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image/jpeg: add more theHuffmanSpec comments

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Change-Id: I2c68dde6e968e0643109161e52a76189e48b4d19
Reviewed-on: https://go-review.googlesource.com/c/go/+/624715
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Nigel Tao <nigeltao@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
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Nigel Tao 2024-11-03 23:09:45 +11:00 committed by Gopher Robot
parent 989eed2849
commit 9c93d99c61
1 changed files with 12 additions and 4 deletions
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16
src/image/jpeg/writer.go
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@ -50,8 +50,8 @@ const (
// unscaledQuant are the unscaled quantization tables in zig-zag order. Each // unscaledQuant are the unscaled quantization tables in zig-zag order. Each
// encoder copies and scales the tables according to its quality parameter. // encoder copies and scales the tables according to its quality parameter.
// The values are derived from section K.1 after converting from natural to // The values are derived from section K.1 of the spec, after converting from
// zig-zag order. // natural to zig-zag order.
var unscaledQuant = [nQuantIndex][blockSize]byte{ var unscaledQuant = [nQuantIndex][blockSize]byte{
// Luminance. // Luminance.
{ {
@ -89,14 +89,22 @@ const (
// huffmanSpec specifies a Huffman encoding. // huffmanSpec specifies a Huffman encoding.
type huffmanSpec struct { type huffmanSpec struct {
// count[i] is the number of codes of length i bits. // count[i] is the number of codes of length i+1 bits.
count [16]byte count [16]byte
// value[i] is the decoded value of the i'th codeword. // value[i] is the decoded value of the i'th codeword.
value []byte value []byte
} }
// theHuffmanSpec is the Huffman encoding specifications. // theHuffmanSpec is the Huffman encoding specifications.
// This encoder uses the same Huffman encoding for all images. //
// This encoder uses the same Huffman encoding for all images. It is also the
// same Huffman encoding used by section K.3 of the spec.
//
// The DC tables have 12 decoded values, called categories.
//
// The AC tables have 162 decoded values: bytes that pack a 4-bit Run and a
// 4-bit Size. There are 16 valid Runs and 10 valid Sizes, plus two special R|S
// cases: 0|0 (meaning EOB) and F|0 (meaning ZRL).
var theHuffmanSpec = [nHuffIndex]huffmanSpec{ var theHuffmanSpec = [nHuffIndex]huffmanSpec{
// Luminance DC. // Luminance DC.
{ {