Specimen Authoring

Specimen Authoring

How to build a Specimen for the Embody Collection -- a transparent, reusable TDXN network that demonstrates a TouchDesigner technique. Two specimens set the bar: reaction-diffusion (generative, a GPU feedback simulation) and kaleidoscope (compositing, a reusable polar-mirror component).

The bar -- every Specimen must clear it

1. Clear use -- a user can finish "I'd use this for ___" (a VJ loop, a drop-in component, a texture/displacement source, a real learning reference).
2. Non-obvious technique -- a "how'd they do that?" moment, not a one-TOP drag.
3. Striking -- worth opening and exploring.
4. Drop-in -- clean input/output, exposed parameters, self-contained, bounded for performance.

Generic noise plus a colorize is NOT a specimen. If a beginner makes it by accident, cut it.

Workflow (one at a time)

1. Build in the sandbox COMP /specimen_lab/<name> (NOT inside the Embody COMP -- the toe file, somewhere neutral). Iterate freely there.
2. Gate performance (see performance.md): baseline get_project_performance before, re-check after each heavy step. Feedback loops and GLSL are the usual cost. fps below ~90% of target is a stop condition -- optimize before continuing. Beware: a concurrent capture_top or numpyArray() stalls the GPU and makes the fps reading dip; take a clean reading without them.
3. Verify it actually animates and cooks before believing it -- see "Cook demand" below. Never claim animation from a single forced capture.
4. Judge the look by actually looking -- capture_top then read the frame; load visual-aesthetics. capture_top force-cooks, so it can show a frame your live (undemanded) viewer is NOT showing -- a stale viewer and a fresh capture are different frames; that mismatch is real, not a hallucination.
5. Ask the user to review before persisting. Persist only when it is genuinely good, animating, and clean.

Cook demand -- the trap that bit both specimens

A specimen that references time only cooks when its output is demanded (see td-python.md Cook Model). Consequences:

• In the sandbox nothing views out1, so add a temporary frame-driver: an Execute DAT in /specimen_lab (OUTSIDE the COMP) with onFrameStart cooking the specimen's out1. It is not part of the spec; living outside the COMP means it never exports.
• Verify animation with two frames: capture, let real seconds pass (a background sleep), capture again, compare. If the content differs (beyond a pure rotation), the motion is real. A single frame proves nothing.
• The shipped specimen needs no driver: when a user views out1, or the build pipeline cooks warmup_frames, the time-dependence runs it.

Performance: static source + cheap motion

A high-detail generator (domain-warped fBm, a large sim) cannot re-cook every frame at 1280px+. Make it static (remove every time reference so it cooks once and caches) and animate a cheap downstream op -- drift/rotate/warp the sample coordinates, not the source. Confirm glsl_source.cookedThisFrame == False and the animated op == True. (Reaction-diffusion is the exception: the feedback loop must cook every frame, so keep it bounded -- <=512x512, 32-bit float.)

Procedural terrain (why fbm looks generic and ridged does not)

• Ridged multifractal, not plain fbm. Plain Perlin/Simplex fbm reads as "generic noise"; a ridged multifractal reads as real mountains. The recipe: ridge transform n = 1 - abs(2*noise - 1) (sharp ridgelines + V-valleys), pow(n, sharp) to sharpen, multifractal weighting sum += n*amp*prev (rough peaks, smooth valleys = erosion-like), a domain warp (offset the sample coords by another noise) for organic non-grid forms, and offset the height down so valleys drop below the snow line for rock/snow contrast.
• Keep GEOMETRY octaves LOW (3-4, max freq ~8 at a 128 grid) and let the MATERIAL bump carry micro-detail. Ridged geometry is rougher than fbm, so extra octaves alias into speckle.
• In-place 4D morph, not a pan. Use 3D value noise with the 3rd coord = time*rate (a different rate per octave) so the terrain DEFORMS IN PLACE (multi-scale undercurrents). Translating the sample coords by time would pan instead.
• Geometry-roughness vs material-speckle harmonization. Rougher (ridged) geometry makes the material's high-frequency terms (slope-dependent snow, per-pixel bump, fine brightness noise) amplify into salt-and-pepper speckle. Fixes: make snow elevation-based, not slope-based; suppress the bump where there is snow (bump *= 1 - 0.85*snowMask -- smooth snow / rough rock); lower the fine-noise frequency and amount.
• Snow exposure (avoid blowout). High-albedo snow * bright sun clips to flat white. Dim the sun ON SNOW (sun *= 1 - 0.55*snowMask), nudge snow albedo off pure white, and cut the specular.
• Atmospheric perspective depth. A saturating exponential fog reads flat / all-or-nothing. Instead normalize distance across the scene (depth = (dist - near) / range) and use a POWER curve (pow(depth, 1.7)*max) so the foreground stays clear and haze builds with distance; gate any valley mist to distant low areas only.

