Measuring Body Dimensions from a Single Frontal Image Without a Reference Object

Accurately measuring body dimensions from a single frontal image—without any reference object—is a fascinating yet complex challenge. While it may seem daunting, this task can be approached systematically through a two-step process: estimating body proportions and establishing a real-world scale.

Step 1: Estimating Body Proportions

The foundation of this method lies in extracting reliable body proportions from the image. Pose estimation is the most effective technique for this purpose. It identifies key body joints and landmarks, allowing us to infer relative distances and angles between different body parts.

For my experiments, I utilized the pose estimation tool, which delivers precise and detailed keypoint detections. These keypoints serve as the essential building blocks for calculating proportional measurements of the body.

Step 2: Establishing a Reference Scale

Since the image contains no explicit reference object with known size, the next challenge is to convert these relative proportions into real-world measurements. This requires establishing a scale, which I achieved by combining depth estimation with knowledge of the camera’s focal length.

Understanding Focal Length in Pixels

A camera sensor has a fixed physical size and resolution, meaning each pixel corresponds to a specific physical distance on the sensor (e.g., micrometers). The focal length expressed in pixels can be computed as:

Using this, the real-world length of any segment in the image can be approximated by:

Mathematical Model for Real-World Distance Between Two Points

Consider two points in image pixel coordinates:

Their corresponding depth values (distance from the camera) are:

With the camera focal length in pixels denoted as , the real-world 3D coordinates for each point can be calculated by projecting the pixel coordinates scaled by depth and normalized by focal length:

The real-world Euclidean distance between these two points is then:

Depth Estimation

To obtain the depth values and , I employed the state-of-the-art depth estimation model, which predicts the distance of objects in the image relative to the camera.

---

By seamlessly integrating pose estimation for body proportions with depth data and camera focal length, it becomes possible to estimate accurate, real-world body measurements from a single frontal image—without relying on any physical reference object. This approach opens exciting possibilities for applications in health, fitness, fashion, and beyond.