问题

I am currently working with Javacv which makes use of the public void onPreviewFrame(byte[] data, Camera camera) camera function.

Since camera is deprecated, I have been looking into camera2 and MediaProjection. Both of these libraries make use of the ImageReader class.

Currently I instantiate such an ImageReader with the following code:

ImageReader.newInstance(DISPLAY_WIDTH, DISPLAY_HEIGHT, PixelFormat.RGBA_8888, 2);

And attach an OnImageAvailableListener like this:

 private final ImageReader.OnImageAvailableListener mOnImageAvailableListener             = new ImageReader.OnImageAvailableListener() {          @Override         public void onImageAvailable(ImageReader reader) {              mBackgroundHandler.post(new processImage(reader.acquireNextImage()));         }

I have tried using the RGBA_8888 format with Javacv as per this thread: https://github.com/bytedeco/javacv/issues/298 but that doesn't work for me.

So instead I was thinking about using Render script to convert these Images to NV21(YUV_420_SP) format (which is the default output of camera in the onPreviewFrame function) as that worked for me with the camera library.

I have also read posts such as this one and this website to do the conversion, but these didn't work for me and I fear they will be too slow. Furthermore, my knowledge of C is severely limited. Basically it looks like I want the reverse operation of https://developer.android.com/reference/android/renderscript/ScriptIntrinsicYuvToRGB.html

So how can you go from an Image to a byte array that would match the output of the onPreviewFrame function i.e. NV21(YUV_420_SP) format? Preferably using Renderscript as it's faster.

Edit 1:

I have tried using ImageFormat.YUV_420_888 but to no avail. I kept getting errors like The producer output buffer format 0x1 doesn't match the ImageReader's configured buffer format. I switched back to PixelFormat.RGBA_8888 and discovered that there is only one plane in the ImageObject. The byte buffer of this plane is of size width*height*4 (one byte for R,G,B,A respectively). So I tried to convert this to NV21 format.

I modified code from this answer to produce the following function:

void RGBtoNV21(byte[] yuv420sp, byte[] argb, int width, int height) {     final int frameSize = width * height;      int yIndex = 0;     int uvIndex = frameSize;      int A, R, G, B, Y, U, V;     int index = 0;     int rgbIndex = 0;      for (int i = 0; i < height; i++) {         for (int j = 0; j < width; j++) {              R = argb[rgbIndex++];             G = argb[rgbIndex++];             B = argb[rgbIndex++];             A = argb[rgbIndex++];              // RGB to YUV conversion according to             // https://en.wikipedia.org/wiki/YUV#Y.E2.80.B2UV444_to_RGB888_conversion             Y = ((66 * R + 129 * G + 25 * B + 128) >> 8) + 16;             U = ((-38 * R - 74 * G + 112 * B + 128) >> 8) + 128;             V = ((112 * R - 94 * G - 18 * B + 128) >> 8) + 128;              // NV21 has a plane of Y and interleaved planes of VU each sampled by a factor             // of 2 meaning for every 4 Y pixels there are 1 V and 1 U.             // Note the sampling is every other pixel AND every other scanline.             yuv420sp[yIndex++] = (byte) ((Y < 0) ? 0 : ((Y > 255) ? 255 : Y));             if (i % 2 == 0 && index % 2 == 0) {                 yuv420sp[uvIndex++] = (byte) ((V < 0) ? 0 : ((V > 255) ? 255 : V));                 yuv420sp[uvIndex++] = (byte) ((U < 0) ? 0 : ((U > 255) ? 255 : U));             }             index++;         }     } }

and invoke it using:

int mWidth = mImage.getWidth(); int mHeight = mImage.getHeight();  byte[] rgbaBytes = new byte[mWidth * mHeight * 4]; mImage.getPlanes()[0].getBuffer().get(rgbaBytes); mImage.close();  byte[] yuv = new byte[mWidth * mHeight * 3 / 2]; RGBtoNV21(yuv, rgbaBytes, mWidth, mHeight);

Here mImage is an Image object produced by my ImageReader.

Yet this produces a result similar to this image:

which is clearly malformed. Seems like my conversion is off but I cannot figure what exactly.

回答1:

@TargetApi(19) public static byte[] yuvImageToByteArray(Image image) {      assert(image.getFormat() == ImageFormat.YUV_420_888);      int width = image.getWidth();     int height = image.getHeight();      Image.Plane[] planes = image.getPlanes();     byte[] result = new byte[width * height * 3 / 2];      int stride = planes[0].getRowStride();     assert (1 == planes[0].getPixelStride());     if (stride == width) {         planes[0].getBuffer().get(result, 0, width*height);     }     else {         for (int row = 0; row < height; row++) {             planes[0].getBuffer().position(row*stride);             planes[0].getBuffer().get(result, row*width, width);         }     }      stride = planes[1].getRowStride();     assert (stride == planes[2].getRowStride());     int pixelStride = planes[1].getPixelStride();     assert (pixelStride == planes[2].getPixelStride());     byte[] rowBytesCb = new byte[stride];     byte[] rowBytesCr = new byte[stride];      for (int row = 0; row < height/2; row++) {         int rowOffset = width*height + width/2 * row;         planes[1].getBuffer().position(row*stride);         planes[1].getBuffer().get(rowBytesCb);         planes[2].getBuffer().position(row*stride);         planes[2].getBuffer().get(rowBytesCr);          for (int col = 0; col < width/2; col++) {             result[rowOffset + col*2] = rowBytesCr[col*pixelStride];             result[rowOffset + col*2 + 1] = rowBytesCb[col*pixelStride];         }     }     return result; }

I have published another function with similar requirements. That new implementation tries to take advantage of the fact that quite often, YUV_420_888 is only NV21 in disguise.

来源：https://stackoverflow.com/questions/39067828/android-convert-imagereader-image-to-ycbcr-420-sp-nv21-byte-array-using-render

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