Evaluating image segmentation models for background removal for Images

Last week, we wrote about face cropping for Images, which runs an open-source face detection model in Workers AI to automatically crop images of people at scale.

It wasn’t too long ago when deploying AI workloads was prohibitively complex. Real-time inference previously required specialized (and costly) hardware, and we didn’t always have standard abstractions for deployment. We also didn’t always have Workers AI to enable developers — including ourselves — to ship AI features without this additional overhead.

And whether you’re skeptical or celebratory of AI, you’ve likely seen its explosive progression. New benchmark-breaking computational models are released every week. We now expect a fairly high degree of accuracy — the more important differentiators are how well a model fits within a product’s infrastructure and what developers do with its predictions.

This week, we’re introducing background removal for Images. This feature runs a dichotomous image segmentation model on Workers AI to isolate subjects in an image from their backgrounds. We took a controlled, deliberate approach to testing models for efficiency and accuracy.

Here’s how we evaluated various image segmentation models to develop background removal.

In computer vision, image segmentation is the process of splitting an image into meaningful parts.

Segmentation models produce a mask that assigns each pixel to a specific category. This differs from detection models, which don’t classify every pixel but instead mark regions of interest. A face detection model, such as the one that informs face cropping, draws bounding boxes based on where it thinks there are faces. (If you’re curious, our post on face cropping discusses how we use these bounding boxes to perform crop and zoom operations.)

Salient object detection is a type of segmentation that highlights the parts of an image that most stand out. Most salient detection models create a binary mask that categorizes the most prominent (or salient) pixels as the “foreground” and all other pixels as the “background”. In contrast, a multi-class mask considers the broader context and labels each pixel as one of several possible classes, like “dog” or “chair”. These multi-class masks are the basis of content analysis models, which distinguish which pixels belong to specific objects or types of objects.

_{In this photograph of my dog, a detection model predicts that a bounding box contains a dog; a segmentation model predicts that some pixels belong to a dog, while all other pixels don’t.}

For our use case, we needed a model that could produce a soft saliency mask, which predicts how strongly each pixel belongs to either the foreground (objects of interest) or the background. That is, each pixel is assigned a value on a scale of 0–255, where 0 is completely transparent and 255 is fully opaque. Most background pixels are labeled at (or near) 0; foreground pixels may vary in opacity, depending on its degree of saliency.

In principle, a background removal feature must be able to accurately predict saliency across a broad range of contexts. For example, e-commerce and retail vendors want to display all products on a uniform, white background; in creative and image editing applications, developers want to enable users to create stickers and cutouts from uploaded content, including images of people or avatars.

In our research, we focused primarily on the following four image segmentation models:

• U²-Net (U Square Net): Trained on the largest saliency dataset (DUST-TR) of 10,553 images, which were then horizontally flipped to reach a total of 21,106 training images.

• IS-Net (Intermediate Supervision Network): A novel, two-step approach from the same authors of U2-Net; this model produces cleaner boundaries for images with noisy, cluttered backgrounds.

• BiRefNet (Bilateral Reference Network): Specifically designed to segment complex and high-resolution images with accuracy by checking that the small details match the big picture.

• SAM (Segment Anything Model): Developed by Meta to allow segmentation by providing prompts and input points.

Different scales of information allow computational models to build a holistic view of an image. Global context considers the overall shape of objects and how areas of pixels relate to the entire image, while local context traces fine details like edges, corners, and textures. If local context focuses on the trees and their leaves, then global context represents the entire forest.

U²-Net extracts information using a multi-scale approach, where it analyzes an image at different zoom levels, then combines its predictions in a single step. The model analyzes global and local context at the same time, so it works well on images with multiple objects of varying sizes.

IS-Net introduces a new, two-step strategy called intermediate supervision. First, the model separates the foreground from the background, identifying potential areas that likely belong to objects of interest — all other pixels are labeled as the background. Second, it refines the boundaries of the highlighted objects to produce a final pixel-level mask.

The initial suppression of the background results in cleaner, more precise edges, as the segmentation focuses only on the highlighted objects of interest and is less likely to mistakenly include background pixels in the final mask. This model especially excels when dealing with complex images with cluttered backgrounds.

Both models output their predictions in a single direction for scale. U²-Net interprets the global and local context in one pass, while Is-Net begins with the global context, then focuses on the local context.

In contrast, BiRefNet refines its predictions over multiple passes, moving in both contextual directions. Like Is-Net, it initially creates a map that roughly highlights the salient object, then traces the finer details. However, BiRefNet moves from global to local context, then from local context back to global. In other words, after refining the edges of the object, it feeds the output back to the large-scale view. This way, the model can check that the small-scale details align with the broader image structure, providing higher accuracy on high-resolution images.

U²-Net, IS-Net, and BiRefNet are exclusively saliency detection models, producing masks that distinguish foreground pixels from background pixels. However, SAM was designed to be more extensible and general; its primary goal is to segment any object based on specified inputs, not only salient objects. This means that the model can also be used to create multi-class masks that label various objects within an image, even if they aren’t the primary focus of an image.

In most saliency datasets, the actual location of the object is known as the ground-truth area. These regions are typically defined by human annotators, who manually trace objects of interest in each image. This provides a reliable reference to evaluate model predictions.

_{Photograph by Allen Fang}

Each model outputs a predicted area (where it thinks the foreground pixels are), which can be compared against the ground-truth area (where the foreground pixels actually are).

Models are evaluated for segmentation accuracy based on common metrics like Intersection over Union, Dice coefficient, and pixel accuracy. Each score takes a slightly different approach to quantify the alignment between the predicted and ground-truth areas (“P” and “G”, respectively, in the formulas below).

Intersection over Union (IoU), also called the Jaccard index, measures how well the predicted area matches the true object. That is, it counts the number of foreground pixels that are shared in both the predicted and ground-truth masks. Mathematically, IoU is written as:

_{Jaccard formula}

The formula divides the intersection (P∩G), or the pixels where the predicted and ground-truth areas overlap, by the union (P∪G), or the total area of pixels that belong to either area, counting the overlapping pixels only once.

IoU produces a score between 0 and 1. A higher value indicates a closer overlap between the predicted and ground-truth areas. A perfect match, although rare, would score 1, while a smaller overlapping area brings the score closer to 0.

The Dice coefficient, also called the Sørensen–Dice index, similarly compares how well the model’s prediction matches reality, but is much more forgiving than the IoU score. It gives more weight to the shared pixels between the predicted and actual foreground, even if the areas differ in size. Mathematically, the Dice coefficient is written as:

_{Sørensen–Dice formula}

The formula divides twice the intersection (P∩G) by the sum of pixels in both predicted and ground-truth areas (P+G), counting any overlapping pixels twice.

Like IoU, the Dice coefficient also produces a value between 0 and 1, indicating a more accurate match as it approaches 1.