Visual Ground Truth: The Only Way to Map Non-Deterministic Legacy UI Behaviors

The most dangerous line of code in your enterprise isn't the one that breaks—it's the one that works, but nobody knows why. In the labyrinth of legacy systems, particularly those spanning decades in financial services or healthcare, the source code is often a poor reflection of reality. We call this "The Documentation Gap," and according to Replay's analysis, 67% of legacy systems lack any form of accurate documentation. When you are tasked with modernizing a monolithic application, relying on the source code alone is a recipe for disaster. To truly capture the nuance of a system, you must rely on visual ground truth only.

TL;DR: Legacy modernization fails 70% of the time because teams try to interpret "zombie code" instead of observing runtime reality. By using Replay’s Visual Reverse Engineering, enterprises can bypass the documentation gap, reducing the time to modernize from years to weeks. By focusing on visual ground truth only, you eliminate the guesswork of non-deterministic UI behaviors and generate production-ready React components with 70% average time savings.

The $3.6 Trillion Technical Debt Trap#

The global technical debt has ballooned to a staggering $3.6 trillion. For the Senior Architect, this isn't just a number; it's the weight of thousands of undocumented "quirks" in a legacy UI that have become load-bearing features for the business.

When we talk about non-deterministic behaviors, we are referring to UI states that depend on specific sequences of user interaction, race conditions with ancient backend APIs, or CSS hacks that only work in Internet Explorer 11. If you attempt to rewrite these by reading the old code, you will miss the "Visual Ground Truth"—the actual behavior the user experiences.

Industry experts recommend moving away from manual code audits. Why? Because the average enterprise rewrite takes 18 months, and most of that time is spent in "Discovery." By shifting your strategy to prioritize visual ground truth only, you move from archaeological digging to precision engineering.

Learn more about modernizing legacy systems

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Why Visual Ground Truth Only Can Map Non-Deterministic UI#

Non-deterministic UI behavior is the "ghost in the machine." It’s the button that only enables when a specific sequence of three tabs is hit, or the form validation that triggers based on a hidden global variable set five screens ago.

Visual ground truth is the definitive record of what happened on the screen during a real user session.

Video-to-code is the process of capturing these visual sessions and programmatically converting the rendered elements, styles, and state transitions into modern code structures.

By focusing on visual ground truth only, architects can bypass the "how" of the legacy code (which is often broken or over-engineered) and focus on the "what" of the user experience. This is where Replay excels. Instead of manual translation, Replay records the workflow and generates a documented React component library.

Comparison: Manual Discovery vs. Visual Ground Truth (Replay)#

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The Technical Reality: Mapping State to Components#

To understand why visual ground truth only works where static analysis fails, we have to look at how state is managed in legacy environments (often jQuery or vanilla JS) versus modern React.

In a legacy environment, state is often "leaky." It lives in the DOM, in global window objects, or in hidden input fields.

The Legacy "Zombie" State Example#

Imagine a legacy insurance claim form where the "Submit" button logic looks like this:

typescript
// The "Black Box" Legacy Logic
// Found in a 5,000 line file named global_v2_final.js
function checkSubmit() {
    if (window.user_type === 'admin' && $('#hidden_field_4').val() !== '') {
        if ($('.error-text').is(':hidden')) {
            document.getElementById('btn_submit').disabled = false;
        }
    }
}
// Problem: When is this called? Who knows. It's triggered by 15 different listeners.

If you try to port this logic directly, you carry over the technical debt. However, by observing the visual ground truth only, Replay identifies that the button enables when three specific data conditions are met visually. It then generates a clean, declarative React component.

The Replay-Generated Modern Component#

Replay's AI Automation Suite analyzes the recording and produces structured, type-safe code:

tsx
import React, { useState, useEffect } from 'react';
import { Button, TextField } from '@/components/ui';

interface ClaimFormProps {
  userRole: 'admin' | 'user';
  initialData?: any;
}

/**
 * Modernized via Replay Visual Reverse Engineering
 * Mapping visual ground truth from Legacy Claim Module
 */
export const ClaimForm: React.FC<ClaimFormProps> = ({ userRole, initialData }) => {
  const [claimId, setClaimId] = useState(initialData?.id || '');
  const [isValid, setIsValid] = useState(false);

  // Replay identified this dependency from visual state transitions
  useEffect(() => {
    const validate = () => {
      const hasId = claimId.length > 0;
      const isAdmin = userRole === 'admin';
      return hasId && isAdmin;
    };
    setIsValid(validate());
  }, [claimId, userRole]);

  return (
    <div className="p-6 space-y-4">
      <TextField 
        label="Claim ID" 
        value={claimId} 
        onChange={(e) => setClaimId(e.target.value)} 
      />
      <Button disabled={!isValid} variant="primary">
        Submit Claim
      </Button>
    </div>
  );
};

Read about our Component Library automation

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Implementation: The Replay Workflow#

Modernizing a $100M revenue application requires more than just a screen recorder. It requires an architecture-first approach. Replay structures this through three core pillars:

1. The Flows (Architecture Mapping)#

Before writing code, you must map the user journey. In legacy systems, "Flows" are often undocumented. By recording real user workflows, Replay builds a visual map of the application architecture. This ensures that when you modernize, you aren't just moving buttons—you are preserving business logic that has been refined over decades.

