14aa1e136f
* hashcompute_test: Add benchmark * dct: Improve DCT1D to O(nlogn) algorithm AS-IS: BenchmarkPerceptionHash-8 500 2893930 ns/op 456698 B/op 4455 allocs/op TO-BE: BenchmarkPerceptionHash-8 2000 890306 ns/op 456382 B/op 4455 allocs/op reference: DCT type II, unscaled. Algorithm by Byeong Gi Lee, 1984.
76 lines
1.6 KiB
Go
76 lines
1.6 KiB
Go
// Copyright 2017 The goimagehash Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package transforms
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import (
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"math"
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"sync"
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)
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// DCT1D function returns result of DCT-II.
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// DCT type II, unscaled. Algorithm by Byeong Gi Lee, 1984.
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func DCT1D(input []float64) []float64 {
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temp := make([]float64, len(input))
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forwardTransform(input, temp, len(input))
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return input
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}
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func forwardTransform(input, temp []float64, Len int) {
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if Len == 1 {
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return
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}
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halfLen := Len / 2
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for i := 0; i < halfLen; i++ {
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x, y := input[i], input[Len-1-i]
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temp[i] = x + y
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temp[i+halfLen] = (x - y) / (math.Cos((float64(i)+0.5)*math.Pi/float64(Len)) * 2)
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}
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forwardTransform(temp, input, halfLen)
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forwardTransform(temp[halfLen:], input, halfLen)
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for i := 0; i < halfLen-1; i++ {
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input[i*2+0] = temp[i]
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input[i*2+1] = temp[i+halfLen] + temp[i+halfLen+1]
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}
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input[Len-2], input[Len-1] = temp[halfLen-1], temp[Len-1]
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}
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// DCT2D function returns a result of DCT2D by using the seperable property.
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func DCT2D(input [][]float64, w int, h int) [][]float64 {
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output := make([][]float64, h)
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for i := range output {
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output[i] = make([]float64, w)
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}
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wg := new(sync.WaitGroup)
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for i := 0; i < h; i++ {
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wg.Add(1)
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go func(i int) {
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cols := DCT1D(input[i])
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output[i] = cols
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wg.Done()
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}(i)
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}
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wg.Wait()
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for i := 0; i < w; i++ {
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wg.Add(1)
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in := make([]float64, h)
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go func(i int) {
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for j := 0; j < h; j++ {
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in[j] = output[j][i]
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}
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rows := DCT1D(in)
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for j := 0; j < len(rows); j++ {
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output[j][i] = rows[j]
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}
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wg.Done()
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}(i)
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}
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wg.Wait()
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return output
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}
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