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:

uint16_t buf;
while (read(0, &buf, sizeof buf)) {
    unsigned char red = (buf & 0xf800) >> 11;
    unsigned char green = (buf & 0x07c0) >> 5;
    unsigned char blue = buf & 0x003f;
    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.

(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?