GLSL specifics

• Uniforms are set on the GLSL TOP via vec0name + vec0valuex/y/z/w (one vec4), or const0name/const0value. The values take expressions -- bind them to COMP params or time: gl.par.vec0name='uParams'; gl.par.vec0valuex.expr="parent().par.Segments.eval()", ...valuew.expr="absTime.seconds*parent().par.Flowspeed.eval()". Declare uniform vec4 uParams; and pack four values per vec.
• Vector uniforms are a parameter SEQUENCE on both glslMAT and glslPOP. Set the count with op.seq.vec.numBlocks = N -- NOT op.par.vec (a Sequence-style par whose .eval() always reads 0 and will mislead you). Set the type with vecNtype (e.g. 'vec4'). vecNvaluex/y/z/w accept expressions; bind them to params.
• Uniform declaration differs by op type. In a glslMAT or glslTOP you DO declare uniform vec4 uShape; manually. In a glslPOP compute shader the custom uniforms are AUTO-DECLARED from the Vectors page -- do NOT write uniform vec4 uShape; there or it fails to compile.
• Reach the specimen's params by shortcut, not by depth. Bindings live on ops, but the params live on the specimen COMP. Set par.parentshortcut = 'Specimen' on the specimen COMP once, then every descendant binds with parent.Specimen.par.X regardless of how deep it sits -- a POP inside geo and a MAT directly inside the COMP use the identical expression, and nesting one more level later breaks nothing. Only fall back to parent(N).par.X when the shortcut cannot be set, and then verify the depth with parent().path before trusting it -- parent(2) encodes the hierarchy and silently resolves to the wrong COMP the moment anything moves.
• Direction params (sun / light): expose azimuth + elevation, convert to a UNIT vector in the value expression -- x=cos(el)*sin(az), y=sin(el), z=cos(el)*cos(az) via math.cos/math.radians. A raw XYZ slider can be zeroed to (0,0,0) and NaN the shader's normalize(); az/el is unit by construction.
• TD's built-in noise: TDSimplexNoise(vec4) (also vec2/vec3) is available in every GLSL TOP/MAT shader -- no 80-line Ashima paste. Seamless loop = walk a circle in the zw plane (radius = how much it morphs per loop; scale zw slowly per octave or the fine detail flickers). Verified 2026-09-12, prismatic-strata.
• Multiple render targets: set numcolorbufs on the GLSL TOP, declare layout(location = 1) out vec4 fragField; beside fragColor, and pull buffer N with a Render Select TOP (top = the GLSL TOP, bufferindex = N). Its format defaults to rgba8fixed: set a float format, and encode signed data as 0.5 + 0.5*x anyway.
• Alpha as a data channel: the GLSL TOP's "Pre-Multiply RGB by Alpha" toggle (premultrgbbyalpha) is ON by default and multiplies the color by your mask -- turn it off on that op. capture_top then flags the intermediate fully_transparent; expected, judge out1.
• Ramp TOP keys: write the docked keys table with set_dat_content(rows=...) (header row pos r g b a). Rows appended with appendRow in the same execute_python that also changed the ramp's own parameters came back reverted to the default two keys (2026-09-12).
• Loop-frame captures: 24 synchronous TOP.save() PNGs at 1080p blow the 30 s MCP budget. Schedule each frame with run(code, delayFrames=k*3) (set the phase uniform to a constant, cook(force=True), save JPEG), restore the expression in a last run, then analyze the folder offline.
• Matching a reference clip: extract its frames and compare NUMBERS, not impressions -- luminance histogram, hue histogram, warm/cool fractions per tone band, x/y autocorrelation (feature shape), high-pass energy, thin-vertical energy, temporal autocorrelation and the loop seam. A stats loop converged where eyeballing oscillated (prismatic-strata, 2026-09-12).
• Boilerplate (TOP/MAT): out vec4 fragColor;, sample inputs texture(sTD2DInputs[0], vUV.st), texel size uTD2DInfos[0].res.xy. Bound every for/while with a constant (GLSL crash rule).
• A generator needs outputresolution='custom' + resolutionw/h; a simulation buffer needs format='rgba32float' or it bands and dies.
• Every GLSL op you create docks DATs that scatter onto neighbors -- a GLSL TOP docks pixel/compute/info; a GLSL MAT docks vertex/pixel/info; a GLSL POP docks compute/info. Hug them AND run get_network_layout in the SAME execute_python that creates the op. NEVER defer to a later cleanup pass: this exact trap has recurred repeatedly because the build loop is heads-down on the shader, and the scattered docks silently pile onto the camera/light/render until someone points at the mess. The Verify step is the only thing that catches it -- run it every time, not just at the end.

