Android NDK gl使用Java Nio字节缓冲区获取纹理图像

Question

I am using Java side direct nio buffer which is holding pixel data from a bitmap and use it in the NDK side as a gl texture. Basically I am not able to read the Java nio buffer pixels correctly in the c/c++ gl draw call.

It appears that the Java.ByteBuffer that is filled with pixels on Java side is not directly compatible with NDK side gl which requires unsigned bytes ( java byte is apparently 32 bit)

so a single white pixel on Java side :

int size = 1;
ByteBuffer vv = ByteBuffer.allocateDirect(size_t*4);
vv.order(ByteOrder.nativeOrder());
vv.put((byte)255); // R
vv.put((byte)255); // G
vv.put((byte)255); // B
vv.put((byte)255); // A
vv.position(0);
... //code to send the buffer address to JNI/NDK gl side
... //

will be drawn on NDK side as a black pixel;

I realize that the actual unsigned byte value of this buffer maybe be negative - how do I correct for this. Also - I will need to convert byte[] that is an image data derived from a Bitmap on Java side to be used in the NDK side texture once I solve this issue.

Thanks in advance!

Answer 1

First "byte" in Java is 8 bit (signed) not 32 bit. I don't use nio.ByteBufffer (though one can). byte[] array works much better and simpler.

in AndroidBitmap.java

public class AndroidBitmap {

    public static native void updateBitmap(android.graphics.Bitmap bitmap, byte[] data, int w, int h, int bpp);

}

in AndroidBitmap.c

jboolean Java_jni_AndroidBitmap_updateBitmap(JNIEnv* env, jobject that, jobject bitmap, jbyteArray data, jint w, jint h, jint bpp) {

    jbyte* a = (*env)->GetByteArrayElements(env, data, NULL);
    jsize  bytes = (*env)->GetArrayLength(env, data);

    AndroidBitmapInfo info = {0};
    int r = AndroidBitmap_getInfo(env, bitmap, &info);
    if (r != 0) {
        // … "AndroidBitmap_getInfo() failed ! error=%d", r
        return false;
    }
    int width = info.width;
    int height = info.height;
    if (info.format != ANDROID_BITMAP_FORMAT_RGBA_8888 && info.format != ANDROID_BITMAP_FORMAT_A_8) {
        // "Bitmap format is not RGBA_8888 or A_8"
        return false;
    }
    int bytesPerPixel = info.format == ANDROID_BITMAP_FORMAT_RGBA_8888 ? 4 : 1;
    void* pixels = null;
    r = AndroidBitmap_lockPixels(env, bitmap, &pixels);
    if (r != 0) {
        // ..."AndroidBitmap_lockPixels() failed ! error=%d", r
        return false;
    }
    if (w == width && h == height && bytesPerPixel == bpp) {
        memcpy(pixels, a, width * height * bytesPerPixel);
    } else if (bytesPerPixel == 4 && bpp == 1) {
        grayscaleToRGBA(pixels, &info, data, w, h);
    } else {
        assertion(bytesPerPixel == 4 && bpp == 1, "only grayscale -> RGBA is supported bytesPerPixel=%d bpp=%d", bytesPerPixel, bpp);
    }
    AndroidBitmap_unlockPixels(env, bitmap);
    (*env)->ReleaseByteArrayElements(env, data, a, 0);
    return true;
}

Hope this helps.