Source Range From Segment Plan

Once your LTX-2 extension plan exists, someone has to answer the question "which frames of the source footage do I feed this segment?" That's IAMCCS_SourceRangeFromSegmentPlan. Given a segment's plan numbers, it computes the inclusive frame range into the source that the segment should consume - including the overlap backtrack so continuation segments re-see the tail of the previous chunk.

It's a small arithmetic node that sits between the planner family and the frame-slicing / start-image loading steps in the extension workflow. If you're doing source-video-to-video or frame-by-frame source alignment, this is the node that turns "segment 3 starts at frame 480" into "load frames 471 through 720" without you doing the off-by-overlap math by hand.

How it works

The key subtlety is the overlap backtrack. A continuation segment generates current_segment_raw_frames total, but only current_segment_unique_frames are genuinely new - the difference is overlap carried from the previous segment. So the range has to start earlier than the segment's unique start to include that overlap: the node computes overlap_delta = raw - unique, backtracks overlap_delta - 1 frames from current_segment_start_frames (and backtracks nothing for segment 0, since the first segment has no overlap), then returns range_start_index through range_end_index covering range_count frames. Every number is clamped to be safe, and report shows the full breakdown.

Inputs and outputs

Inputs (all straight from a planner's current-segment outputs, or the derivation node):

• segment_index - which segment you're on (drives the backtrack rule).
• current_segment_raw_frames - frames this segment will actually generate.
• current_segment_unique_frames - new content frames.
• current_segment_start_frames - where the segment begins in the timeline.

Outputs:

• range_start_index - first source frame to load
• range_end_index - last (exclusive)
• range_count - how many frames that is
• report - the math, spelled out

Installing

ComfyUI Manager → search "IAMCCS", or

cd ComfyUI/custom_nodes
git clone https://github.com/IAMCCS/IAMCCS-nodes.git

Restart. No models, no pip deps - pure integer arithmetic on the plan.

Gotchas

The classic error is feeding it planner outputs from a different plan than the one driving the sampler - the overlap backtrack silently shifts by a frame or two and you get a barely-perceptible jump at every segment boundary that's brutal to diagnose. Also note range_end_index is exclusive and range_count is end - start: when you're looping over the range to slice frames, use range(start, end), not range(start, start + raw). And segment 0's backtrack is zero by design - don't "fix" it by hand or you'll load one frame early.