GLSL POP as a geometry generator

A GLSL POP runs a compute shader over points -- it is the GPU way to displace / generate geometry (it replaces a stack of fbm noisePOPs). Compute-shader API:

void main(){
  const uint id = TDIndex();
  if(id >= TDNumElements()) return;
  vec3 pos = TDIn_P(0, id);   // read input attribute -- NOT TDInPoint_P (that name fails to compile)
  // ...displace pos...
  P[id] = pos;                // write the output array directly -- NOT oTDPoint_P
}

• The output array is undeclared and EMPTY until you set outputattrs to * (or P). Without it, P[id] does not exist. initoutputattrs=on seeds the outputs from the inputs.
• The canonical template is in the docked compute DAT of a freshly created glslPOP -- it ships with //P[id] = TDIn_P();. Read that DAT to confirm the exact API for any build.
• Custom uniforms here are auto-declared (see GLSL specifics above) -- bind uShape, uTime, etc. to params via the vec sequence.

GPU particle systems and flocking (POPs)

A GPU particle sim is a POP feedback loop: spherePOP/gridPOP -> particlePOP -> ...forces... -> nullPOP, where the Particle POP's targetpop parameter points at the downstream Null (a relative ref, null_sim, never absolute) so last frame's state feeds the next.

• Transport is Initialize -> Start -> Play, not just play. The Particle POP has initializepulse (reset to empty), startpulse (begin emission), and play (toggle). Pulsing initializepulse alone leaves it initialized-but-not-emitting (count stays 0). After wiring targetpop or changing topology: pulse initializepulse THEN startpulse, and confirm play=True. Births accrue over real frames - a synchronous cook(force=True) loop does not advance them; add the temp frame-driver and let wall-clock pass.
• Forces live INSIDE the loop (between the Particle POP and the feedback Null - anything writing PartForce/PartVel/P). Topology-changing or render ops (Trail, color, render) live OUTSIDE it, on a side branch off the Null.
• Particle attributes: P, PartVel, PartForce, PartMass, PartDrag, PartAge, PartLifeSpan, PartId. With timeintegration ON the Particle POP integrates PartForce->PartVel->P itself (with its own drag/damping); your force op just WRITES PartForce each frame (overwrite, don't accumulate stale fed-back force).

