FLUX Video Edit [fast]

Introduction

Light and location are the two things a shoot commits to hardest. A property clip filmed at noon cannot be re-filmed at golden hour without a second visit. A fitness campaign shot in July cannot be reshot in November for the winter push. A spokesperson recorded against the only clean wall in the office is stuck with that wall in every market.

Changing the setting is the edit that moves the camera's world without moving the camera. The subject, the motion and the timing stay put, and everything around them is re-rendered.

The sofa has not moved and neither has the camera. The sun has. This guide covers relighting, seasons, background swaps behind a person, and the parts of a shot that will not survive being relocated.

The request

A setting change is the same call as any other edit. Everything distinctive is in the sentence.

import { createClient } from '@runware/sdk'

const client = await createClient({ apiKey: process.env.RUNWARE_API_KEY })
await client.connect()

const [result] = await client.run({
  model: 'bfl:flux@video-edit-fast',
  positivePrompt: 'Make it golden hour, with low warm sunlight raking through the window and long soft shadows across the floor.',
  inputs: {
    video: 'https://vm.runware.ai/video/os/a14d18/ws/2/vi/5c9e1a74-8b26-4d30-a7f5-3e1c6d9b4a25.mp4'
  },
  deliveryMethod: 'async'
})

import asyncio
import os

from runware import Runware


async def main():
    async with Runware(api_key=os.environ["RUNWARE_API_KEY"]) as client:
        results = await client.run({
            "model": "bfl:flux@video-edit-fast",
            "positivePrompt": "Make it golden hour, with low warm sunlight raking through the window and long soft shadows across the floor.",
            "inputs": {
                "video": "https://vm.runware.ai/video/os/a14d18/ws/2/vi/5c9e1a74-8b26-4d30-a7f5-3e1c6d9b4a25.mp4"
            },
            "deliveryMethod": "async"
        })


asyncio.run(main())

curl https://api.runware.ai/v1 \
  -H "Authorization: Bearer $RUNWARE_API_KEY" \
  -H "Content-Type: application/json" \
  -d '[
    {
      "taskType": "videoInference",
      "taskUUID": "1f6a3c92-7d45-4b08-9e21-4a8c2d5f7b13",
      "model": "bfl:flux@video-edit-fast",
      "positivePrompt": "Make it golden hour, with low warm sunlight raking through the window and long soft shadows across the floor.",
      "inputs": {
        "video": "https://vm.runware.ai/video/os/a14d18/ws/2/vi/5c9e1a74-8b26-4d30-a7f5-3e1c6d9b4a25.mp4"
      },
      "deliveryMethod": "async"
    }
  ]'

runware run bfl:flux@video-edit-fast \
  positivePrompt="Make it golden hour, with low warm sunlight raking through the window and long soft shadows across the floor." \
  inputs.video=https://vm.runware.ai/video/os/a14d18/ws/2/vi/5c9e1a74-8b26-4d30-a7f5-3e1c6d9b4a25.mp4 \
  deliveryMethod=async

{
  "taskType": "videoInference",
  "taskUUID": "1f6a3c92-7d45-4b08-9e21-4a8c2d5f7b13",
  "model": "bfl:flux@video-edit-fast",
  "positivePrompt": "Make it golden hour, with low warm sunlight raking through the window and long soft shadows across the floor.",
  "inputs": {
    "video": "https://vm.runware.ai/video/os/a14d18/ws/2/vi/5c9e1a74-8b26-4d30-a7f5-3e1c6d9b4a25.mp4"
  },
  "deliveryMethod": "async"
}

[
  {
    "taskType": "videoInference",
    "taskUUID": "1f6a3c92-7d45-4b08-9e21-4a8c2d5f7b13",
    "videoUUID": "7d3f5b18-4a92-4c60-b1e7-9c2a5d3f8e24",
    "videoURL": "https://vm.runware.ai/video/os/a14d18/ws/2/vi/7d3f5b18-4a92-4c60-b1e7-9c2a5d3f8e24.mp4"
  }
]

Relighting and time of day

This is the highest-confidence setting change, because the geometry of the scene does not move. The model keeps every surface where it is and changes what falls on them.

One source, three times of day:

All three prompts share a structure worth reusing: a named time or condition, then the direction and quality of the light, then what that light does to a surface in the frame.

• Name the light's direction. "Raking through the window" is a vector the model can apply. "Warmer" is a grade adjustment, and if a grade is what you want, do it in your edit rather than by regenerating the frame.
• Name a surface the light lands on. Long shadows on the floor, rain on the glass, lamps in the room. This is what stops a relight from reading as a color filter over the original lighting.
• Say what the light source is at night. A dark room with no named source gives the model no reason to put a highlight anywhere in particular. "Warm table lamps" and "city lights beyond the glass" tell it where the light lives.

