av/codec/mjpeg/jpeg.go

/*
DESCRIPTION
  jpeg.go contains constants, structure and functions specific to the JPEG.

AUTHOR
  Saxon Nelson-Milton <saxon@ausocean.org>

LICENSE
  Copyright (C) 2017 the Australian Ocean Lab (AusOcean)

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  for more details.

  You should have received a copy of the GNU General Public License
  along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
*/

package mjpeg

import (
	"fmt"
	"io"
)

// JPEG marker codes.
const (
	soi  = 0xd8 // Start of image.
	dri  = 0xdd // Define restart interval.
	dqt  = 0xdb // Define quantization tables.
	dht  = 0xde // Define hierarchical progression.
	sos  = 0xda // Start of scan.
	app0 = 0xe0 // TODO: find out what this is.
	sof0 = 0xc0 // Baseline
)

// writeMarker writes an JPEG marker with code to w.
func writeMarker(w io.Writer, code byte) error {
	_, err := w.Write([]byte{0xff, code})
	if err != nil {
		return err
	}
	return nil
}

// writeHuffman write a JPEG huffman table to w.
func writeHuffman(w io.Writer, class, id int, bits, values []byte) error {
	_, err := w.Write([]byte{byte(class<<4 | id)})
	if err != nil {
		return fmt.Errorf("could not write class and id: %w", err)
	}

	var n int
	for i := 1; i <= 16; i++ {
		n += int(bits[i])
	}

	_, err = w.Write(bits[1:17])
	if err != nil {
		return fmt.Errorf("could not write first lot of huffman bytes: %w", err)
	}

	_, err = w.Write(values[0:n])
	if err != nil {
		return fmt.Errorf("could not write second lot of huffman bytes: %w", err)
	}

	return nil
}