Loops#

For-each loops and frame loops.

9 nodes.

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Contents#

• For-Each Begin (PCG) — Starts a for-each loop block and hands each piece, point, primitive or iteration index to the loop body.
• For-Each End (PCG) — Ends a for-each loop block and gathers the results of all iterations.
• For-Each Frame Begin (PCG) — Starts a per-frame loop that re-evaluates the loop body for each frame and outputs the frame number and time.
• For-Each Frame End (PCG) — Ends a per-frame loop and collects each frame's geometry and/or COP image into sequences.
• Geo Sequence Pick (PCG) — Picks one frame from a geometry sequence by index, wrapping around at the end.
• Retime (PCG) — Remaps the timing of a geometry sequence to a new sequence, by speed, by target length or by a time-warp curve.
• Select Best (PCG) — Scores every geometry in a sequence with an expression and outputs the best one, the top K and a ranked report.
• Time Blend (PCG) — Extracts one geometry at a fractional frame of a sequence by interpolating between the two neighbouring frames.
• Trail (PCG) — Builds a trail from the previous frames of a geometry sequence, as stacked copies, polylines, a swept mesh or a velocity attribute.

Nodes#

For-Each Begin (PCG)#

PCG_ForEachBegin · category PCG/loop

Starts a for-each loop block and hands each piece, point, primitive or iteration index to the loop body.

Equivalent of Houdini's Block Begin. It decides how the input is cut into pieces and how many times the chain between Begin and For-Each End runs. Connect Begin's loop output to For-Each End's loop input; that link is how End finds its Begin, and without it there is no loop.

Iteration Method: By Pieces makes one iteration per distinct value of Piece Attribute (typically name or class), on primitives or points; By Primitives and By Points make one iteration per element; By Connectivity makes one per connected component; By Count does not cut the geometry and simply runs Iterations times with the whole input. Group restricts the elements that are cut (all other modes except count). Pieces on points keep isolated points and only primitives whose points are all in the piece.

Method controls what the geo output carries: the current piece (Extract Piece or Point), the result of the previous iteration (Fetch Feedback, the original input on the first iteration), or always the original input (Fetch Input, equal to the input output). The input output always carries the uncut input, so no second Begin is needed for that.

Iteration metadata is written as detail attributes on geo: iteration, numiterations, value (string) and ivalue, so Wrangles inside the loop can read them with detail(). value is the piece attribute value in piece mode and the element or iteration number otherwise; ivalue is that value as an integer when it is numeric, else the iteration index.

The loop is realised by expanding the loop body one iteration at a time, so with many iterations the node ids grow longer; Max Iterations is the safety stop. Nested loops are supported. If there are zero pieces, the body still runs once on an empty geometry that keeps the input's attribute layout, and End outputs an empty result.

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Compared with Houdini

In Houdini, Iteration Method, Piece Attribute and similar settings live on Block End, and Block Begin looks them up. Here they are on Begin and End keeps only Gather Method, so the settings are part of Begin's cache signature. The extra input output replaces a second Begin set to Fetch Input, and the Fetch Metadata method is replaced by detail attributes on geo. Zero iterations cannot skip the loop body entirely; it runs once on empty geometry.

Tips

• Use Single Pass to debug the loop body on one piece before running everything.
• To process fractured pieces separately, run Voronoi Fracture into By Pieces on the name attribute.

For-Each End (PCG)#

PCG_ForEachEnd · category PCG/loop

Ends a for-each loop block and gathers the results of all iterations.

Equivalent of Houdini's Block End. The geometry connected to geo is the result of one iteration. loop must come directly from the paired For-Each Begin; it tells End which Begin to loop back to.

With Merge Each Iteration every iteration's result is merged into one geometry. With Feedback Each Iteration only the last result is kept; combined with Begin's Fetch Feedback method this makes an iterative feedback loop. The number of iterations and how the input is split are set on Begin, not here.

