In this example, unstained cells are detected based on their morphology by multi-template matching. A representative cell is selected (the template) and all structures on the image that have a certain similarity to the template are identified. This is a simple method that does not require deep programming knowledge. However, it is limited to recognizing only objects that are morphologically very similar.
The original publication can be found here: https://bmcbioinformatics.biomedcentral.com/articles/10.1186/s12859-020-3363-7