Photo in a Timeline slot ignores source_in and motion: why

When a Timeline video slot points at an image, the render holds that image for the slot's whole duration and ignores two fields that only make sense on video: source_in and motion. Each ignored field produces a soft warning, still_source_in_ignored or motion_ignored, and the job still completes.

The Timeline docs put it directly: stills are static holds, and motion is accepted and ignored with motion_ignored. The code adds the other half, the in-point.

What we saw when we compiled it

We compiled a two-slot, 12-second timeline whose first slot is a photo with source_in: 2. The output warning read: "Segment 0 is a still image; source_in 2.000s was ignored and the still is held for the whole slot. Timeline position unchanged." Nothing was cropped from the start, nothing was shortened.

A still is looped into its slot, so it is also never too short. A 6-second slot on a one-frame image does not trigger the short-source pad or loop warnings that a video clip would.

If you wanted movement

Timeline does not animate stills. Camera moves such as a push-in or a pan have to come from a video clip. Generate the move with an image-to-video model on the video router, then drop the resulting clip into the slot. Choosing the model and checking its price is covered in the Ken Burns post.

If you wanted the photo to appear later in time, that is the slot's start, not source_in. start is where on the audio spine the slot begins; source_in is where inside the file to start reading, and a one-frame file has no inside.

Clean up the document

Remove source_in and motion from image slots, and keep fit set deliberately. A photo with a ratio different from the output is cropped by the default cover, so use contain or blur if the whole picture must stay visible. Then the warnings list is empty and any warning that remains is worth reading.