Flocking: true per-neighbor Reynolds on the GPU

• Neighbor POP, two output modes. nebroutput='avg' gives neighbor-AVERAGE attributes (NebrP, NebrPartVel) + NumNebrs. nebroutput='nebr' gives a per-point neighbor INDEX list attribute Nebr (size maxneighbors) + NumNebrs. numhashbuckets ~= particle count; incquerypt=False excludes self.
• Centroid-only forces cannot break a symmetric clump. Cohesion/separation built from NebrP (the local centroid) net to ~zero at the center of a dense symmetric cluster, so the core never spreads - you get a blob no matter how you tune. The fix is true per-neighbor separation: in a GLSL POP, read the Nebr index list and random-access each neighbor (uint nIdx = TDIn_Nebr(0, id, i); vec3 nP = TDIn_P(0, nIdx);), then sum an inverse-square push clamp(k/(dist*dist),0,cap) * normalize(P-nP). The closest neighbor dominates, so it never cancels -> even, uniform spacing.
• TDIn_Nebr(...) returns uint (declare uint, not int). centroid is a reserved GLSL word - name it nbrCenter. Random access TDIn_P(0, nIdx) into the input buffer is allowed and is how you reach other points.
• A coherent body needs a soft containment (pull back when dist(P, attractor) > Bodyradius) + linear drag (force -= V*k), applied AFTER the force clamp so they keep authority. Without drag, velocities coast and the swarm flies apart; a strong global attractor (not local cohesion) is what compresses it.
• Verifying a flock is coupled. Forces are not independent - ablating one reshapes the whole emergent state (removing alignment can cause collapse, which RAISES coherence). Measure local residual alignment (subtract the swarm-mean velocity, then correlate neighbor residuals) to isolate alignment from global drift. A uniform random 3D swarm has median nearest-neighbor ~1.9% of the bbox diagonal (Poisson) - don't expect a "3% floor" without grid-like packing. Read attributes in bulk via a Pop-to CHOP + numpyArray() (CPU-resident, reliable); per-point pop.point('P',i) works only when the GPU is idle (pause the driver first) and is flaky for a second attribute in the same call.

Rendering a POP swarm

• Render Simple TOP renders a POP directly (pop param, auto camera/light, bgcolor) but has no point-size control and no additive blend - fine for a probe, not for a hero look. For glowing sprites set materialsource='matnode' and assign a Point Sprite MAT (mat).
• Point Sprite MAT: pointscaleattrib reads a per-point PointScale; pointsize is the constant multiplier; additive glow = blending=on, blendop=add, srcblend=one, destblend=one, depthtest=off, depthwriting=off. For soft round dots, feed a radial-falloff texture into colormap (a tiny glslTOP: a = smoothstep(1.0, 0.15, length(vUV.st*2-1)); fragColor = vec4(vec3(a),1)); a square sprite otherwise.
• To add a Color/PointScale attribute a glslPOP doesn't have, use its New Attribute sequence (seq.attr.numBlocks, attrNcustomname, attrNnumcomps), then write Color[id]/PointScale[id] in the shader. Map length(PartVel) to a 3-stop ramp for a speed-colored swarm. Additive blending blows out in dense moments - keep per-sprite brightness low (multiply the color down) and let Bloom carry the glow.

TDXN round-trips single-component custom-parameter VALUES (verified v6.0.26)

A specimen's exposed parameters export their definitions (default, range, help). A current value is written as an explicit value: only when it differs from the default; when value == default the value: is omitted. The importer covers both cases: it applies any explicit value:, and -- for a single-component par (Float, Int, Toggle, Str, Menu; no component suffixes, size 1) -- it seeds .val from the default: when no value was stored (TDXNExt.py ~L3362-3377, the elif ('values' not in par_def and 'default' in par_def ...) branch). So a specimen authored with default == intended value drops in live with correct values -- no onInitTD value-setter or one-time "revert to default" is needed.

Verified v6.0.26: re-importing plasma-interference via ImportNetwork brought every Float param back at its authored value (Scale.val == 6.0, not 0), and the network rendered and compiled identically to the original. (This is the fix the older "imports inert" warning anticipated -- that warning described pre-fix behavior and is now obsolete.)

The one real gap -- multi-component default-valued params. The default->value seeding is single-component only; a multi-component def (RGB / RGBA / XYZ / WH -- anything with component suffixes) carries one default that does not map cleanly across components, so a color/vector par whose value == default still imports at 0/min. If a specimen exposes such a par, give it a value that differs from the default (forcing an explicit value:/values: on export) or set it in an init hook. Single-component params -- which is almost everything a specimen exposes to drive uniforms -- are fine.

