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91 lines
4.1 KiB
C++
91 lines
4.1 KiB
C++
/*
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* Copyright (c) 2023, Nico Weber <thakis@chromium.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include <AK/Debug.h>
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#include <AK/Endian.h>
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#include <AK/Format.h>
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#include <AK/MemoryStream.h>
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#include <AK/Vector.h>
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#include <LibGfx/ImageFormats/WebPLoaderLossy.h>
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// Lossy format: https://datatracker.ietf.org/doc/html/rfc6386
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namespace Gfx {
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// https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossy
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// https://datatracker.ietf.org/doc/html/rfc6386#section-19 "Annex A: Bitstream Syntax"
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ErrorOr<VP8Header> decode_webp_chunk_VP8_header(ReadonlyBytes vp8_data)
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{
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if (vp8_data.size() < 10)
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return Error::from_string_literal("WebPImageDecoderPlugin: 'VP8 ' chunk too small");
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// FIXME: Eventually, this should probably call into LibVideo/VP8,
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// and image decoders should move into LibImageDecoders which depends on both LibGfx and LibVideo.
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// (LibVideo depends on LibGfx, so LibGfx can't depend on LibVideo itself.)
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// https://datatracker.ietf.org/doc/html/rfc6386#section-4 "Overview of Compressed Data Format"
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// "The decoder is simply presented with a sequence of compressed frames [...]
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// The first frame presented to the decompressor is [...] a key frame. [...]
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// [E]very compressed frame has three or more pieces. It begins with an uncompressed data chunk comprising 10 bytes in the case of key frames"
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u8 const* data = vp8_data.data();
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// https://datatracker.ietf.org/doc/html/rfc6386#section-9.1 "Uncompressed Data Chunk"
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u32 frame_tag = data[0] | (data[1] << 8) | (data[2] << 16);
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bool is_key_frame = (frame_tag & 1) == 0; // https://www.rfc-editor.org/errata/eid5534
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u8 version = (frame_tag & 0xe) >> 1;
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bool show_frame = (frame_tag & 0x10) != 0;
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u32 size_of_first_partition = frame_tag >> 5;
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if (!is_key_frame)
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return Error::from_string_literal("WebPImageDecoderPlugin: 'VP8 ' chunk not a key frame");
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if (!show_frame)
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return Error::from_string_literal("WebPImageDecoderPlugin: 'VP8 ' chunk has invalid visibility for webp image");
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if (version > 3)
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return Error::from_string_literal("WebPImageDecoderPlugin: unknown version number in 'VP8 ' chunk");
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u32 start_code = data[3] | (data[4] << 8) | (data[5] << 16);
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if (start_code != 0x2a019d) // https://www.rfc-editor.org/errata/eid7370
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return Error::from_string_literal("WebPImageDecoderPlugin: 'VP8 ' chunk invalid start_code");
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// "The scaling specifications for each dimension are encoded as follows.
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// 0 | No upscaling (the most common case).
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// 1 | Upscale by 5/4.
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// 2 | Upscale by 5/3.
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// 3 | Upscale by 2."
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// This is a display-time operation and doesn't affect decoding."
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u16 width_and_horizontal_scale = data[6] | (data[7] << 8);
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u16 width = width_and_horizontal_scale & 0x3fff;
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u8 horizontal_scale = width_and_horizontal_scale >> 14;
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u16 heigth_and_vertical_scale = data[8] | (data[9] << 8);
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u16 height = heigth_and_vertical_scale & 0x3fff;
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u8 vertical_scale = heigth_and_vertical_scale >> 14;
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dbgln_if(WEBP_DEBUG, "version {}, show_frame {}, size_of_first_partition {}, width {}, horizontal_scale {}, height {}, vertical_scale {}",
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version, show_frame, size_of_first_partition, width, horizontal_scale, height, vertical_scale);
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return VP8Header { version, show_frame, size_of_first_partition, width, horizontal_scale, height, vertical_scale, vp8_data.slice(10) };
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}
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ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8_contents(VP8Header const& vp8_header, bool include_alpha_channel)
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{
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auto bitmap_format = include_alpha_channel ? BitmapFormat::BGRA8888 : BitmapFormat::BGRx8888;
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// Uncomment this to test ALPH decoding for WebP-lossy-with-alpha images while lossy decoding isn't implemented yet.
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#if 0
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return Bitmap::create(bitmap_format, { vp8_header.width, vp8_header.height });
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#else
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// FIXME: Implement webp lossy decoding.
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(void)vp8_header;
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(void)bitmap_format;
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return Error::from_string_literal("WebPImageDecoderPlugin: decoding lossy webps not yet implemented");
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#endif
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}
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}
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