LibGfx/WebP: Move lossy decoder to its own file

Pure code move (except of removing `static` on the two public functions in the new header), not behavior change. There isn't a lot of lossy decoder yet, but it'll make implementing it more convenient. No behavior change.
2025-06-01 10:38:13 +00:00 · 2023-05-08 00:48:23 -04:00 · 2023-05-08 00:48:23 -04:00 · bc207fd0a0
commit bc207fd0a0
parent 83da7ad875
4 changed files with 120 additions and 81 deletions
--- a/Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
+++ b/Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
@ -0,0 +1,87 @@
+/*
+ * Copyright (c) 2023, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Debug.h>
+#include <AK/Endian.h>
+#include <AK/Format.h>
+#include <AK/MemoryStream.h>
+#include <AK/Vector.h>
+#include <LibGfx/ImageFormats/WebPLoaderLossy.h>
+
+// Lossy format: https://datatracker.ietf.org/doc/html/rfc6386
+
+namespace Gfx {
+
+// https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossy
+// https://datatracker.ietf.org/doc/html/rfc6386#section-19 "Annex A: Bitstream Syntax"
+ErrorOr<VP8Header> decode_webp_chunk_VP8_header(ReadonlyBytes vp8_data)
+{
+    if (vp8_data.size() < 10)
+        return Error::from_string_literal("WebPImageDecoderPlugin: 'VP8 ' chunk too small");
+
+    // FIXME: Eventually, this should probably call into LibVideo/VP8,
+    // and image decoders should move into LibImageDecoders which depends on both LibGfx and LibVideo.
+    // (LibVideo depends on LibGfx, so LibGfx can't depend on LibVideo itself.)
+
+    // https://datatracker.ietf.org/doc/html/rfc6386#section-4 "Overview of Compressed Data Format"
+    // "The decoder is simply presented with a sequence of compressed frames [...]
+    //  The first frame presented to the decompressor is [...] a key frame.  [...]
+    //  [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
+
+    u8 const* data = vp8_data.data();
+
+    // https://datatracker.ietf.org/doc/html/rfc6386#section-9.1 "Uncompressed Data Chunk"
+    u32 frame_tag = data[0] | (data[1] << 8) | (data[2] << 16);
+    bool is_key_frame = (frame_tag & 1) == 0; // https://www.rfc-editor.org/errata/eid5534
+    u8 version = (frame_tag & 0xe) >> 1;
+    bool show_frame = (frame_tag & 0x10) != 0;
+    u32 size_of_first_partition = frame_tag >> 5;
+
+    if (!is_key_frame)
+        return Error::from_string_literal("WebPImageDecoderPlugin: 'VP8 ' chunk not a key frame");
+
+    // FIXME: !show_frame does not make sense in a webp file either, probably?
+
+    u32 start_code = data[3] | (data[4] << 8) | (data[5] << 16);
+    if (start_code != 0x2a019d) // https://www.rfc-editor.org/errata/eid7370
+        return Error::from_string_literal("WebPImageDecoderPlugin: 'VP8 ' chunk invalid start_code");
+
+    // "The scaling specifications for each dimension are encoded as follows.
+    //   0     | No upscaling (the most common case).
+    //   1     | Upscale by 5/4.
+    //   2     | Upscale by 5/3.
+    //   3     | Upscale by 2."
+    // This is a display-time operation and doesn't affect decoding.
+    u16 width_and_horizontal_scale = data[6] | (data[7] << 8);
+    u16 width = width_and_horizontal_scale & 0x3fff;
+    u8 horizontal_scale = width_and_horizontal_scale >> 14;
+
+    u16 heigth_and_vertical_scale = data[8] | (data[9] << 8);
+    u16 height = heigth_and_vertical_scale & 0x3fff;
+    u8 vertical_scale = heigth_and_vertical_scale >> 14;
+
+    dbgln_if(WEBP_DEBUG, "version {}, show_frame {}, size_of_first_partition {}, width {}, horizontal_scale {}, height {}, vertical_scale {}",
+        version, show_frame, size_of_first_partition, width, horizontal_scale, height, vertical_scale);
+
+    return VP8Header { version, show_frame, size_of_first_partition, width, horizontal_scale, height, vertical_scale, vp8_data.slice(10) };
+}
+
+ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8_contents(VP8Header const& vp8_header, bool include_alpha_channel)
+{
+    auto bitmap_format = include_alpha_channel ? BitmapFormat::BGRA8888 : BitmapFormat::BGRx8888;
+
+    // Uncomment this to test ALPH decoding for WebP-lossy-with-alpha images while lossy decoding isn't implemented yet.
+#if 0
+    return Bitmap::create(bitmap_format, { vp8_header.width, vp8_header.height });
+#else
+    // FIXME: Implement webp lossy decoding.
+    (void)vp8_header;
+    (void)bitmap_format;
+    return Error::from_string_literal("WebPImageDecoderPlugin: decoding lossy webps not yet implemented");
+#endif
+}
+
+}