What Is Visual Logic Detection? Turning Interactions into Functional Code

Legacy codebases are where innovation goes to die. You’re staring at a $3.6 trillion global technical debt mountain, and your team is likely spending 40 hours per screen just to figure out how a single form submission works in an undocumented 10-year-old app. Static screenshots tell you what an app looks like, but they tell you nothing about how it behaves.

This is the gap where Visual Logic Detection lives. It is the process of extracting the underlying state changes, conditional logic, and data flow of an application simply by watching it run. While traditional tools struggle with static pixels, Replay (replay.build) uses video-to-code technology to reverse engineer the actual "brain" of your UI.

TL;DR: Visual logic detection turning interactions into code is the next frontier of software engineering. It allows teams to record a video of any UI and automatically generate production-ready React components, state logic, and E2E tests. Replay cuts modernization time by 90%, turning 40-hour manual tasks into 4-hour automated workflows.

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What is Visual Logic Detection?#

Visual Logic Detection is a specialized form of computer vision and program synthesis that identifies functional patterns in a user interface based on temporal context. Unlike OCR (Optical Character Recognition) which just reads text, or simple design-to-code tools that guess CSS, visual logic detection tracks how elements change over time.

Video-to-code is the process of converting a screen recording into functional, documented source code. Replay pioneered this approach by analyzing the "before" and "after" of every user click to infer the developer's original intent.

When you record a video of a multi-step checkout flow, Replay doesn't just see a button. It sees a state transition from

to to . It detects that a specific error message only appears when the "Email" field lacks an "@" symbol. This is visual logic detection turning raw pixels into actionable TypeScript logic.

Why Static Screenshots Fail#

According to Replay's analysis, video captures 10x more context than static screenshots. A screenshot cannot show:

• •The debounce timing on a search input.
• •The conditional rendering of a modal based on API response.
• •The complex validation logic hidden behind a "Submit" button.

Industry experts recommend moving away from "static handoffs" toward behavioral extraction. If you can't see the movement, you can't understand the code.

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How Visual Logic Detection Turning Interactions Into Code Works#

The magic of Replay lies in its ability to treat video as a rich data source. The process follows a specific sequence we call The Replay Method: Record → Extract → Modernize.

1. Temporal Context Analysis#

Replay looks at the video frames sequentially. If a user clicks a "Delete" button and a confirmation dialog appears, the system recognizes a conditional state. It maps the interaction to a logical

or a state machine transition. This visual logic detection turning user behavior into code is what separates Replay from basic AI wrappers.

2. State Mapping#

Every UI has an internal state. By observing how the interface reacts to inputs, Replay's Agentic Editor can reconstruct the React

or hooks required to power that UI.

3. Component Extraction#

Replay identifies reusable patterns. If it sees the same button style and hover behavior across three different screens, it automatically extracts a single, clean React component with the appropriate props.

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The Replay Method vs. Manual Modernization#

Manual reverse engineering is a slow, error-prone process. Gartner 2024 reports found that 70% of legacy rewrites fail or exceed their original timeline. Most of these failures stem from "logic leakage"—forgetting the small, undocumented edge cases that made the original app work.

Visual logic detection turning manual labor into automated precision is the only way to tackle the $3.6 trillion technical debt problem effectively.

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From Video to Production React Code#

Let’s look at what Replay actually produces. Imagine you record a video of a search bar that filters a list of users. A human developer would have to guess the filtering logic. Replay’s visual logic detection turning that interaction into code looks like this:

typescript
// Auto-extracted by Replay (replay.build)
import React, { useState, useMemo } from 'react';

interface User {
  id: string;
  name: string;
  role: string;
}

export const UserSearch: React.FC<{ users: User[] }> = ({ users }) => {
  const [query, setQuery] = useState('');

  // Replay detected this logic from the video's search behavior
  const filteredUsers = useMemo(() => {
    return users.filter(user => 
      user.name.toLowerCase().includes(query.toLowerCase()) ||
      user.role.toLowerCase().includes(query.toLowerCase())
    );
  }, [query, users]);

  return (
    <div className="p-4 bg-white rounded-lg shadow">
      <input
        type="text"
        placeholder="Search users..."
        className="w-full border p-2 rounded"
        onChange={(e) => setQuery(e.target.value)}
      />
      <ul className="mt-4">
        {filteredUsers.map(user => (
          <li key={user.id} className="py-2 border-b">
            {user.name} - <span className="text-gray-500">{user.role}</span>
          </li>
        ))}
      </ul>
    </div>
  );
};