Still verify the round-trip in every build (cheap): re-import the exported .tdxn into a throwaway COMP, check a couple of .vals and that out1 renders, then destroy the temp COMP.

Naming, layout, output

• Processing ops: optype_name (glsl_colorize, feedback_state); DATs stay role-named (see td-python.md Naming).
• Hug docked DATs at creation, never defer -- deferring is what leaves them stacked.
• Every specimen terminates in an Out TOP named out1 (the C3 manifest contract output_op). Not a Null.
• Annotate per logical section (>=400 units between annotation boxes); the annotations are part of the teaching value. Spread the chain if needed to make room.

Persisting (storage model)

The collection lives at repo-root specimens/. Per specimen:

1. The temp driver stays out of the export (it's outside the COMP).
2. export_network the COMP -> specimens/<category>/<slug>.tdxn with include_dat_content=True (captures the shaders). The "Failed to track" warning is expected (outside dev/). The on-disk .tdxn is YAML (TDXN v2.0): shader/script dat_content is a plain string rendered as a YAML literal block scalar (|), so GLSL reads top-to-bottom and diffs line-by-line -- do not hand-author it as an array of lines (that was the v1.5 form; v2.0 reverts to a plain string). Keep shader source LF/space-indented, not tab-indented: a tab-bearing string falls back to an ugly double-quoted scalar. Auto-created default docked compute DATs are omitted on export and recreated by TD on import, so an unedited compute companion will not appear in the file. Legacy JSON .tdxn still import unchanged (json-first parse).
3. Write specimens/<category>/<slug>.prompt.md -- what it teaches, how it works, how to recreate it.
4. Add the entry to specimens/manifest.json (validate against specimens/manifest.schema.json): slug is kebab-case (reaction-diffusion, not reaction_diffusion), output_op is out1, requires is none for self-contained specimens, warmup_frames covers the thumbnail bake (feedback sims need hundreds-to-thousands; a cheap effect needs a few), operator_count = all operators including those nested inside sub-COMPs (annotations not counted).
5. Bake the RESULT thumbnail. build_specimens.py does not exist yet, so bake by hand -- this is required, not optional; an unbaked specimen shows "preview coming soon" in the gallery: a. Drive the specimen for its warmup_frames first. A single capture_top of an undriven feedback/particle sim grabs frame 0 (the cook-model trap -- this is what produced a dim/empty murmuration and an undeveloped reaction-diffusion). Schedule real frames: for i in range(1, warmup_frames+1): run("op('<comp>/out1').cook(force=True)", delayFrames=i), wait it out, then capture. Static shaders (kaleido, plasma, mandelbulb) need ~0; feedback sims need hundreds-to-thousands (reaction-diffusion wanted ~1800 to show real Gray-Scott mitosis). b. capture_top the out1, Read the image and judge it against visual-aesthetics (not black, real value range, clear subject). Re-drive + re-capture until the frame is strong. c. Save it BESIDE the network as specimens/<category>/<slug>.jpg (long edge <= 1200px, under 500 KB -- the same cap the site's upload route enforces; the generator refuses a larger file) and set the manifest's thumbnail_path to <category>/<slug>.jpg (repo-relative, like tdxn_path). d. Run python3 platform/apps/web/scripts/build-specimen-data.py and commit its output with the image. The cover then flows exactly like a community upload: the generator hashes it, seeds thumbnail_key = thumbnails/<sha256>, the uploaders put it in R2, and the site serves it from the row via /api/specimens/<slug>/thumbnail. Nothing in the site lists slugs -- the hardcoded BAKED_* sets and public/specimens/*.jpg were removed 2026-09-13; never reintroduce them. (When build_specimens.py lands it automates a-c from warmup_frames + thumbnail_path; until then this manual path is the procedure.)

After persisting

If you discovered a TD behavior that contradicts a rule or skill, update that rule/skill (and its shipped template if it has one) -- that is exactly how this skill came to exist.