End's geo input has to be downstream of Begin: a link that comes from outside the loop body raises an error, because the loop would silently recompute the same result each time.

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Compared with Houdini

Iteration settings are on For-Each Begin rather than End; see the note on For-Each Begin.

For-Each Frame Begin (PCG)#

PCG_ForEachFrameBegin · category PCG/loop

Starts a per-frame loop that re-evaluates the loop body for each frame and outputs the frame number and time.

The way to make geometry change over frames. Connect frame, frame_f, t or seconds to parameters inside the loop body (convert the widget to an input; put an Expression in between to shape a curve) and the geometry follows the frame. The whole body is re-evaluated for every frame; nodes outside the block run once and stay cached. Connect loop to the loop input of For-Each Frame End.

Method: input starts every frame from the original input, so frame N depends only on frame (keyframe animation, procedural deformation) and frames are independent. feedback starts every frame from the previous frame's result and frame 0 from the original input, which is what Houdini's SOP Solver does (growth, accumulation, progressive fracture). Feedback is truly sequential.

Cams: connect a Camera Path and cam outputs the camera for the current frame, so a Render Pass in the body gets both moving geometry and a moving camera. The camera count must equal the frame count, or be 1 for a fixed camera; anything else is an error.

Frame metadata is also stored as detail attributes on geo: frame, frameindex, numframes, t and time, readable with detail() in a Wrangle. If no geometry is connected, an empty geometry is used.

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Compared with Houdini

The feedback method corresponds to a SOP Solver. The timeline is not read from a global $F; time comes only from this node's outputs.

For-Each Frame End (PCG)#

PCG_ForEachFrameEnd · category PCG/loop

Ends a per-frame loop and collects each frame's geometry and/or COP image into sequences.

Collects the result of every frame into sequences. Connect geo, cop or both; at least one is required, and whatever is connected is collected. A typical setup sends cop to Sequence Export for video and geoseq to Sequence Motion for motion vectors.

The accumulated frames are held in memory as literals in the expanded graph. Memory is counted as the loop runs, and when it exceeds about 1 GB the node raises an error that explains the options: lower the resolution, keep fewer layers, use fewer frames or run in several segments. (240 frames of 1024x1024 with five layers is about 4.7 GB.)

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Geo Sequence Pick (PCG)#

PCG_GeoSequencePick · category PCG/loop

Picks one frame from a geometry sequence by index, wrapping around at the end.

Takes one geometry out of a geometry sequence such as the geoseq output of For-Each Frame End. The index wraps around (modulo the sequence length) instead of raising an error, because going out of range is normal while scrubbing an index. An empty sequence is an error.

A common use is checking an animation: pick frame 0 and frame 5 into two Viewports and compare.

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Parameters

Retime (PCG)#

PCG_Retime · category PCG/loop · Houdini: Time Shift (SOP)

Remaps the timing of a geometry sequence to a new sequence, by speed, by target length or by a time-warp curve.

Builds a new sequence by choosing, for each output frame i, a (possibly fractional) source frame.

• Speed: source = start + i * speed. A speed of 0.5 is half speed, a negative speed plays backwards.
• Output Length: spreads frames output frames evenly over the whole source sequence. Changes duration only, not content.
• Time Warp Channel: source = a keyframe channel evaluated at i, for time warps such as rush, hold, ease out: 0:0 6:12 14:12 24:23. This is the only mode that can express a pause.

Interpolation: Integer Frames (Hold) takes the nearest source frame, Blend Subframes interpolates between source frames and requires all frames to have the same topology (an error names the offending frame). Integer, boolean and string attributes are not interpolated but taken from the nearer frame. Source frames outside the sequence are clamped to the ends, or wrapped when Loop is on.

An identity mapping is exactly the identity: with channel 0:0 n-1:n-1 (or speed 1 and start 0) every output frame is bit-identical to the input frame, with no multiply-add.

