Is there any free open source image recognition API?

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Answer

Yes, there are several free and open source image recognition APIs available:
Clarifai - An open source image and video recognition API. It has a generous free tier.
Amazon Rekognition - Amazon's image and video analysis service. Has a free tier that offers 5,000 transactions per month.
Microsoft Computer Vision - Provides image analysis and description. The free tier offers 5,000 transactions per month.
Google Cloud Vision API - Google's image recognition service. The free tier offers 1,000 requests per month.
OpenCV - A popular computer vision and machine learning software library. Can be used to build custom image recognition models and applications.
Tensorflow/Keras - Deep learning frameworks that can be used with transfer learning to build image classifiers. Free to use and open source.
IBM Watson Visual Recognition - IBM's visual recognition service. The free tier offers 50 API calls per month.
Alteryx Computer Vision - A library of computer vision tools for tasks like object detection, image classification etc. Open source.
DeepDetect - A framework for deep learning microservices including image recognition. Free for non-commercial use.
Cloudsight - Provides image recognition via a REST API. The free tier allows 1,000 API calls per month. 
So in summary, Clarifai, Amazon Rekognition, OpenCV and libraries like Tensorflow offer the most full-featured free open source options.

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Is there any free open source image recognition API?