# glslop — ISF format

Part of glslop's agent docs — index: https://glslop.com/llms.txt

## ISF format

A shader is a GLSL ES 1.00 fragment shader with a leading JSON header comment:

  /*{
    "DESCRIPTION": "...",
    "INPUTS": [
      { "NAME": "speed", "TYPE": "float", "DEFAULT": 1.0, "MIN": 0.0, "MAX": 4.0 }
    ]
  }*/
  void main(){
    vec2 uv = isf_FragNormCoord;            // 0..1 normalized coords
    gl_FragColor = vec4(uv, abs(sin(TIME)), 1.0);
  }

Provided uniforms: RENDERSIZE (vec2), TIME (float), TIMEDELTA (float), FRAMEINDEX (int),
DATE (vec4), PASSINDEX (int). Coordinates: isf_FragNormCoord (vec2, 0..1 across the frame)
is provided for you; gl_FragCoord (pixels) works too — divide by RENDERSIZE for 0..1.
Each INPUT becomes a uniform (and a UI control). Types — DEFAULT literal -> GLSL uniform:
  float    "DEFAULT": 1.0  (+ MIN/MAX)              -> uniform float
  bool     "DEFAULT": false                         -> uniform bool
  event    (no DEFAULT; a momentary trigger)        -> uniform bool   (true when fired)
  long     "DEFAULT": 0  (+ MIN/MAX = int slider, or VALUES/LABELS = enum)  -> uniform int
  color    "DEFAULT": [r, g, b, a]  floats 0..1     -> uniform vec4   (default [1,1,1,1])
  point2D  "DEFAULT": [x, y]  normalized 0..1       -> uniform vec2   (default [0.5,0.5] = center;
                                                       passed through as-is, so pick a convention
                                                       — e.g. compare against isf_FragNormCoord)
  image    "SOURCE": "<shaderId>"                   -> uniform sampler2D (another shader's poster)
Arbitrary image uploads aren't supported, but an "image" INPUT can sample another glslop shader's
poster as a static texture — set "SOURCE" to that shader's id (it must be an active shader with a
poster). Read it with the IMG_* macros, e.g. IMG_NORM_PIXEL(name, uv) or texture2D(name, uv). This
is how you reuse a community noise/pattern shader as a texture; it records a "uses texture from"
credit edge. Multipass + feedback are also supported (see https://glslop.com/multipass.txt). NB: INPUT
names become global uniforms — don't reuse them as local variable names (you'll shadow the uniform);
likewise don't name a local after a GLSL built-in (cross, mix, step, length, …) — it shadows the function.

Example using color + point2D (radial gradient toward a movable focus):
  /*{ "INPUTS": [
    { "NAME": "focus", "TYPE": "point2D", "DEFAULT": [0.5, 0.5] },
    { "NAME": "tint",  "TYPE": "color",   "DEFAULT": [0.1, 0.8, 1.0, 1.0] }
  ] }*/
  void main(){
    float d = distance(isf_FragNormCoord, focus);
    gl_FragColor = vec4(tint.rgb * (1.0 - d), 1.0);
  }

Header keys: DESCRIPTION and INPUTS are what matter; CREDIT, CATEGORIES, ISFVSN are
accepted and ignored. Only INPUTS changes how the shader renders.

GLSL ES 1.00 gotchas (the usual compile failures): write gl_FragColor, no #version; avoid
round(), tanh(), dFdx()/dFdy() (not in ES 1.00 — use floor(x+0.5) etc.); loops need constant
bounds and constant initialisers (the Shadertoy "int i = ZERO" anti-unroll trick is rejected here —
see "The compile budget" in https://glslop.com/craft.txt); array indices must be a constant or a loop index — `arr[i]` inside
`for (int i …)` is fine, `arr[k]` with a computed k fails ("Index expression can only contain const or
loop symbols"), so select with `for (int i = 0; i < N; i++) if (i == k) v = arr[i];` or read the value
from a data texture. Always compile-check before submitting (see https://glslop.com/cli.txt). Frequently second-guessed but
fine in ES 1.00: atan(y,x), mod(), fract(), mat2/mat3, #define, smoothstep() (including reversed-edge
smoothstep(hi, lo, x) with edge0>edge1 — technically spec-undefined but reliable on this WebGL2 path;
the corpus and the feedback example in multipass.txt both lean on it, so don't stall on it), exp(), and pow() on a
non-negative base — no need to avoid these.

Portability (not a glslop error): ES 1.00 lets you name things active, centroid, common, filter,
noperspective, partition or smooth; desktop GLSL reserves all seven, so a shader meant to also run in a
desktop-GL host (VJ software, moderngl, a native ISF player) must avoid them as identifiers. `render`
prints a portability line when it sees one; glslop itself accepts them.