VisLog (Visual Logger)—most prominently known as a built-in feature in Unreal Engine—is a powerful debugging tool designed to capture the real-time gameplay and state of your code objects (actors) and represent them visually.
Instead of staring at thousands of lines of raw text logs, VisLog allows you to scrub through time, see 3D geometry paths, and view specific object properties precisely when a bug occurs. Core Components of VisLog
VisLog accelerates troubleshooting by breaking your data into three scannable panels:
Actor List (Left Panel): Shows a list of all game objects or code actors that have logged data during the session.
Visual Logger Viewport (Center Panel): Displays 3D visual representations of logs, such as paths traveled, collision boxes, or target locations.
Status & Text Log (Right/Bottom Panel): Displays the traditional text logs and detailed variable states of the selected actor at that specific frame. Step-by-Step: How to Troubleshoot Faster 1. Instrument Your Code with VisLog Macros
Instead of standard printf or Log statements, use visual logging macros in your code. This tells VisLog exactly what data to grab. For example, in Unreal Engine, you use specific macros to capture data shapes:
UE_VLOG(): Logs standard text categorized by a specific actor.
UE_VLOG_SEGMENT(): Draws a line segment in the 3D space (e.g., shooting trajectories or sightlines).
UE_VLOG_LOCATION(): Places a visual 3D point in the world (e.g., target destinations).
UE_VLOG_BOX() / UE_VLOG_CYLINDER(): Renders bounds to visualize triggers or collision areas. 2. Record the Issue
Open the Visual Logger via your editor UI (e.g., Tools > Debug > Visual Logger in newer Unreal versions) and click Record. Run your application or play through your game until the complex bug occurs, then stop the recording. 3. Scrub Through Time (Frame-by-Frame Isolation)
Complex code bugs often happen over a single frame. VisLog records a timeline at the bottom of the screen. You can slide the timeline marker backward and forward to watch exactly how the variables, locations, and states changed leading up to the crash or failure. 4. Filter by Specific Actor
If you have hundreds of objects running simultaneously, the text log becomes unreadable. In VisLog, simply click on the specific Actor causing the issue in the list. The tool instantly filters out all other log noise, showing only the pathing and text logs relevant to that object. 5. Compare Expected vs. Actual Visual State
Look at the 3D viewport. If an AI character is supposed to walk to a door but gets stuck, VisLog will visually show you its path line. If the line plunges through the floor or points to the wrong coordinates, you instantly know your vector math or pathfinding logic is broken without having to print coordinates to a console. Why VisLog is Faster Than Traditional Text Debugging Traditional Text Logs VisLog (Visual Logger) Data Format Raw text wall, timestamps. 3D visual shapes + text timelines. Reproducibility Hard to catch one-frame bugs. Easy; capture once, scrub frame-by-frame. Filtering Manual Ctrl+F text searching. Click the object to filter everything. Spatial Awareness Must infer coordinates from numbers. Directly see vectors, paths, and bounding boxes. Pro-Tips for Maximizing Speed
Auto-Reset Sessions: Turn on Reset Data with New Session in your editor preferences so old logs don’t clutter your new testing runs.
Snapshot State: Take advantage of VisLog’s ability to cache the entire “status snapshot” of an actor, allowing you to view its internal struct variables on any given frame without setting tedious breakpoints.
Save Log Files: You can export .vlog files. If a QA tester encounters a rare bug, they can send you the VisLog file, allowing you to debug the exact session on your own machine without needing to reproduce it live. Visual Logger in Unreal Engine – Epic Games Developers
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