Struct JpegDecoder 

pub struct JpegDecoder<S: Source> { /* private fields */ }

Decodes a JPEG stream.

Baseline frames are decoded by this crate, one MCU row at a time. Progressive frames are handed to a wrapped decoder (ADR-0004) and are internally buffered — the distinction is not visible through Decoder, which is the point of the trait boundary.

Implementations

impl<S: Source> JpegDecoder<S>

pub fn new(source: S, limits: Limits) -> Result<Self>

Read every header up to and including the scan header.

No coefficient is decoded here for a baseline frame, so this is the probe()/metadata path as well as the start of a decode (SPEC §Guarantees 3). A progressive frame is decoded in full, because it has no prefix that yields a finished row.

Errors

Returns PixelsError::Malformed for a stream that is not a well-formed JPEG, PixelsError::Unsupported for a variant neither this crate nor the wrapped decoder handles (arithmetic coding, 12-bit, CMYK), or PixelsError::LimitExceeded if the frame exceeds limits.

pub fn with_scale(source: S, limits: Limits, scale: Scale) -> Result<Self>

Read every header, and decode at a reduced resolution.

At M/8 scale the decoder inverse-transforms only the low-frequency corner of each block, so a thumbnail costs a fraction of the arithmetic and — the point of it — the full-resolution image is never materialized. Entropy decoding is unchanged: every coefficient is still read, because the format gives no way to skip one.

Decoder::descriptor reports the scaled size, so a caller that asks for a reduced decode is told what it will actually receive.

scale is ignored for a progressive frame, which the wrapped decoder produces at full size; Decoder::descriptor then reports that size, so the caller is never misled about what it is getting.

Errors

As JpegDecoder::new.

pub const fn scale(&self) -> Scale

The resolution this decoder produces, as eighths of full size.

Always Scale::Full for a progressive frame.

pub const fn is_progressive(&self) -> bool

Whether this stream was progressive, and so decoded by the wrapped codec rather than by this crate.

Trait Implementations

impl<S: Debug + Source> Debug for JpegDecoder<S>

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

Formats the value using the given formatter.

impl<S: Source + Debug> Decoder for JpegDecoder<S>

fn icc_profile(&self) -> Option<&[u8]>

The ICC profile, joined from its APP2 segments.

fn orientation(&self) -> Orientation

The EXIF orientation, from the first EXIF APP1 segment.

fn descriptor(&self) -> ImageDescriptor

The shape of the image, known after the header is parsed.

fn capability(&self) -> DecodeCapability

What this decoder can produce without a full decode.

fn reduced_descriptor(&self, target: (u32, u32)) -> Option<ImageDescriptor>

What this decoder would produce if asked for target or larger, when the format lets it reach that size more cheaply than a full decode.

fn reduce_to(&mut self, descriptor: ImageDescriptor) -> Result<()>

Commit to producing descriptor, which Decoder::reduced_descriptor must have returned.

fn read_row(&mut self, out: &mut [u8]) -> Result<()>

Decode the next row, top to bottom, into out.

fn animation(&self) -> Option<Animation>

The source’s animation, if it has more than one frame.

fn read_region( &mut self, region: Region, out: &mut TileMut<'_>, ) -> Result<(), PixelsError>

Decode an arbitrary region into out.