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72 lines
2.1 KiB
72 lines
2.1 KiB
4 months ago
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/*
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* Copyright 2021 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "phash_fingerprinter.h"
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#include "array2d.h"
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#include "fast_dct-inl.h"
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#include "fast_dct.h"
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#include "phash_config.h"
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namespace android {
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constexpr int kResizedLength = 8;
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int64_t PhashFingerprinter::GenerateFingerprint(const uint8_t* frame) {
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// 1. verify input
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if (frame == nullptr) {
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return 0;
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}
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// 2. Compute the DCT
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Array2D<const uint8_t> dct_in(frame);
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Array2D<int16_t> dct_out(dct_buf_);
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FastDCT<const uint8_t, int16_t> dct(kImageLength);
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dct.ForwardTransform2D(dct_in, &dct_out);
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// 3. Simplify the DCT by only using the coefficients in the top-left 8x8
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// window (i.e. those for the lowest frequencies).
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double total = 0;
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for (int x = 0; x < kResizedLength; x++) {
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for (int y = 0; y < kResizedLength; y++) {
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total += dct_out(x, y);
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}
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}
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// Remove the DC component at (0, 0) from total.
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total -= dct_out(0, 0);
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// 4. Compute the average
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double average = total / ((kResizedLength * kResizedLength) - 1);
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// 5. Further simplify the DCT by setting each value to 0 or 1 based on
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// whether the DCT value is larger than the average, then store each value as
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// a bit in an int64.
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int64_t fingerprint = 0;
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for (int x = 0; x < kResizedLength; x++) {
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for (int y = 0; y < kResizedLength; y++) {
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fingerprint <<= 1;
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fingerprint |= (dct_out(x, y) > average ? 1 : 0);
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}
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}
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// Remove the DC component at (0, 0)
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fingerprint &= ~(1ULL << 63);
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return fingerprint;
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}
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} // namespace android
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