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LibGfx/WebPWriter: Separate symbol generation from statistics collection

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We now do this in two passes instead of in one. This is virtually free
performance-wise, and allows nicer factoring.

Perf numbers after this change (see previous commit for perf numbers
before):

    Benchmark 1: image -o sunset-retro.webp sunset-retro.bmp
      Time (mean ± σ): 26.7 ms ± 0.8 ms

    Benchmark 1: animation -o 7z7c.webp 7z7c.gif
      Time (mean ± σ): 14.5 ms ± 0.6 ms

    Benchmark 1: animation -o wow.webp wow.gif
      Time (mean ± σ): 108.2 ms ± 2.2 ms
This commit is contained in:
Nico Weber 2024-06-30 13:57:06 +02:00
parent 85739def89
commit 580134241e
1 changed files with 14 additions and 11 deletions
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25
Userland/Libraries/LibGfx/ImageFormats/WebPWriterLossless.cpp
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@ -284,12 +284,6 @@ static ErrorOr<void> write_VP8L_coded_image(ImageKind image_kind, LittleEndianOu
Vector<Symbol> symbols;
TRY(symbols.try_ensure_capacity(bitmap.size().area()));
Array<Array<u16, 256>, 4> symbol_frequencies {};
static constexpr auto saturating_increment = [](u16& value) {
if (value < UINT16_MAX)
value++;
};
auto emit_literal = [&](ARGB32 pixel) {
Symbol symbol;
@ -298,11 +292,6 @@ static ErrorOr<void> write_VP8L_coded_image(ImageKind image_kind, LittleEndianOu
symbol.b = pixel;
symbol.a = pixel >> 24;
symbols.append(symbol);
saturating_increment(symbol_frequencies[0][symbol.green_or_length_or_index]);
saturating_increment(symbol_frequencies[1][symbol.r]);
saturating_increment(symbol_frequencies[2][symbol.b]);
saturating_increment(symbol_frequencies[3][symbol.a]);
};
for (ARGB32 const* it = bitmap.begin(), * end = bitmap.end(); it != end; ++it) {
@ -313,6 +302,20 @@ static ErrorOr<void> write_VP8L_coded_image(ImageKind image_kind, LittleEndianOu
// We do use huffman coding by writing a single prefix-code-group for the entire image.
// FIXME: Consider using a meta-prefix image and using one prefix-code-group per tile.
Array<Array<u16, 256>, 4> symbol_frequencies {};
static constexpr auto saturating_increment = [](u16& value) {
if (value < UINT16_MAX)
value++;
};
for (Symbol const& symbol : symbols) {
saturating_increment(symbol_frequencies[0][symbol.green_or_length_or_index]);
saturating_increment(symbol_frequencies[1][symbol.r]);
saturating_increment(symbol_frequencies[2][symbol.b]);
saturating_increment(symbol_frequencies[3][symbol.a]);
}
Array<Array<u8, 256>, 4> code_lengths {};
for (int i = 0; i < 4; ++i) {
// "Code [0..15] indicates literal code lengths." => the maximum bit length is 15.