How do we accurately measure objects in the real world using Computer Vision without any reference object?

the problem is the following given an image like this how do we estimate foot length and width using image processing or deep learning techniques?

1. `I have tried image processing techniques but the results weren't good at all`
2. `I have used yolov8 + FastSAM for zero-shot segmentation along with a reference object and it works perfectly but the client said I don't want to use any reference object so what will be the solution for such a case?` …

computer computer vision computervision deep learning deep learning techniques good image image processing object objects processing reference results vision world