forge-pbr-textures

name	forge-pbr-textures
description	Add PBR texture loading with separate roughness/metallic support to an SDL GPU project using forge_scene.h

Load PBR material texture sets (albedo, normal, roughness, metallic, AO, emissive) from the asset pipeline and render them with a Cook-Torrance BRDF shader. Supports both packed metallic-roughness (glTF convention) and separate single-channel textures (ambientCG convention).

When to use

• Loading PBR materials from .fmat sidecar files
• Rendering textured PBR materials on scene models
• Supporting both packed and separate metallic-roughness workflows
• Adding multiple material support to a scene

Material loading pattern

/* Parse .fmat sidecar */
ForgePipelineMaterialSet mat_set;
if (!forge_pipeline_load_materials("assets/materials/Rock026/Rock026.fmat", &mat_set)
    || mat_set.material_count == 0) {
    SDL_Log("Failed to load Rock026 material");
    return false;
}
const ForgePipelineMaterial *mat = &mat_set.materials[0];

/* Load textures from processed assets.
 * vram tracks GPU memory usage — must be non-NULL. */
ForgeSceneVramStats vram = {0};
ForgeSceneModelTextures tex = {0};

/* Base color — sRGB (authored color) */
tex.base_color = forge_scene_load_pipeline_texture(
    scene, &vram, "assets/materials/Rock026/Rock026_Color.png", true, false);

/* Normal map — linear, is_normal_map=true for BC5 */
tex.normal = forge_scene_load_pipeline_texture(
    scene, &vram, "assets/materials/Rock026/Rock026_NormalGL.png", false, true);

/* Separate roughness — linear */
tex.roughness = forge_scene_load_pipeline_texture(
    scene, &vram, "assets/materials/Rock026/Rock026_Roughness.png", false, false);

/* AO — linear, R channel */
tex.occlusion = forge_scene_load_pipeline_texture(
    scene, &vram, "assets/materials/Rock026/Rock026_AmbientOcclusion.png", false, false);

8-sampler PBR pipeline

The shader uses 8 texture slots to support both packed and separate workflows:

Slot	Texture	Color space	Fallback
0	Base color	sRGB	White
1	Normal map	Linear	Flat (+Z)
2	Packed metallic-roughness	Linear	White
3	Occlusion (AO)	Linear	White
4	Emissive	sRGB	Black
5	Shadow map	Depth	Scene shadow
6	Separate roughness	Linear	White
7	Separate metallic	Linear	White

Separate MR flag

The shininess field in ForgeSceneModelFragUniforms (offset 80, unused by PBR) is repurposed as use_separate_mr:

ForgeSceneModelFragUniforms fu;
forge_scene__fill_model_frag_uniforms(scene, mat, &fu);
fu.shininess = (tex.roughness || tex.metallic) ? 1.0f : 0.0f;

In the shader:

if (use_separate_mr > 0.5) {
    roughness = roughness_tex.Sample(roughness_smp, input.uv).r * roughness_factor;
    metallic  = metallic_tex.Sample(metallic_smp, input.uv).r * metallic_factor;
} else {
    float2 mr = mr_tex.Sample(mr_smp, input.uv).bg;
    metallic  = mr.x * metallic_factor;
    roughness = mr.y * roughness_factor;
}

sRGB vs linear

Data type	Color space	Why
Base color, emissive	sRGB	Authored as visible color — GPU linearizes on sample
Normal, roughness, metallic, AO	Linear	Physical parameters — gamma would distort values

Common mistakes

• Loading roughness as sRGB — makes 0.5 appear as ~0.22, materials look too shiny
• Loading normal maps with sRGB — corrupts direction vectors, surface detail is lost
• Forgetting the use_separate_mr flag — shader reads packed MR texture (white fallback = all 1.0), making everything fully metallic and rough
• Not checking metallic_roughness_texture before loading separate — if both packed and separate exist, prefer packed

Reference

See Lesson 52 — PBR Textures for the full implementation.