageFormat.YV12: return PIXEL_FORMAT_YUV420P;
case ImageFormat.RGB_565: return PIXEL_FORMAT_RGB565;
case ImageFormat.JPEG: return PIXEL_FORMAT_JPEG;
default: return null;
}
}
可能有同学问了“这是设置的预览的格式啊，怎么保证回调回来的数据也是这个格式呢？” 因为 previewCallback 的作用就是 used to deliver copies of preview frames as they are displayed. 拷贝预览帧。
API 2
预览回调是通过增加一个ImageReader 实时获取预览数据，设置的格式不区分预览和拍照，因为拍照也是通过ImageReader 的onImageAvailable 回调获得数据。
previewReader = ImageReader.newInstance(previewSize.getWidth(), previewSize.getHeight
(), ImageFormat.YUV_420_888, 2);
这里点名不支持NV21
* @param width The default width in pixels of the Images that this reader
* will produce.
* @param height The default height in pixels of the Images that this reader
* will produce.
* @param format The format of the Image that this reader will produce. This
* must be one of the {@link android.graphics.ImageFormat} or
* {@link android.graphics.PixelFormat} constants. Note that not
* all formats are supported, like ImageFormat.NV21.
* @param maxImages The maximum number of images the user will want to
* access simultaneously. This should be as small as possible to
* limit memory use. Once maxImages Images are obtained by the
* user, one of them has to be released before a new Image will
* become available for access through
* {@link #acquireLatestImage()} or {@link #acquireNextImage()}.
* Must be greater than 0.
* @see Image
*/
public static ImageReader newInstance(int width, int height, int format, int maxImages) {
return new ImageReader(width, height, format, maxImages, BUFFER_USAGE_UNKNOWN);
}
那API 1 中通过setPreviewFormat() 设置的ImageFormat.NV21 预览格式，在API 2 中设置成哪个格式呢？ 答案是 ImageFormat.YUV_420_888。至于为什么，源码跳到jni 里面去了，我没跟到后面的源码。 从dumpsys media.camera 的信息中看，preview-format 的值和API 1 中设置NV21 是一样的，都是 yuv420sp。
$ adb shell dumpsys media.camera | grep -E "preview-format|Latest set parameters|Device"
Device 0 is open. Client instance dump:
Latest set parameters:
preview-format: yuv420sp
preview-format-values: yuv420sp,yuv420p,nv12-venus
Device 1 is closed, no client instance
preview-format: yuv420sp
preview-format-values: yuv420sp,yuv420p,nv12-venus
Device 2 is open. Client instance dump:
Latest set parameters:
preview-format: yuv420p
preview-format-values: yuv420sp,yuv420p,nv12-venus
Device 3 is closed, no client instance
在API 2 上可以使用下面的方法查询支持输出的格式：
StreamConfigurationMap map = characteristics.get(
CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP);
Log.d(TAG, "get supported out put format: " + map.getOutputFormats());
通常来讲如果你设置一个不是getOutputFormats() 返回的格式，你是不能成功创建预览的（可以open），但不排除部分厂商在底层做了workaround（比如小米8）。