Question

Hi my task is to display a set of decoded frames in android. In my native code i have a character(char ) pointer which holds the address of the decoded frame. I want to display this frame on my device so i got a hint from:
Displaying YUV Image in Android
hence in my activity class i wrote the following function:

public void displayFrame(byte[] data, int fwidth,int fheight){

    ImageView frameImgView=(ImageView) findViewById(R.id.imageView2);

    ByteArrayOutputStream out=new ByteArrayOutputStream();
    YuvImage yuvimg=new YuvImage(data, ImageFormat.NV21, fwidth, fheight, null);
    yuvimg.compressToJpeg(new Rect(0, 0, fwidth, fheight), 100, out);
    byte[] imageBytes = out.toByteArray();
    Bitmap image = BitmapFactory.decodeByteArray(imageBytes, 0, imageBytes.length);
    frameImgView.setImageBitmap(image);
    return;
}

Hence from my native code i need to call this function in java. From the link:
http://www.altdevblogaday.com/2011/12/09/running-native-code-on-android-part-1/
i got to know how to call a java function from native c.
However to call a java function we need an environment variable to do that. Im my java class i hav declared the main function as native and generated a header for that main function:

JNIEXPORT jint JNICALL Java_com_example_decoder_Decoder_mainFunction(JNIEnv *env, jclass jobj, jint argc, jstring argv1, jstring argv2);

however my mainFunction calls a function “fwriter(some pointer, some pointer)” which has a pointers as parameters. How do i get environment variable for the same. I know that in java class i need to declare “fwriter” function as native but what do i represent pointers as??

eg: my c function is:

void fwriter(int *ptr, char *ptr)
{
  ....
}

In my java class how do i declare this function as native??
Please help.
Any other methods to diaplay YUV in java/Android will also be appreciated.
Thank you.

Answer 1

Whilst JPEG encoding and decoding may be hardware accelerated on most phones, I would expect a simple RGB->YUV conversion for-loop you wrote yourself to be much faster (fast enough even in Java without using a native library) and simpler.

Here is the formula. If you are using OpenGLES to display, consider using a fragment shader to do the conversion on-the-fly.

Finally, here’s the integer-only code you can use on the CPU:

Ytmp =      4768 * (Y - 16);
R = (Ytmp + 6537 * (V - 128)) >> 12;
G = (Ytmp - 3330 * (V - 128) - 1602 * (U - 128)) >> 12;
B = (Ytmp + 8266 * (U - 128)) >> 12;