Convert RGB565 to Greyscale (8 bits)

I am trying to convert a rgb565 image (video stream from the Android phone camera) into a greyscale (8 bits) image.

So far I got to the following code (the conversion is computed in native code using the Android NDK). Note that my input image is 640*480 and I want to crop it to make it fit in a 128*128 buffer.

#define RED(a)      ((((a) & 0xf800) >> 11) << 3)
#define GREEN(a)    ((((a) & 0x07e0) >> 5) << 2)
#define BLUE(a)     (((a) & 0x001f) << 3)

typedef unsigned char byte;

void toGreyscale(byte *rgbs, int widthIn, int heightIn, byte *greyscales)
{
    const int textSize = 128;
    int x,y;
    short* rgbPtr = (short*)rgbs;
    byte *greyPtr = greyscales;

    // rgbs arrives in RGB565 (16 bits) format开发者_StackOverflow
    for (y=0; y<textSize; y++)
    {
        for (x=0; x<textSize; x++)
        {
            short pixel = *(rgbPtr++);
            int r = RED(pixel);
            int g = GREEN(pixel);
            int b = BLUE(pixel);

            *(greyPtr++) = (byte)((r+g+b) / 3);
        }
        rgbPtr += widthIn - textSize;
    }
}

The image is sent to the function like this

jbyte* cImageIn = env->GetByteArrayElements(imageIn, &b);
jbyte* cImageOut = (jbyte*)env->GetDirectBufferAddress(imageOut);
toGreyscale((byte*)cImageIn, widthIn, heightIn, (byte*)cImageOut);

The result I get is a horizontally-reversed image (no idea why...the UVs to display the result are correct...), but the biggest problem is that only the red channel is actually correct when I display them separately. The green and blue channels are all messed up and I have no idea why. I checked on the Internet and all the resources I found showed that the masks I am using are correct. Any idea where the mistake could be?

Thanks!

May be an endianess issue?

You could check quickly by reversing the two bytes of your 16 bits word before shifting out the RGB components.