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LibGfx/WebPWriter: Use one-element huffman tree for opaque images

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That way, we can write 0 instead of 8 bits for every alpha byte.
Reduces the size of sunset-retro.png when saved as a webp file
from 3 MiB to 2.25 MiB, without affecting encode speed.

Once we use CanonicalCodes we'll get this for free for all channels,
but opaque images are common enough that it feels worth it to do this
before then.
This commit is contained in:
Nico Weber 2024-05-04 20:19:44 -04:00 committed by Tim Flynn
parent 7d93f7fc99
commit 51fc51d0b5
1 changed files with 16 additions and 2 deletions
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18
Userland/Libraries/LibGfx/ImageFormats/WebPWriter.cpp
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@ -116,7 +116,10 @@ static ErrorOr<void> write_VP8L_image_data(Stream& stream, Bitmap const& bitmap)
if (alphabet_sizes[0] > 288)
return Error::from_string_literal("Invalid alphabet size");
for (int i = 0; i < 4; ++i) {
bool all_pixels_are_opaque = are_all_pixels_opaque(bitmap);
int number_of_full_channels = all_pixels_are_opaque ? 3 : 4;
for (int i = 0; i < number_of_full_channels; ++i) {
TRY(bit_stream.write_bits(0u, 1u)); // Normal code length code.
// Write code length codes.
@ -146,6 +149,14 @@ static ErrorOr<void> write_VP8L_image_data(Stream& stream, Bitmap const& bitmap)
// (This is different from deflate, which needs 1 bit per element.)
}
if (all_pixels_are_opaque) {
// Use a simple 1-element code.
TRY(bit_stream.write_bits(1u, 1u)); // Simple code length code.
TRY(bit_stream.write_bits(0u, 1u)); // num_symbols - 1
TRY(bit_stream.write_bits(1u, 1u)); // is_first_8bits
TRY(bit_stream.write_bits(255u, 8u)); // symbol0
}
// For code #5, use a simple empty code, since we don't use this yet.
TRY(bit_stream.write_bits(1u, 1u)); // Simple code length code.
TRY(bit_stream.write_bits(0u, 1u)); // num_symbols - 1
@ -164,7 +175,10 @@ static ErrorOr<void> write_VP8L_image_data(Stream& stream, Bitmap const& bitmap)
TRY(bit_stream.write_bits(Compress::reverse8_lookup_table[g], 8u));
TRY(bit_stream.write_bits(Compress::reverse8_lookup_table[r], 8u));
TRY(bit_stream.write_bits(Compress::reverse8_lookup_table[b], 8u));
TRY(bit_stream.write_bits(Compress::reverse8_lookup_table[a], 8u));
// If all pixels are opaque, we wrote a one-element huffman table for alpha, which needs 0 bits per element.
if (!all_pixels_are_opaque)
TRY(bit_stream.write_bits(Compress::reverse8_lookup_table[a], 8u));
}
// FIXME: Make ~LittleEndianOutputBitStream do this, or make it VERIFY() that it has happened at least.