algorithm - How does one convert 16-bit RGB565 to 24-bit RGB888?

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 want to map each of these from a 5/6 bit space to an 8 bit space.

• 5 bits = 32 values
• 6 bits = 64 values
• 8 bits = 256 values

The code you're using is taking the naive approach that x5 * 256/32 = x8 where 256/32 = 8 and multiplying by 8 is left shift 3 but, as you say, this doesn't necessarily fill the new number space "correctly". 5 to 8 for max value is 31 to 255 and therein lies your clue to the solution.

x8 = 255/31 * x5
x8 = 255/63 * x6

where x5, x6 and x8 are 5, 6 and 8 bit values respectively.

Now there is a question about the best way to implement this. It does involve division and with integer division you will lose any remainder result (round down basically) so the best solution is probably to do floating point arithmetic and then round half up back to an integer.

This can be sped up considerably by simply using this formula to generate a lookup table for each of the 5 and 6 bit conversions.