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third_party/cpp_base64/README.md

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+base64 library from github: https://github.com/ReneNyffenegger/cpp-base64, merged .cpp and .h files together.
+
+**NOTE**
+
+used for encoding image to base64 string when interact with mLLM in VidepPipe.
+
+test file `base64_test.cpp`, which convert cv::Mat to base64 string and convert back to cv::Mat
+```
+mkdir build
+cd build
+cmake ..
+make -j8
+./base64_test
+```