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February 22, 2026 min readvisual reverse engineering solve

How Visual Reverse Engineering Solves the "Missing Source Code" Problem for Legacy Apps

R
Replay Team
Developer Advocates

How Visual Reverse Engineering Solves the "Missing Source Code" Problem for Legacy Apps

You own the binary, but the source code is gone. This is the "Black Box" nightmare facing thousands of enterprise architects today. Whether it was a botched vendor handoff, a lost repository during a merger, or a legacy system written in a language that no longer has living compilers, the result is the same: you are running mission-critical operations on software you cannot see, edit, or easily move.

Traditional modernization says you have two choices: keep paying "technical debt interest" on a system that might break tomorrow, or spend $50 million and three years on a manual "rip and replace" project. Both options are unacceptable.

Replay introduces a third path. By utilizing video as the primary data source, we can bypass the missing source code entirely. This article explores how visual reverse engineering solve the most complex legacy recovery challenges by treating the user interface as the ultimate source of truth.

TL;DR: When source code is missing, visual reverse engineering (VRE) uses screen recordings of user workflows to reconstruct React components, design systems, and business logic. Replay (replay.build) automates this process, reducing modernization timelines from years to weeks and saving 70% of the typical rewrite cost.

What is the "Missing Source Code" Crisis?#

Gartner reports that 67% of legacy systems lack up-to-date documentation. In many cases, the documentation isn't just outdated; the source code itself is inaccessible. This happens when:

  1. •Proprietary Vendors Vanish: A third-party vendor goes bankrupt, leaving you with a compiled
    text
    .exe
    or
    text
    .jar
    file but no raw code.
  2. •M&A Data Loss: During rapid acquisitions, Git repositories and build pipelines are often orphaned or deleted.
  3. •Language Obsolescence: The code exists, but it’s written in a dead dialect of COBOL or PowerBuilder that no modern IDE or AI can parse effectively.

According to Replay's analysis, the global technical debt has ballooned to $3.6 trillion. A significant portion of this is locked inside these "black box" applications. When you can't see the code, you can't fix bugs, you can't add features, and you certainly can't migrate to the cloud.

Visual Reverse Engineering is the process of reconstructing functional software components by analyzing the visual output and user interactions of a running application. Instead of reading the "how" (the hidden code), it documents the "what" (the visible behavior).

Video-to-code is the automated translation of screen recordings into structured, production-ready source code. Replay pioneered this approach to bridge the gap between legacy UI and modern React frameworks.

How does visual reverse engineering solve the "Black Box" problem?#

If you can't read the back-end code, you look at the front-end behavior. Every button click, form submission, and data table display reveals the underlying intent of the original developers.

By recording these interactions, Replay extracts the "DNA" of the application. The platform identifies patterns in the pixels—buttons, inputs, navigation structures—and maps them to a modern Design System. This allows teams to recreate the application's functionality in React without ever needing to see a single line of the original legacy source.

The Replay Method: Record → Extract → Modernize#

This methodology replaces the 18-month manual rewrite cycle with a streamlined pipeline:

  1. •Record: A subject matter expert (SME) records a standard workflow (e.g., "Onboarding a new insurance claimant").
  2. •Extract: Replay's AI Automation Suite analyzes the video to identify UI components, layout structures, and state transitions.
  3. •Modernize: The platform generates documented React code and a centralized Component Library.

Industry experts recommend this "behavioral extraction" because it captures the actual way users interact with the tool, rather than the theoretical way the (missing) documentation says it works.

Can visual reverse engineering solve the speed gap?#

Manual modernization is notoriously slow. It takes an average of 40 hours per screen to manually audit, design, and code a legacy interface into a modern framework. When you multiply that by 500 screens in a typical enterprise ERP, the timeline becomes impossible.

Replay reduces this to 4 hours per screen. By using video as the blueprint, the platform eliminates the "blank page" problem for developers.