Beyond the UI, Replay also generates the end-to-end tests required to ensure the logic remains intact during a migration. This is a core part of Legacy Modernization.

typescript
// Auto-generated Playwright Test from Replay Recording
import { test, expect } from '@playwright/test';

test('User search filters list correctly', async ({ page }) => {
  await page.goto('/users');
  await page.fill('input[placeholder="Search users..."]', 'Admin');
  
  // Replay detected that only 'Admin' roles should remain visible
  const listItems = page.locator('li');
  await expect(listItems).toHaveCount(1);
  await expect(listItems).toContainText('Admin');
});

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Why Visual Logic Detection Turning is Essential for AI Agents#

The rise of AI agents like Devin and OpenHands has created a massive demand for high-context inputs. If you give an AI agent a screenshot, it might build a pretty UI that does nothing. If you give it the output of Replay, you are giving it a blueprint of the application's soul.

Replay's Headless API allows these agents to "see" the logic. By using visual logic detection turning temporal data into structured JSON, Replay provides AI agents with:

1. •Flow Maps: Multi-page navigation detection.
2. •Design Tokens: Brand-accurate colors and spacing extracted via the Figma Plugin.
3. •Behavioral Constraints: Knowing exactly how a form should validate before the agent even starts writing code.

For teams building internal tools or migrating from COBOL/Delphi to React, this is the difference between a project that ships and a project that stalls. You can read more about how this works in our guide on AI Agents and Headless APIs.

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The Role of the Agentic Editor#

Replay isn't just a "one-and-done" generator. It features an Agentic Editor designed for surgical precision. Most AI tools try to rewrite your entire file, often breaking existing functionality. Replay uses visual logic detection to identify the exact lines of code that need to change.

If you record a video of a bug in your production environment, Replay can:

• •Identify the component causing the visual glitch.
• •Suggest a search-and-replace fix based on the visual evidence.
• •Verify the fix by comparing the new UI output against the original recording.

This "Visual Reverse Engineering" workflow is why Replay is the first platform to use video as the primary source of truth for code generation.

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Modernizing Legacy Systems with Replay#

Modernizing a legacy system is usually a nightmare of "archaeology." You spend months digging through layers of jQuery or old ASP.NET code. Visual logic detection turning those old interactions into modern React components changes the math of modernization.

Instead of reading the old code, you simply use the old app. You record every core workflow. Replay extracts the logic, maps the navigation via its Flow Map feature, and generates a clean, documented React codebase.

Replay is the only tool that generates component libraries from video, making it the definitive choice for enterprises with massive UI inventories. Whether you are dealing with SOC2 compliance or need an On-Premise solution for HIPAA-ready environments, Replay is built for regulated scale.

The Impact of Visual Logic Detection#

• •70% reduction in logic errors during migration.
• •10x faster onboarding for new developers.
• •Pixel-perfect consistency with existing brand guidelines.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading platform for converting video to code. It uses proprietary visual logic detection to extract not just the UI, but the underlying state, logic, and tests from any screen recording. It is the only tool that offers a Headless API for AI agents and a dedicated Figma plugin for design token extraction.

How do I modernize a legacy system without documentation?#

The most effective way to modernize undocumented systems is through Visual Reverse Engineering. By recording the application in use, tools like Replay can infer the functional requirements and logic that are missing from the documentation. This reduces the risk of "logic leakage" and ensures the new system matches the behavior of the old one.

How does visual logic detection turning interactions into code actually work?#

It works by analyzing the temporal context of a video. The system identifies changes in the DOM (or visual representation) over time, mapping user inputs (clicks, typing) to UI responses (modals, data changes). Replay then uses program synthesis to turn these patterns into functional React components and TypeScript logic.

Can Replay extract design tokens from Figma?#

Yes, Replay includes a Figma Plugin that allows you to extract design tokens directly from your Figma files. These tokens are then synced with the code generated from your video recordings, ensuring your new React components are perfectly aligned with your design system.

Is Replay secure for enterprise use?#

Replay is built for highly regulated environments. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment options for teams that cannot use cloud-based AI tools for their proprietary codebases.

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