quality-knob-as-wgsl-specialization-constant

name	quality-knob-as-wgsl-specialization-constant
description	When a RenderQuality knob changes infrequently (at config load, not per-frame) and needs to reach a WGSL shader, use a specialization constant (override) set at pipeline creation rather than a UBO field — avoids UBO plumbing and per-frame write_buffer
source	auto-skill
extracted_at	2026-06-15T21:07:04.805Z

Quality knob as WGSL specialization constant

When a RenderQuality knob changes at config-load time (not per-frame) and the WGSL shader reads it at pipeline creation, use a WGSL specialization constant (override) set via PipelineCompilationOptions::constants rather than adding a UBO field.

Why: A UBO field requires: extending the UBO struct (Rust + WGSL), writing the value per-frame in queue.write_buffer, and consuming a binding slot. A specialization constant requires only: declaring override in WGSL, passing the value in ShaderModuleDescriptor::compilation_options.constants at pipeline creation. The value is baked into the compiled shader — it cannot change without pipeline re-creation, but for quality knobs that change at config reload (not per-frame), pipeline re-creation (~ms) is cheaper than per-frame UBO writes (~µs × N_frames).

How to apply:

1. Declare in WGSL (top of shader file):

override vsm_pcf_kernel: u32;

2. Set at pipeline creation in Rust:

let shader = device.create_shader_module(wgpu::ShaderModuleDescriptor {
    label: Some("stratum_shading.wgsl"),
    source: wgpu::ShaderSource::Wgsl(include_str!("shaders/stratum_shading.wgsl").into()),
    compilation_options: wgpu::PipelineCompilationOptions {
        constants: &[("vsm_pcf_kernel".to_string(), quality.vsm_pcf_kernel as f64)],
        zero_initialize_workgroup_memory: true,
    },
});

3. The shader body uses the constant directly (no UBO binding needed):

if vsm_pcf_kernel <= 3u {
    vis = textureSampleCompare(...);
} else {
    // Gather4-based larger kernel
}

When NOT to use: If the value changes every frame (e.g., camera position, time, per-frame jitter), use a UBO. Pipeline re-creation every frame is too expensive. If the value changes at higher frequency than config reload (e.g., dynamic quality scaling responding to frame time), use a UBO with queue.write_buffer.