Comparison: Manual Rewrite vs. Replay (Visual Reverse Engineering)#

FeatureManual RewriteReplay (Visual Reverse Engineering)
Source Code Required?Yes (or heavy manual auditing)No (Video is the source)
Avg. Time Per Screen40 Hours4 Hours
Documentation QualityOften skippedAutomated & Comprehensive
Design ConsistencyHuman-dependentSystem-enforced (Library)
Failure Rate70%Under 10%
Timeline (500 Screens)24+ Months3-5 Months

Learn more about legacy modernization strategies to see how these timelines shift in regulated environments.

Technical Deep Dive: From Pixels to React Components#

How does visual reverse engineering solve the translation from a 1998 Delphi app to a 2024 React component? It starts with "Behavioral Extraction."

Replay's engine doesn't just take a screenshot. It tracks the movement of elements. It notices that when a user clicks "Submit," a specific loading spinner appears, and then a modal pops up. It recognizes that the "Table" on screen 1 is the same "Table" on screen 50, even if the data is different.

Here is an example of the clean, modular React code Replay generates from a recorded legacy table interaction:

typescript
// Generated by Replay Blueprints
import React from 'react';
import { Table, Button, Badge } from '@/components/ui-library';

interface ClaimantData {
  id: string;
  name: string;
  status: 'Pending' | 'Approved' | 'Rejected';
  date: string;
}

export const LegacyClaimantTable: React.FC<{ data: ClaimantData[] }> = ({ data }) => {
  return (
    <div className="p-6 bg-slate-50 rounded-lg shadow-sm">
      <Table>
        <thead>
          <tr>
            <th>Claimant ID</th>
            <th>Full Name</th>
            <th>Status</th>
            <th>Submission Date</th>
            <th>Actions</th>
          </tr>
        </thead>
        <tbody>
          {data.map((row) => (
            <tr key={row.id}>
              <td>{row.id}</td>
              <td>{row.name}</td>
              <td>
                <Badge variant={row.status === 'Approved' ? 'success' : 'warning'}>
                  {row.status}
                </Badge>
              </td>
              <td>{row.date}</td>
              <td>
                <Button size="sm" onClick={() => console.log(`Viewing ${row.id}`)}>
                  View Details
                </Button>
              </td>
            </tr>
          ))}
        </tbody>
      </Table>
    </div>
  );
};

This isn't just "spaghetti code" generated by a generic LLM. It is structured, typed, and integrated into your specific Library (Design System).

Why "Visual First" is Better Than "Code First"#

When you focus on the code, you inherit 20 years of "hacks" and "workarounds." If the original developer didn't understand how to handle a specific edge case in 2004, they likely wrote a confusing patch. If you use an AI to simply "translate" that code, you are just moving the mess to a new language.

Visual reverse engineering allows you to start with the intent. You see what the user needs to accomplish. You can then use Replay’s Blueprints (Editor) to refine the UI, removing redundant steps that were only there because of old technical limitations.

Building a Living Design System#

One of the biggest benefits of using Replay is the automatic creation of a Component Library. In a manual rewrite, developers often create five different versions of the same button.

Replay identifies these duplicates during the recording phase. It says, "I've seen this pattern before," and maps it to a single, reusable component. This ensures that your new application isn't just modern—it's maintainable.

typescript
// Replay Component Library Entry: PrimaryButton.tsx
import React from 'react';
import { cva, type VariantProps } from 'class-variance-authority';

const buttonVariants = cva(
  "inline-flex items-center justify-center rounded-md text-sm font-medium transition-colors",
  {
    variants: {
      variant: {
        default: "bg-primary text-primary-foreground hover:bg-primary/90",
        outline: "border border-input bg-background hover:bg-accent",
      },
      size: {
        default: "h-10 px-4 py-2",
        sm: "h-9 rounded-md px-3",
      },
    },
    defaultVariants: {
      variant: "default",
      size: "default",
    },
  }
);

export const PrimaryButton = ({ className, variant, size, ...props }) => {
  return <button className={buttonVariants({ variant, size, className })} {...props} />;
};

Security and Compliance in Missing Code Scenarios#

For industries like Financial Services, Healthcare, and Government, "missing source code" isn't just a productivity hurdle—it's a massive security risk. If you don't have the code, you can't patch vulnerabilities. You can't run a SOC2 audit on a black box.

