ALPH chunks are only used to give lossy webp frames an alpha channel,
and lossy decompression isn't implemented yet. So this can currently
never be hit in practice -- but for debugging and testing, I put in
some code behind `#if 0` for now that fake-decompresses a lossy webp
frame by returning an empty bitmap.
But this also doesn't implement compressed ALPH chunks yet, and I
couldn't find any lossy-webp-with-alpha files that use uncompressed
alpha channels. So the code here isn't really tested.
If someone comes along who wants to implement lossy webp decoding,
they now only need to implement decode_webp_chunk_VP8() and everything
might Just Work.
It also makes it possible to implement alpha chunk decoding before
implementing lossy decoding (by making decode_webp_chunk_VP8()
return an empty black bitmap for testing).
That way, animated and non-animated webp files use the same code path
to decode images. That will make it easier to add handling for lossy
decompression and for alpha chunk handling.
No behavior change.
With this, lossless animated webp files work :^)
(Missing: Loop count handling is not yet implemented, and alpha blending
between frames isn't done in linear space.)
read_webp_first_chunk() sensibly assumes that if decode_webp_header()
succeeds, there are at least sizeof(WebPFileHeader) bytes available.
But if the file size in the header was less than the size of the header,
decode_webp_header() would truncate the data to less than that and
happily report success. Now it no longer does that.
Found by clusterfuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=57843&sort=-opened&can=1&q=proj%3Aserenity
Prefix code decoding seems to work fairly well and produces a ton of
log output with `#define WEBP_DEBUG 1`, so remove the log lines.
(If needed it's always possible to just locally revert this commit.)
No behavior change, since WEBP_DEBUG isn't usually defined.
WebP lossless files that use a color indexing transform with <= 16
colors use pixel bundling to pack 2, 4, or 8 pixels into a single pixel.
If the image's width doesn't happen to be an exact multiple of the
bundling factor, we need to:
1. Use ceil_div() instead of just dividing the width by the bundling
factor
2. Remember the original width and use it instead of computing
reduced width times bundling factor
This does these changes, and adds a simple test for it -- it at least
checks that the decoded images have the right size.
(I created these images myself in Photoshop, and used the same
technique as for Tests/LibGfx/test-inputs/catdog-alert-*.webp
to create images with a certain number of colors.)
See the lengthy comment added in this commit for details.
With this, the webp lossless decoder is feature complete :^)
(...except for bug fixes and performance improvements, as always.)
...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).
