shader-programming

SKILL.md

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

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.

Track coordinate spaces. Positions move model → world → view → clip space;
normals belong in world or view space. Mixing spaces is the most common bug.

Drive effects with UVs and time. UVs are 0..1 texture coordinates;
offset, scale, or distort them, and animate with a time uniform.

Work per pixel, branch-light. Prefer mix, step, smoothstep, and
clamp over if where possible; GPUs run pixels in lockstep and dislike
divergent branches.

Pass data via uniforms (constant per draw) and varyings (interpolated
vertex→fragment). Keep texture samples few; they dominate cost.

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-style fragment snippets (close to Godot's canvas_item/spatial
shaders and OpenGL). 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;
    if (n < amount) discard;                 // cut away dissolved pixels
    float e = smoothstep(amount, amount + edge, n);  // 0 at the edge -> 1 inside
    frag = mix(edge_color, c, e);            // HLSL: lerp(edge_color, c, e)
}

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

// Rim = 1 where the surface faces away from 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.

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, column- vs row-major matrices. See the reference mapping.

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.