shader-programming

Shader programming (cross-engine)

Shaders are small programs that run per vertex and per pixel on the GPU. The concepts — the pipeline, coordinate spaces, UVs, and how common effects are built — port across engines; only the language dialect and built-in variable names change. This skill teaches those portable fundamentals in GLSL with HLSL equivalents; use godot-shaders (or Unity/Unreal material docs) for the exact engine syntax and built-ins.

When to use

• Use to understand or write vertex/fragment shaders and to reason about UVs, coordinate spaces, and the GPU pipeline.
• Use to build common effects: tint/recolor, scrolling textures, dissolve, outlines, fresnel/rim light, vignette, color grading.
• Use to translate a shader concept between GLSL and HLSL, or between engines.

When not to use: for an engine's exact shader language and built-ins, use godot-shaders (Godot shading language) or the engine's material docs. For full particle VFX systems, see unreal-niagara. For post-process stacks, defer to the engine's renderer settings.

Core workflow

1. Know which stage you're in. The vertex shader transforms each vertex into clip space and passes data (UVs, normals) onward; the fragment/pixel shader runs per rasterized pixel and outputs a color. Most game effects live in the fragment stage.
2. Track coordinate spaces. Positions move model → world → view → clip space; normals belong in world or view space. Mixing spaces is the most common bug.
3. Drive effects with UVs and time. UVs are 0..1 texture coordinates; offset, scale, or distort them, and animate with a time uniform.
4. Work per pixel, branch-light. Prefer mix, step, smoothstep, and clamp over if where possible; GPUs run pixels in lockstep, so a branch whose condition differs between neighbouring pixels runs both sides. A branch on a uniform (same for the whole draw) is cheap.
5. Pass data via uniforms (constant per draw) and varyings (interpolated vertex→fragment). Keep texture samples few; they dominate cost.
6. Validate source before visual testing when the tools are already available. For GLSL, run glslangValidator path/to/shader.frag with the matching stage suffix; for WGSL, run naga path/to/shader.wgsl. Check Godot .gdshader files by opening the project in Godot, which has no standalone validator for them.
7. Verify visually and on target hardware. Shaders that look right on desktop can break on mobile (precision, missing features). Test where it ships.

Patterns

GLSL fragment snippets for desktop #version 330 (each passes glslangValidator with that header; close to Godot's canvas_item/spatial shaders). For GLSL ES 3.00 — WebGL2, three.js ShaderMaterial, PixiJS filters — add precision highp float; and set uniform defaults from code: ES rejects uniform float edge = 0.05; with "cannot initialize this type of qualifier" (Godot's shading language accepts it). See references/effects.md for the HLSL equivalents and the full outline/fresnel/vignette shaders.

1. Fragment basics: sample, tint, and combine

// Per-pixel: read the texture at this UV, multiply by a color (tint), keep alpha.
uniform sampler2D tex;
uniform vec4 tint;          // e.g. (1,0,0,1) reddens; multiply is non-destructive
in vec2 uv;                 // interpolated 0..1 texture coordinate (a "varying")
out vec4 frag;
void main() {
    vec4 c = texture(tex, uv);   // HLSL: tex.Sample(samp, uv)
    frag = c * tint;             // component-wise multiply tints without clipping
}

2. Scrolling UVs (animated texture) — frame-rate independent

// Add time * speed to the UV to scroll. fract() wraps it into 0..1 so it tiles.
uniform sampler2D tex;
uniform float time;          // seconds, supplied by the engine
uniform vec2 scroll_speed;   // UV units per second, e.g. (0.1, 0.0)
in vec2 uv;
out vec4 frag;
void main() {
    vec2 scrolled = fract(uv + scroll_speed * time);  // HLSL: frac(...)
    frag = texture(tex, scrolled);
}
// Drive with a real time uniform, not a per-frame accumulator, so speed is stable.

3. Dissolve (threshold a noise map, glow the edge)

// Hide pixels where noise < threshold; tint a thin band at the boundary.
uniform sampler2D tex;
uniform sampler2D noise_tex;     // grayscale noise, 0..1
uniform float amount;            // 0 = fully visible, 1 = fully dissolved
uniform float edge = 0.05;       // width of the glowing edge band
uniform vec4 edge_color;
in vec2 uv;
out vec4 frag;
void main() {
    vec4 c = texture(tex, uv);
    float n = texture(noise_tex, uv).r;
    float cut = amount * (1.0 + edge) - edge;  // edge band starts below 0 -> none at amount 0
    if (n <= cut) discard;                     // cut away dissolved pixels (all of them at 1)
    float e = smoothstep(cut, cut + edge, n);  // 0 at the edge -> 1 inside; keep edge > 0
    frag = mix(edge_color, c, e);              // HLSL: lerp(edge_color, c, e)
}
// WRONG: thresholding at `amount` directly — at amount 0 every pixel with
// n < edge is still tinted, so the "fully visible" sprite shows glowing specks.

4. Fresnel rim light (3D) — brighten glancing angles

// Rim = 1 where the surface is edge-on to the camera (silhouette glow).
in vec3 world_normal;        // normalized, world space (from the vertex stage)
in vec3 view_dir;            // normalized, surface -> camera, world space
uniform float power = 3.0;
uniform vec3 rim_color;
out vec4 frag;
void main() {
    float f = pow(1.0 - clamp(dot(world_normal, view_dir), 0.0, 1.0), power);
    frag = vec4(rim_color * f, 1.0);   // add to lighting; f peaks at the silhouette
}
// Correctness: normal and view_dir MUST be in the same space and normalized.

Pitfalls

• Mixing coordinate spaces (lighting a world-space normal against a view-space light) yields subtly wrong shading. Pick one space and convert everything into it.
• Forgetting to normalize interpolated normals/directions: interpolation shortens vectors, so dot() results drift. normalize() in the fragment stage.
• UV assumptions across engines. Some engines flip V (top-left vs bottom-left origin); a texture may appear upside-down. Know your engine's convention.
• Heavy branching / dynamic loops stall GPUs. Prefer step/smoothstep/ mix; reserve if/discard for genuinely cheap early-outs.
• discard defeats early-Z and can hurt performance on tiled mobile GPUs; prefer alpha blending where you can.
• Reversed smoothstep edges. The GLSL spec leaves smoothstep(e0, e1, x) undefined when e0 >= e1, so smoothstep(1.0, 0.5, d) may work on one GPU and not another. Write 1.0 - smoothstep(0.5, 1.0, d).
• Precision on mobile: highp vs mediump matters; large UVs or time values in low precision shimmer. Use adequate precision for coordinates and time.
• Assuming GLSL == HLSL. mix↔lerp, fract↔frac, texture()↔.Sample(), vec2↔float2, M * v↔mul(M, v), mod↔fmod (different for negatives). See the reference mapping.
• A GLSL ShaderMaterial that renders nothing in three.js under WebGPURenderer: that renderer only runs node materials (TSL), on its WebGPU and WebGL 2 backends alike. Keep WebGLRenderer for raw GLSL or port to TSL.

References

• references/effects.md — full outline (2D sprite + 3D), vignette, and color grading shaders; the GLSL↔HLSL function/type mapping table; per-engine notes (Godot canvas_item/spatial, Unity ShaderLab/HLSL, Unreal material nodes).

Related skills

• godot-shaders — Godot shading language syntax, built-ins, and screen-reading.
• unreal-niagara — GPU particle VFX (a different shader use).
• procedural-gen — the noise that drives dissolve and procedural texturing.