Implementation of QOI encoding and decoding in Go.

For more info about QOI, see:

• the specification at https://qoiformat.org/qoi-specification.pdf
• the reference implementation at https://github.com/phoboslab/qoi

Even though this implementation is very straightforward and not particularly optimised (there is not much to optimise with such a simple format anyway), I found that it is much faster than the existing Go implementations:

Full benchmark comparisons can be found here.

I think a lot of the difference is caused by these libraries making huge amounts of unneeded allocations.

Compared to the reference C implem, I didn't make a full benchmark, but I have some evidence that this package would be about 20-50% slower. Encoding especially takes a hit because it needs to compare pixels a lot and Go doesn't have (C-style) union types, so 4 bytes cannot be cast to an uint32 for free to make a single comparison, it needs to do 4 compares.

The only thing I did that I would call "optimisation" (i.e. apart from doing sensible allocations) simply consists of replacing the check for whether we can use an OP_DIFF in the encoder (and similarly for OP_LUMA):

if dr >= -2 && dr <= 1 && dg >= -2 && dg <= 1 && db >= -2 && db <= 1 { ...

with this equivalent version:

if uint8((dr+2)|(dg+2)|(db+2))&0xfc == 0 { ...

This avoids having too many branches and for photographic images where OP_DIFF and OP_LUMA are plentiful, this can speed up encoding by more than 10 %.