How does one convert 16-bit RGB565 to 24-bit RGB88

Question

I’ve got my hands on a 16-bit rgb565 image (specifically, an Android framebuffer dump), and I would like to convert it to 24-bit rgb888 for viewing on a normal monitor.

The question is, how does one convert a 5- or 6-bit channel to 8 bits? The obvious answer is to shift it. I started out by writing this:

puts("P6 320 480 255");
uint16_t buf;
while (read(0, &buf, sizeof buf)) {
    unsigned char red = (buf & 0xf800) >> 11;
    unsigned char green = (buf & 0x07e0) >> 5;
    unsigned char blue = buf & 0x001f;
    putchar(red << 3);
    putchar(green << 2);
    putchar(blue << 3);
}

However, this doesn’t have one property I would like, which is for 0xffff to map to 0xffffff, instead of 0xf8fcf8. I need to expand the value in some way, but I’m not sure how that should work.

The Android SDK comes with a tool called ddms (Dalvik Debug Monitor) that takes screen captures. As far as I can tell from reading the code, it implements the same logic; yet its screenshots are coming out different, and white is mapping to white.

Here’s the raw framebuffer, the smart conversion by ddms, and the dumb conversion by the above algorithm. Note that the latter is slightly darker and greener.

(By the way, this conversion is implemented in ffmpeg, but it’s just performing the dumb conversion listed above, leaving the LSBs at all zero.)

I guess I have two questions:

• What’s the most sensible way to convert rgb565 to rgb888?
• How is DDMS converting its screenshots?

Answer 1

There is an error jleedev !!!

unsigned char green = (buf & 0x07c0) >> 5;
unsigned char blue = buf & 0x003f;

the good code

unsigned char green = (buf & 0x07e0) >> 5;
unsigned char blue = buf & 0x001f;

Cheers, Andy

Answer 2

I used the following and got good results. Turned out my Logitek cam was 16bit RGB555 and using the following to convert to 24bit RGB888 allowed me to save as a jpeg using the smaller animals ijg: Thanks for the hint found here on stackoverflow.

// Convert a 16 bit inbuf array to a 24 bit outbuf array
BOOL JpegFile::ByteConvert(BYTE* inbuf, BYTE* outbuf, UINT width, UINT height)
{     UINT row_cnt, pix_cnt;     
      ULONG off1 = 0, off2 = 0;
      BYTE  tbi1, tbi2, R5, G5, B5, R8, G8, B8;

      if (inbuf==NULL)
          return FALSE;

      for (row_cnt = 0; row_cnt <= height; row_cnt++) 
      {     off1 = row_cnt * width * 2;
            off2 = row_cnt * width * 3;
            for(pix_cnt=0; pix_cnt < width; pix_cnt++)
            {    tbi1 = inbuf[off1 + (pix_cnt * 2)];
                 tbi2 = inbuf[off1 + (pix_cnt * 2) + 1];
                 B5 = tbi1 & 0x1F;
                 G5 = (((tbi1 & 0xE0) >> 5) | ((tbi2 & 0x03) << 3)) & 0x1F;
                 R5 = (tbi2 >> 2) & 0x1F;
                 R8 = ( R5 * 527 + 23 ) >> 6;
                 G8 = ( G5 * 527 + 23 ) >> 6;
                 B8 = ( B5 * 527 + 23 ) >> 6;
                 outbuf[off2 + (pix_cnt * 3)] = R8;
                 outbuf[off2 + (pix_cnt * 3) + 1] = G8;
                 outbuf[off2 + (pix_cnt * 3) + 2] = B8;
            }
       }
       return TRUE;
}