How calculate recall for images retrieval based euclidean distance method?

I have images belongs to 5 categories. Each category consist of 100 images. I have calculated three features for each image. The euclidean distance is calculated between query image and training images and retrieved top 20 images. How to calculate recall in this case?

[Yuvaraj Venkataswamy]

https://in.mathworks.com/matlabcentral/fileexchange/37758-performance-measures-for-classification?focused=5240671&tab=function