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Compared with Houdini

Based on the Retime SOP; the time-warp channel mode takes the place of animating the retime with a curve.

Select Best (PCG)#

PCG_SelectBest · category PCG/loop

Scores every geometry in a sequence with an expression and outputs the best one, the top K and a ranked report.

Closes the loop for automatic variant selection: run many variants (for example a For-Each over N seeds), collect them as a geometry sequence, and let the node choose.

The score is an expression evaluated for each geometry. It can use every metric from Geo Metrics (prims, closed, flipped_edges, uv_overlap, genus ...), i and n (index and count) and the geometry-reading functions of Wrangle / Switch (detail("score"), npoints(), prim("area", 0, 0)).

Aesthetic scores (CLIP, aesthetic predictors, human preference) are image-side and not computed here. Write them in the loop as a detail attribute on the geometry (Attribute Create, detail class), then weight them with structural criteria in one expression, e.g. detail("score") * 2 - nonmanifold_edges * 0.1 - degenerate_prims. Carrying the score on the geometry keeps score and geometry together; separate parallel lists drift out of step.

Ties are broken by taking the lower index, so the same input always selects the same variant. report is a JSON list sorted by rank with the score and the metrics named in Report Metrics.

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Tips

• Make a hard gate by multiplying the score by a metric such as closed.

Time Blend (PCG)#

PCG_TimeBlend · category PCG/loop · Houdini: Sequence Blend (SOP)

Extracts one geometry at a fractional frame of a sequence by interpolating between the two neighbouring frames.

The counterpart of Houdini's Time Blend: samples a sequence at a possibly fractional frame. Typical uses are rendering a 24 fps sequence at 48 fps, and slow motion, where frame is driven by a Channel whose curve advances slower than one frame per frame.

Blend Subframes requires the whole sequence to have the same topology (point i is the same point in every frame) and raises an error naming the frame that differs. Sequences whose topology changes, such as fracturing or growth, must use Integer Frames (Hold), which takes the nearest integer frame without interpolating. Integer, boolean and string attributes (id, class, name) are never interpolated and come from the nearer frame, as do groups; they therefore switch when the position passes halfway between two frames.

Outside the sequence the frame is clamped to the first or last frame, or wrapped when Loop is on.

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Compared with Houdini

Corresponds to Time Blend. Houdini also requires matching topology but reports a mismatch less clearly; here the error names the offending frame.

Trail (PCG)#

PCG_Trail · category PCG/loop · Houdini: Trail (SOP)

Builds a trail from the previous frames of a geometry sequence, as stacked copies, polylines, a swept mesh or a velocity attribute.

Takes the frame at index as the current frame and looks back length frames, stepping increment frames at a time. Frame numbers outside the sequence are clamped to the ends (or wrapped with Loop), not dropped, so the trail has constant length at the start of the sequence.

Modes: Preserve Original merges every frame of the trail unchanged, for ghosting or motion-blur reference, and adds a point attribute trailframe (0 is the current frame, -1 the previous one, ...) to fade by. Connect as Polygons joins the positions of each point over the trail into one open polyline, with a point attribute trailu from 0 (oldest) to 1 (current). Connect as Mesh sweeps every primitive edge between consecutive trail frames into a quad (a curve becomes a ribbon, an outline becomes a tube wall); an edge shared by two faces is swept once, and geometry with no edges, such as a point cloud, falls back to the polylines. It also writes trailu. Compute Velocity outputs the current frame with a velocity attribute in units per frame: a backward difference from the previous frame (spaced increment apart), or a forward difference on the first frame of the sequence.

The Connect modes require the frames used to have the same topology; Compute Velocity requires the two frames compared to match.

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Compared with Houdini

Corresponds to Trail. Houdini's Connect as Mesh on point clouds is the same as Connect as Polygons here; on geometry with edges, Connect as Mesh sweeps edges into quads.