decode_quirks.wuffs

// Copyright 2017 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//
// SPDX-License-Identifier: Apache-2.0 OR MIT

// --------

// Quirks are discussed in (/doc/note/quirks.md).
//
// The base38 encoding of "gif." is 0x0E_A964. Left shifting by 10 gives
// 0x3AA5_9000.
pri const QUIRKS_BASE : base.u32 = 0x3AA5_9000

// --------

// When this quirk is enabled, when skipping over frames, the number of frames
// visited isn't incremented when the last byte of the N'th frame is seen.
// Instead, it is incremented when the first byte of the N+1'th frame's header
// is seen. There may be zero or more GIF extensions between the N'th frame's
// payload and the N+1'th frame's header.
//
// For a well-formed GIF, this won't have much effect. For a malformed GIF,
// this can affect the number of valid frames, if there is an error detected in
// the extensions between one frame's payload and the next frame's header.
//
// Some other GIF decoders don't register the N'th frame as complete until they
// see the N+1'th frame's header (or the end-of-animation terminator), so that
// e.g. the API for visiting the N'th frame can also return whether it's the
// final frame. Enabling this quirk allows for matching that behavior.
pub const QUIRK_DELAY_NUM_DECODED_FRAMES : base.u32 = 0x3AA5_9000 | 0x00

// When this quirk is enabled, the background color of the first frame is set
// to black whenever that first frame has a local (frame-specific) palette.
// That black can be either opaque black or transparent black, depending on
// whether or not that first frame is opaque: whether that local palette
// contains a transparent color.
//
// This has no effect unless QUIRK_HONOR_BACKGROUND_COLOR is also enabled.
//
// There isn't really much of a rationale for this, other than it matches the
// behavior of another GIF implementation.
pub const QUIRK_FIRST_FRAME_LOCAL_PALETTE_MEANS_BLACK_BACKGROUND : base.u32 = 0x3AA5_9000 | 0x01

// When this quirk is enabled, the background color is taken from the GIF
// instead of always being transparent black. If the background color index in
// the GIF header is non-zero but less than the global palette's size, the
// global background color is that global palette's entry. Otherwise, it is
// opaque black. A frame's background color is transparent if the frame palette
// contains a transparent color. Otherwise, it is the global background color.
// Note that different frames can have different background colors.
//
// Specifically, if the initial frame bounds is smaller than the image bounds,
// those pixels outside the initial frame bounds are assumed to start as that
// frame background color. The frame background color should also be used when
// processing WUFFS_BASE__ANIMATION_DISPOSAL__RESTORE_BACKGROUND. In both
// cases, the caller of Wuffs, not Wuffs itself, is responsible for filling the
// pixel buffer with that color.
pub const QUIRK_HONOR_BACKGROUND_COLOR : base.u32 = 0x3AA5_9000 | 0x02

// When this quirk is enabled, silently ignore e.g. a frame that reports a
// width and height of 6 pixels each, followed by 50 pixel values. In that
// case, we process the first 36 pixel values and discard the excess 14.
pub const QUIRK_IGNORE_TOO_MUCH_PIXEL_DATA : base.u32 = 0x3AA5_9000 | 0x03

// When this quirk is enabled, if the initial frame bounds extends beyond the
// image bounds, then the image bounds stay unchanged. By default (with this
// quirk disabled), the image bounds are adjusted to always contain the first
// frame's bounds (but not necessarily subsequent frame's bounds).
//
// For more discussion, see
// https://github.com/google/wuffs/blob/main/test/data/artificial/gif-frame-out-of-bounds.gif.make-artificial.txt
pub const QUIRK_IMAGE_BOUNDS_ARE_STRICT : base.u32 = 0x3AA5_9000 | 0x04

// When this quirk is enabled, a frame with zero width or height is rejected
// during decode_frame (but accepted during decode_frame_config).
pub const QUIRK_REJECT_EMPTY_FRAME : base.u32 = 0x3AA5_9000 | 0x05

// When this quirk is enabled, a frame with no explicit palette is rejected,
// instead of implicitly having a palette with every entry being opaque black.
pub const QUIRK_REJECT_EMPTY_PALETTE : base.u32 = 0x3AA5_9000 | 0x06

pri const QUIRKS_COUNT : base.u32 = 0x07