Equirectangular IBL Seam Resolution

This document describes the resolution of the vertical seam artifact observed in the IBL (Image Based Lighting) environment maps when using equirectangular projection.

Problem Description

A faint, perfectly straight vertical line was visible in the environment reflection (Specular IBL) and irradiance (Diffuse IBL), most noticeably in "Irradiance (Diff)" debug mode. This line appeared at the UV wrap-around point (where the azimuthal angle \(\phi\) jumps from \(+\pi\) to \(-\pi\)).

The issue was caused by two cooperative factors:

1. Horizontal Wrap Mode: The generated IBL textures (Irradiance Map and Prefiltered Specular Map) were initialized with GL_CLAMP_TO_EDGE for the GL_TEXTURE_WRAP_S parameter. When sampling near the \(u=1.0\) or \(u=0.0\) boundary, linear filtering would "clamp" to the edge pixel instead of blending with the other side of the equirectangular map.
2. GPU Derivative Discontinuity: When using the standard texture() function, the GPU calculates automatic MIP levels based on screen-space derivatives (\(dFdx\) / \(dFdy\)). At the UV wrap-around point, the coordinate jumps from \(1.0\) to \(0.0\) (or vice versa) in a single pixel, creating an enormous gradient. This triggers the GPU to select the coarsest MIP level (highly blurred) for that specific pixel column, creating a "seam".

Resolution

The fix involves two changes:

1. Correcting Wrap Modes (C Code)

In src/pbr.c, the horizontal wrap mode for all equirectangular-mapped IBL textures has been changed from GL_CLAMP_TO_EDGE to GL_REPEAT.

// src/pbr.c
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

This ensures that linear filtering correctly wraps around the "seam" of the sphere.

2. Bypassing Automatic MIP Selection (Shader)

In shaders/pbr_functions.glsl, the sampling of the irradiance map (which is already pre-convolved and does not require complex MIP selection for diffusion) has been updated to use textureLod with an explicit level of 0.0.

// shaders/pbr_functions.glsl
vec3 irradiance = textureLod(irradianceMap, dirToUV(N), 0.0).rgb;

For the specular prefiltered map, which does use MIP levels for roughness, the wrap mode fix in C is usually sufficient to hide the seam, as the derivatives only trigger the wrong MIP at the exact jump point, and GL_REPEAT allows texture() to handle wrap-around derivatives more gracefully on some hardware (though textureLod remains the safest choice for irradiance).

Verification

The fix can be verified by: 1. Enabling "Billboard" mode. 2. Switching to PBR Debug Mode 6 ("Irradiance (Diff)"). 3. Rotating the camera to view the \(X=0\) alignment where the seam previously appeared. 4. Shading should now be perfectly continuous across the entire sphere.

[!NOTE] As IBL maps are generated at application startup, a full restart or environment reload is required to see the effects of the wrap mode change.