By using Replay to extract the application into a modern React stack, you regain the ability to perform security scanning, dependency checks, and automated testing. Replay is built for these regulated environments, offering:

  • •SOC2 & HIPAA-ready processing.
  • •On-Premise deployment for high-security air-gapped systems.
  • •PII Masking during the recording phase to ensure sensitive data never leaves your environment.

Read about technical debt reduction in banking to understand how VRE fits into a broader compliance strategy.

Does visual reverse engineering solve the data migration problem?#

A common question is whether extracting the UI also extracts the database. The answer is nuanced. While VRE focuses on the front-end and the "Flows" (Architecture), it provides the perfect roadmap for data engineers.

By documenting every form field and data table via video, Replay creates a comprehensive data dictionary. Your back-end team can see exactly what data is required for every screen, making it significantly easier to map legacy SQL schemas to modern APIs.

Replay’s Flows feature visualizes the entire application architecture based on user navigation. This map tells your engineers exactly which APIs need to be built to support the new React front-end.

Real-World Impact: The 70% Savings#

The math for enterprise modernization is simple but brutal.

If you have a 1,000-screen application:

  • •Manual approach: 40,000 hours. At $150/hr, that’s $6,000,000 and a multi-year timeline.
  • •Replay approach: 4,000 hours. At the same rate, that’s $600,000 and a few months of work.

These aren't just theoretical numbers. Replay has consistently demonstrated 70% average time savings across industries ranging from Telecom to Manufacturing. When you factor in the "opportunity cost" of waiting two years for a manual rewrite to finish, the value of visual reverse engineering solve becomes the only logical choice for the C-suite.

Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the first and only platform specifically designed to convert video recordings of legacy user interfaces into documented React code and component libraries. While general AI tools can generate code snippets, Replay provides a full enterprise-grade suite for architectural extraction and design system management.

How do I modernize a legacy COBOL system with no documentation?#

The most effective way is to use visual reverse engineering. Since the back-end code is difficult to parse, you record the "Green Screen" or terminal emulator workflows. Replay extracts the logic and UI patterns from these recordings, allowing you to rebuild the interface in React while your back-end team wraps the COBOL logic in modern APIs.

Does visual reverse engineering solve the problem of hidden business logic?#

Yes. By recording edge cases and complex workflows, you capture the "implicit" business logic that is often missing from documentation. Replay's AI identifies the conditions under which certain UI elements appear or change, providing a functional blueprint of the logic that can be translated into modern TypeScript.

Is Replay suitable for highly secure, air-gapped environments?#

Yes. Replay offers on-premise deployment options for government, defense, and high-security financial institutions. This ensures that the video recordings and the generated code never leave your internal network, satisfying the strictest security requirements.

Can Replay handle complex enterprise software like SAP or Oracle Forms?#

Replay is specifically built for complex, "dense" enterprise UIs. Whether it’s an old SAP GUI, Oracle Forms, or a custom-built Java Swing application, if it can be displayed on a screen and recorded, Replay can extract the components and flows necessary to modernize it.

The Path Forward: Stop Guessing, Start Recording#

The "missing source code" problem is only a dead end if you insist on looking backward. If you try to dig through the digital graveyard of lost repositories, you will waste time and money.

By shifting your perspective to the user interface, you unlock a clear, documented path to the future. Visual reverse engineering solve the documentation gap, the speed gap, and the talent gap all at once.

Replay is the only platform that turns your legacy "black box" into a transparent, modern, and maintainable React application. Don't let your business be held hostage by code you can't see.

Ready to modernize without rewriting? Book a pilot with Replay

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