LZDoom: Sprite Clipping Bug Fix Needed!

Introduction

In the world of classic first-person shooters like Doom, the authenticity of the gaming experience heavily relies on accurate rendering and visual presentation. A peculiar issue has surfaced within the LZDoom engine, specifically affecting sprite rendering. The sprite clipping bug manifests when sprites, be they decorative elements or menacing enemies, have edges that extend beyond the boundaries of linedefs (lines that define the structure of the game environment). From certain viewing angles, these parts of the sprite become entirely invisible, leading to a jarring and immersion-breaking experience. This issue is particularly noticeable in custom maps brimming with intricate details and numerous tiny sectors, often seen in doomcute builds.

Understanding the Sprite Clipping Bug

The sprite clipping bug presents itself as an unexpected visual anomaly, where portions of sprites seemingly vanish from the player's view. This isn't a case of simple graphical glitches or minor distortions; instead, entire sections of a sprite abruptly disappear, creating a distracting and often disorienting effect. The primary cause of this issue lies in how the LZDoom engine, when running in software mode, handles the rendering of sprites in relation to the map's linedefs.

Linedefs, fundamental components of the Doom engine, define the boundaries and structural elements of the game world. They dictate the walls, floors, ceilings, and other architectural features that players navigate. When a sprite's edges cross or extend beyond these linedefs, the engine's rendering process sometimes falters, leading to the erroneous clipping of the sprite. This means that the engine incorrectly determines which parts of the sprite should be visible, resulting in the unexpected disappearance of sprite sections.

The angle at which the player views the sprite plays a crucial role in triggering this bug. From certain perspectives, the engine struggles to accurately calculate the visibility of the sprite's edges relative to the linedefs. This is especially prevalent in maps with complex layouts, numerous sectors, and a high density of decorative sprites. The more intricate the environment, the greater the likelihood that sprites will interact with linedefs in ways that expose the clipping issue.

Custom maps, particularly those employing doomcute techniques, exacerbate this problem. Doomcute refers to the practice of creating highly detailed and visually rich environments using a limited set of resources and intricate sector designs. These maps often feature a large number of small sectors and decorative sprites, pushing the engine's rendering capabilities to their limits. As a result, the sprite clipping bug becomes far more noticeable and frequent, detracting from the overall gaming experience.

Reproducing the Bug: A Step-by-Step Guide

To effectively demonstrate and understand the sprite clipping bug in LZDoom, a specific map known as "Doom 2 In Rural Only, Map 10" serves as an excellent example. This map, with its intricate details and numerous decorative elements, provides a clear and easily reproducible scenario for observing the issue.

1. Load the Map: Begin by loading "Doom 2 In Rural Only, Map 10" in LZDoom.
2. Navigate to the Street: Once the map is loaded, navigate your character out of the cave and onto the street.
3. Observe the Trees: Pay close attention to the trees lining the road. These sprites are particularly susceptible to the clipping bug.
4. Adjust Your View Angle: Stand aligned with the trees and slowly rotate your view angle. As you rotate, observe how the trees flicker and parts of them disappear. This flickering and disappearance are the manifestations of the sprite clipping bug.

By following these steps, you can readily witness the sprite clipping bug in action. The trees, acting as prominent decorative sprites, clearly exhibit the issue as their edges interact with the linedefs defining the road and surrounding environment. The bug's visibility is further enhanced by the map's detailed design and the specific angles at which the sprites are viewed.

This reproduction method is not limited to this particular map. The sprite clipping bug can occur in any map with similar characteristics, such as a high density of decorative sprites, intricate sector designs, and complex layouts. However, "Doom 2 In Rural Only, Map 10" provides a convenient and easily accessible means of demonstrating and understanding the issue.

Technical Details

The bug was observed using LZDoom versions ranging from 4.11.4 up to 4.14pre284-g6c28840fcc-x64. The operating system in use was Windows 10 Pro 64-bit, running on hardware that included an NVidia GeForce 1080 Ti and an Intel Core i7-6700 processor.

The user also noted difficulty in locating the configuration file, which is typically stored in the root directory. The absence of the configuration file in its usual location suggests a potential issue with the engine's configuration management or file storage.

Potential Causes and Solutions

The sprite clipping bug in LZDoom likely stems from the way the engine handles sprite rendering in relation to linedefs, particularly in software mode. Here are some potential causes and solutions to address this issue:

• Rendering Order Issues: The engine may be incorrectly determining the order in which sprites and linedefs are rendered. This can lead to situations where the engine attempts to draw a sprite behind a linedef, even when the sprite should be visible. Solution: Implement a more robust rendering order algorithm that accurately prioritizes sprites and linedefs based on their relative positions and distances from the viewer.
• Z-Buffer Inaccuracies: The z-buffer, which stores depth information for each pixel, may have inaccuracies that cause sprites to be incorrectly occluded by linedefs. Solution: Improve the precision and accuracy of the z-buffer calculations to ensure that depth information is properly maintained.
• Clipping Plane Issues: The engine's clipping planes, which define the boundaries of the visible scene, may be incorrectly configured, leading to the premature clipping of sprites. Solution: Adjust the clipping plane settings to ensure that they encompass the entire visible scene and do not inadvertently clip sprites.
• Optimization Strategies: Aggressive optimization strategies may inadvertently introduce clipping artifacts, particularly when dealing with complex scenes and numerous sprites. Solution: Carefully review and refine optimization techniques to minimize the risk of introducing visual errors.

Community Discussion and Further Investigation

This bug report serves as a valuable starting point for further investigation and community discussion. By sharing experiences, insights, and potential solutions, the Doom community can collectively contribute to resolving this issue and enhancing the overall LZDoom experience.

Users are encouraged to:

• Share their experiences: If you have encountered this bug, please share your experiences, including the maps, LZDoom versions, and hardware configurations used.
• Provide additional information: If you have any additional information about the bug, such as specific steps to reproduce it or potential workarounds, please share them with the community.
• Test potential solutions: If you have the technical expertise, consider testing potential solutions and sharing your findings with the community.

By working together, the Doom community can help identify the root cause of this bug and develop effective solutions to address it.

Conclusion

The sprite clipping bug in LZDoom represents a notable issue that detracts from the visual fidelity and immersion of the gaming experience. By understanding the nature of the bug, its causes, and potential solutions, we can work towards resolving it and ensuring that LZDoom remains a top-tier engine for playing and enjoying classic Doom and other source ports. The active involvement of the Doom community is crucial in this endeavor, as collective knowledge and collaborative efforts will pave the way for a more polished and visually appealing gaming experience.

For more information about the Doom engine and its intricacies, visit the Doom Wiki.