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Hanbin Bae 371b802103 Merge pull request #28442 from Anemptyship:optimize/resize-parallel
optimize(dnn): parallelize Resize layer implementation #28442

### optimize(dnn): parallelize Resize layer implementation

This PR addresses the `TODO` in `modules/dnn/src/layers/resize_layer.cpp` regarding the slow implementation of the `Resize` layer when using `opencv_linear` or `nearest` interpolation with specific configurations.

### Changes
- Replaced the serial nested loop (batch x channels) with `cv::parallel_for_`.
- This allows OpenCV to utilize multi-threading for the resizing operation, which was previously single-threaded for these specific paths.

### Performance Results
Benchmark run on 20-thread CPU (AVX2):

| Test Case | Resolution Change | Before (ms) | After (ms) | Speedup |
| :--- | :--- | :--- | :--- | :--- |
| **Upsample Linear** | 64x64 -> 128x128 | 1.47 ms | **0.18 ms** | **~8.1x** |
| **Downsample Nearest** | 128x128 -> 64x64 | 1.55 ms | **0.45 ms** | **~3.4x** |

**Test Configuration:**
- **Upsample**: `[4, 64, 64, 64]` -> `[4, 64, 128, 128]` (Factor: 2.0, Linear)
- **Downsample**: `[4, 128, 128, 128]` -> `[4, 128, 64, 64]` (Factor: 0.5, Nearest)

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      - Added `modules/dnn/perf/perf_resize.cpp` to verify performance gains.
- [ ] The feature is well documented and sample code can be built with the project CMake
2026-01-30 15:27:30 +03:00

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// This file is part of OpenCV project.
// It is subject to the license terms in the LICENSE file found in the top-level directory
// of this distribution and at http://opencv.org/license.html.
#include "perf_precomp.hpp"
namespace opencv_test {
struct Layer_Resize : public TestBaseWithParam<tuple<Backend, Target>>
{
void test_layer(const std::vector<int>& inpShape, int outH, int outW, const String& interp)
{
int backendId = get<0>(GetParam());
int targetId = get<1>(GetParam());
Mat input(inpShape, CV_32FC1);
randu(input, 0.f, 1.f);
Net net;
LayerParams lp;
lp.type = "Resize";
lp.name = "testLayer";
lp.set("interpolation", interp);
lp.set("width", outW);
lp.set("height", outH);
int id = net.addLayerToPrev(lp.name, lp.type, lp);
net.connect(0, 0, id, 0);
// warmup
{
net.setInputsNames({"data"});
net.setInput(input, "data");
net.setPreferableBackend(backendId);
net.setPreferableTarget(targetId);
Mat out = net.forward();
}
TEST_CYCLE()
{
Mat res = net.forward();
}
SANITY_CHECK_NOTHING();
}
};
PERF_TEST_P_(Layer_Resize, Resize_Upsample_Linear)
{
// N=4, C=64, H=64, W=64 -> 128x128 (x2 upsample)
// Common in segmentation/detection heads
test_layer({4, 64, 64, 64}, 128, 128, "opencv_linear");
}
PERF_TEST_P_(Layer_Resize, Resize_Downsample_Nearest)
{
// N=4, C=128, H=128, W=128 -> 64x64 (x0.5 downsample)
test_layer({4, 128, 128, 128}, 64, 64, "nearest");
}
INSTANTIATE_TEST_CASE_P(/**/, Layer_Resize, dnnBackendsAndTargets());
} // namespace opencv_test