2. The Library (Design System Generation)#

Legacy UIs are notorious for "CSS Soup." Five different shades of blue, three different button styles, and inconsistent padding. Replay’s AI Automation Suite extracts these visual tokens to create a unified Design System. According to Replay's analysis, this consolidation alone can reduce future front-end technical debt by 40%.

3. The Blueprints (The Editor)#

Once the visual ground truth only has been captured, the Blueprints editor allows architects to refine the generated code. You can swap out generic divs for your internal UI kit components or adjust the TypeScript interfaces to match your new API schemas.

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Handling Regulated Environments#

For industries like Financial Services, Healthcare, and Government, "cloud-only" is often a deal-breaker. This is why Replay is built for regulated environments.

• •SOC2 & HIPAA Ready: Data privacy is baked into the capture process.
• •On-Premise Availability: Keep your visual ground truth within your own infrastructure.
• •PII Scrubbing: Automatically mask sensitive user data during the recording phase so you are only capturing the structure and behavior of the UI.

In these sectors, the cost of failure is high. A 70% failure rate for rewrites isn't just a statistic; it's a regulatory risk. By relying on visual ground truth only, you provide an audit trail of how the new system matches the old system's behavior, satisfying both stakeholders and auditors.

Strategies for regulated industry modernization

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The "Visual Ground Truth Only" Manifesto#

Why do we insist on visual ground truth only? Because code lies, but behavior doesn't.

When a developer looks at a 20-year-old JSP (JavaServer Pages) file, they see a mess of logic. They might see a conditional statement that looks like it should never execute. They delete it. Suddenly, the production environment breaks because that "impossible" condition was actually handling a specific edge case in the legacy database.

If that developer had used Replay to record the production workflow, they would have seen the UI enter that state. The visual ground truth only approach forces the modernization process to account for reality, not just the "idealized" version of the code found in the repository.

Case Study: Telecom Giant Modernization#

A major telecom provider had a legacy provisioning tool with over 400 screens. Manual estimation for a rewrite was 24 months with a team of 15 developers.

• •The Problem: No original developers were left at the company. No documentation existed.
• •The Solution: They used Replay to record every core workflow.
• •The Result: They mapped the visual ground truth only and generated a React-based UI in just 3 months.
• •Savings: They saved approximately 18,000 man-hours, translating to millions in R&D costs.

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Technical Deep Dive: Extracting Logic from Pixels#

How does Replay turn a video into a React component? It’s not just OCR (Optical Character Recognition). It’s a multi-layered analysis:

1. •DOM Reconstruction: Replay captures the metadata of the DOM during the recording.
2. •State Inference: By analyzing how the UI changes in response to inputs (clicks, typing), Replay infers the underlying state machine.
3. •Componentization: The AI Automation Suite identifies repeating patterns. If a specific table structure appears on 50 screens, Replay identifies it as a single reusable component.
4. •Style Normalization: Replay converts inline styles and legacy CSS into modern Tailwind or CSS-in-JS patterns.

typescript
// Example of Replay's Style Normalization
// From Legacy: <div style="margin-left: 10px; color: #333; font-weight: bold;">
// To Modern: <div className="ml-2.5 text-gray-800 font-bold">

By focusing on visual ground truth only, the output is clean, readable, and—most importantly—maintainable.

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Frequently Asked Questions#

What happens if the legacy UI is already broken?#

If the legacy UI has visual bugs, Replay will capture those as part of the visual ground truth. However, the Blueprints editor allows you to correct these behaviors during the generation phase. You aren't forced to replicate bugs; you are given the truth of the current state so you can make informed decisions on what to fix.

Does Replay require access to my legacy source code?#

No. Replay operates on visual ground truth only. It analyzes the rendered output and the DOM structure at runtime. This is particularly useful for systems where the source code is lost, obfuscated, or written in languages that modern AI tools struggle to parse.

How does this integrate with my existing CI/CD pipeline?#

Replay is designed to fit into modern dev workflows. The generated React components and Design Systems can be exported as standard npm packages or pushed directly to your Git repository. It bridges the gap between the "Old World" and your "New World" DevOps practices.

Can Replay handle complex data visualizations?#

Yes. Because Replay captures the runtime state, it can map complex charts, dashboards, and data grids. It identifies the data structures fueling these visualizations and helps you map them to modern libraries like Recharts or AG-Grid.

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Conclusion: The Path to 70% Faster Modernization#

The enterprise cannot afford to spend 18-24 months on "Discovery." The $3.6 trillion technical debt is a ticking time bomb. By adopting a strategy that prioritizes visual ground truth only, you eliminate the primary cause of rewrite failure: the misunderstanding of legacy logic.

Replay provides the platform to turn recordings into reality. Whether you are in a highly regulated financial environment or a fast-moving manufacturing hub, the ability to convert video to code is the ultimate "cheat code" for enterprise architects.

Ready to modernize without rewriting? Book a pilot with Replay