Derivatives

Derivatives compute per-pixel rate-of-change of a value with respect to screen-space coordinates. They are only valid inside fragment shaders; calling them from any other stage is a WGSL error.

Fine / coarse variants

WGSL provides three precision tiers. wgsl-rs exposes all of them:

The bare dpdx/dpdy/fwidth map to WGSL’s default-precision builtins, which WGSL defines as coarse. Prefer the explicit _fine / _coarse variants when the choice matters for your application.

Common uses

• Mip selection in non-fragment-aware sampling: pass gradients to textureSampleGrad from dpdx/dpdy of the texture coordinates.
• Edge detection / anti-aliasing: fwidth to compute pixel-local width.
• Screen-space dependent branching: compare fwidth against a threshold.

Example

#![allow(unused)]
fn main() {
#[wgsl]
pub mod derivatives_example {
    use wgsl_rs::std::*;

    uniform!(FRAME, Frame);
    texture!(COLOR_TEX, Texture2d);
    sampler!(COLOR_SMP, Sampler);

    pub struct Frame {
        pub time: f32,
    }

    #[fragment]
    pub fn fs(in: VertexOutput) -> Vec4f {
        let uv = in.uv;
        let dx = dpdx_fine(uv);
        let dy = dpdy_fine(uv);
        let w = fwidth_fine(uv);
        let color = textureSampleGrad(COLOR_TEX, COLOR_SMP, uv, dx, dy);
        color
    }
}
}

CPU behavior

On the CPU, derivatives return zero for dpdx/dpdy and the input value’s magnitude for fwidth — enough to keep CPU tests running without panic. The GPU is the source of truth for derivative accuracy.