Struct AlphHeader 

pub struct AlphHeader {
    pub compression: AlphCompression,
    pub filtering: AlphFiltering,
    pub preprocessing: AlphPreprocessing,
    pub reserved: u8,
    pub info_byte: u8,
}

Decoded §2.7.1.2 ALPH info byte plus the offset at which the alpha bitstream begins inside the chunk payload.

Constructed via AlphHeader::parse. The actual alpha bitstream (raw or VP8L-compressed) is not decoded — this layer’s job is to surface the 2-bit Rsv / P / F / C decomposition. The payload after byte 0 — payload[1..] — is the §2.7.1.2 “Alpha bitstream” of Chunk Size - 1 bytes; callers that need it should slice the chunk payload at Self::bitstream_offset.

Fields

compression: AlphCompression

C field — compression method (§2.7.1.2).

filtering: AlphFiltering

F field — filtering method (§2.7.1.2).

preprocessing: AlphPreprocessing

P field — preprocessing hint (§2.7.1.2).

reserved: u8

Rsv field — raw 2-bit value from bits 7..6 of the info byte. §2.7.1.2 says “MUST be 0. Readers MUST ignore this field.” — we surface the raw value for observability without rejecting.

info_byte: u8

Raw info byte, preserved for round-trip and trace assertions.

Implementations

impl AlphHeader

pub fn parse(payload: &[u8]) -> Result<Self, AlphError>

Parse the ALPH chunk payload’s §2.7.1.2 info byte.

payload is the whole §2.3 chunk payload (i.e. the slice returned by crate::container::WebpChunk::payload for a chunk whose FourCC is crate::container::fourcc::ALPH). Only the first byte is consumed by this layer; the remainder is the alpha bitstream callers must hand off to a later VP8L or raw decode pass.

pub const fn bitstream_offset(&self) -> usize

Offset (within the ALPH chunk payload) at which the alpha bitstream begins. Always 1 per §2.7.1.2 — the info byte is followed immediately by the bitstream.

Trait Implementations

impl Clone for AlphHeader

fn clone(&self) -> AlphHeader

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for AlphHeader

impl Debug for AlphHeader

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for AlphHeader

impl PartialEq for AlphHeader

fn eq(&self, other: &AlphHeader) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl StructuralPartialEq for AlphHeader