View Source Pdf.Reader.Images.JPEG (ExPDF v1.0.1)

JPEG (DCTDecode) image utilities for Pdf.Reader.

SOF marker parsing

Reads JPEG dimensions from the Start-of-Frame (SOF) marker without fully decoding the JPEG. This is a read-only scan used to populate %Pdf.Reader.Image{} width and height from the DCT-encoded bytes.

SOF marker table

Source: ITU-T T.81 / ISO/IEC 10918-1, § B.1.1.3 (marker syntax), Table B.1.

Valid SOF markers (0xFF 0xCy):

Marker byteNameNotes
0xC0SOF0Baseline DCT
0xC1SOF1Extended sequential DCT
0xC2SOF2Progressive DCT
0xC3SOF3Lossless (sequential)
0xC5SOF5Differential sequential DCT
0xC6SOF6Differential progressive DCT
0xC7SOF7Differential lossless (sequential)
0xC9SOF9Extended sequential DCT (arithmetic)
0xCASOF10Progressive DCT (arithmetic)
0xCBSOF11Lossless (arithmetic)
0xCDSOF13Differential sequential DCT (arithmetic)
0xCESOF14Differential progressive DCT (arithmetic)
0xCFSOF15Differential lossless (arithmetic)

Non-SOF markers to skip: | 0xC4 | DHT | Huffman table | | 0xC8 | JPG | JPEG extensions | | 0xCC | DAC | Arithmetic coding conditioning |

SOF segment layout (ITU-T T.81 § B.2.2) 

0xFF 0xCy   — marker (2 bytes)
Ls          — segment length in bytes, big-endian uint16 (includes itself, not the marker)
P           — sample precision (1 byte)
Y           — number of lines / height (2 bytes, big-endian)
X           — number of samples per line / width (2 bytes, big-endian)
Nf          — number of components (1 byte)
... (Nf × 3 bytes of component spec — not read here)

dimensions/1 returns {:ok, %{height: y, width: x}} on success.

Summary

Functions

dimensions(arg1)

Scans a JPEG binary for a SOF marker and extracts height and width.

Functions

Link to this function

dimensions(arg1)

View Source 
@spec dimensions(binary()) ::
  {:ok, %{height: non_neg_integer(), width: non_neg_integer()}}
  | {:error, atom()}

Scans a JPEG binary for a SOF marker and extracts height and width.

Returns {:ok, %{height: integer, width: integer}} on success. Returns {:error, :no_sof_marker} if no SOF marker is found. Returns {:error, :not_a_jpeg} if the binary does not start with the JPEG SOI marker.

Source: ITU-T T.81 / ISO/IEC 10918-1, § B.2.2.