...in addition to modifying in-place. This is needed for bitpacking
support for the color indexing transform (and it could also be used
to make the color indexing transform return an indexed bitmap, which
is something we could do if that's the last transform that's applied).
No behavior change.
Reduces the time to run
Build/lagom/image ~/src/libwebp/webp_js/test_webp_wasm.webp -o tmp.png
from 0.5s to 0.25s.
Before, 60% of the time was spent decoding webp and 40% writing png.
Now, 16% of the time was spent decoding webp and 84% writing png.
That means png writing takes 0.2s, and webp decoding time went from
0.3s to 0.05s.
A template expression without explicit return type deduces its return
type as if for a function whose return type is declared auto. That
does deduce return-by-value, while `decltype(auto)` would deduce
return-by-reference. Explictly saying `decltype(auto)` would work
too, but writing out the type is maybe easier to understand.
No behavior change other than being much faster.
The previous attempt was in commit e5e9d3b877, where I thought
max_symbol describes how many code lengths should be read.
But it looks like it instead describes how many code length input
symbols should be read. (The two aren't the same since one code length
input symbol can produce several code lengths.)
I still agree with the commit description of e5e9d3b877 that the spec
isn't very clear on this :)
This time I've found a file that sets max_symbol and with this change
here, that file decodes correctly. (It's Qpalette.webp, which I'm about
to add as a test case.)
WebP lossless differs from deflate in how it handles 1-element codes.
Deflate consumes one bit from the bitstream to produce the element,
while webp lossless consumes 0 bits. Add a wrapper class to handle
this case.
The `static` here meant we always kept the alphabet sizes of the
first image we happened to load -- and a single webp lossless image
can store several helper images used during decoding.
Usually, the helper images wouldn't use a color cache but the main
image would, but the main image would then use the first entry from
the helper images due to the `static`, which led us to not decoding
the codes for the color cache symbols.
The spec is at best misleading here, suggesting that max_symbol should
be set to "num_code_lengths" if it's not explicitly stored.
But num_code_lengths doesn't mean the num_code_lengths mentioned a few
lines further up in the spec, but alphabet_size!
(I had to cheat and look at libwebp instead of the spec for this: See
vp8l_dec.c, ReadHuffmanCode() which passes alphabet_size to
ReadHuffmanCodeLengths() as num_symbols, and ReadHuffmanCodeLengths()
then sets max_symbol to that.)
I haven't yet found a file that uses max_symbol, so this isn't actually
tested. But it's close to what's in libwebp, so maybe it works!
Missing:
* Transform support (used by virtually all lossless webp files)
* Meta prefix / entropy image support
Working:
* Decoding of regular image streams
* Color cache
This happens to be enough to be able to decode
Tests/LibGfx/test-inputs/extended-lossless.webp
The canonical prefix code is very similar to deflate's, enough so that
this can use Compress::CanonicalCode (and take advantage of all the
recent performance improvements there).
