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

You could shift and then or with the most significant bits; i.e.

Red 10101 becomes 10101000 | 101 => 10101101
    12345         12345---   123    12345123

This has the property you seek, but it’s not the most linear mapping of values from one space to the other. It’s fast, though. 🙂

Cletus’ answer is more complete and probably better. 🙂