Season

Moving a clip through the calendar is a relight plus a set dressing change, and it is one of the few edits that genuinely saves a second production. A fitness or lifestyle clip shot once carries a full campaign calendar:

A season instruction needs three registers at once: the foliage, the ground, and the light. Naming only the trees gives you autumn leaves under July sunshine, which reads as a color treatment rather than a month.

Swapping the background behind a person

Background replacement is the setting change with the most direct commercial use: one recording of a spokesperson, several environments, no green screen and no keying step.

Two details decide whether this holds up.

Say how much depth the new background has. "Thrown well out of focus" is not a stylistic preference, it is a way of telling the model that the background is far away and should not compete with the subject for sharpness. A background rendered at the same focus as the person reads as a flat backdrop hung behind her, which is exactly the look green-screen work spends its time avoiding.

Match the new background to the light already on the subject. She is lit softly from the front left, so a background implying a hard source from the right asks the viewer to accept two different rooms in one frame. When the mismatch is unavoidable, stack a lighting instruction alongside the background one so both change together.

Moving the whole scene

Relighting keeps the room. Relocating replaces it. "Put her in a warehouse loft" is a much larger request than "change the wall behind her", and it is the point at which a setting change starts competing with simply generating a new shot.

The honest guidance is about how much of the frame the subject occupies. A tight shot where the subject fills the frame can be relocated convincingly, because the new setting is a small, soft area around the edges. A wide shot where the setting is most of the pixels is being regenerated in all but name, and the source is only supplying the motion.

When you do relocate a wide shot, keep the geometry of the new setting close to the old one. A room with a similar ceiling height, a similar distance to the back wall and a similar floor plane gives the camera move something plausible to travel through. A dolly filmed in a small apartment does not move like a dolly filmed in a warehouse, and the mismatch shows in the parallax rather than in any single frame.

The instruction also needs a second half that is easy to forget: decide what happens to the surfaces the subject touches. Her forearms are resting on a marble island, and preservation by default means an instruction that names only the room leaves that island exactly where it was:

Her hands are correctly on the stone in both, so the contact never breaks. What breaks is the story: a white marble kitchen island is not furniture an industrial loft would contain, and it survived precisely because the prompt did not mention it. The second instruction replaces it with a reclaimed-timber workbench at the same height, which is the clause that lets her keep the exact pose while the object under her changes.

The distinction worth carrying into the next relocation is whether the surface is part of what you are replacing:

• A separate object in the room (an island, a desk, a chair) survives untouched, so name it only when it would look wrong in the new setting. That is a continuity problem, not a physics one.
• A surface that is the setting (a cliff face, a floor, a wall the subject leans against) disappears with everything else, and its replacement has to be named or there is nothing left to touch. Black Forest Labs' climber moves from a cliff to a glass tower, and the instruction has to add metal window ledges at the hand and foot contact points because the rock they were on is gone.

What survives and what does not

A setting change is the edit most likely to alter things you did not ask about, because light touches everything. Expect these to move with it:

• Skin tone and wardrobe color, which are rendered under the new light rather than graded.
• The overall contrast of the frame, since a night version of a daytime shot is a different exposure, not a darker one.
• Reflective surfaces, which now show a scene that was never in front of them.

And expect these to stay: framing, camera path, timing, the subject's motion and performance, and the audio.

Keeping a set consistent across shots

Running the same setting instruction on several clips does not guarantee the same result, because each is an independent generation and there is no seed to lock. Two clips cut from the same shoot, given the identical sentence, can come back with the sun in a different place and the grade pushed to a different depth. Preparing source video shows the failure on the hardest version of the problem: one continuous walkthrough, cut in two, relit with the same words, and the join still visible.

Two things narrow the spread:

• Reuse the instruction verbatim. Not a paraphrase, the same sentence. Small wording differences are the largest controllable source of variation between shots.
• Give the instruction a neutral anchor. "Keep the white walls neutral where the sunlight does not fall" stops one clip reading the request as a warm grade over everything and the next as a warm patch on the floor. Naming what stays unchanged is worth more here than naming the light better.

For a sequence that has to match exactly, the fallback is not a better prompt. It is grading the treated clips together in your edit afterwards, which is a far cheaper way to close a small gap than re-running generations until two of them agree.

Tips

1. Name the time, the direction and a surface. "Golden hour, raking through the window, long shadows on the floor" gives the model a light source and something to prove it on.

2. Give a night scene a light source. Without a named lamp or window, a dark frame has no reason to put a highlight anywhere.

3. Relight shots that contain their own light. A visible window, lamp or sky justifies the change and constrains it.

4. Cover foliage, ground and light for a season. Leaves alone give you autumn color under summer sun.

5. Stack wardrobe with a season change. Snow does not put anyone in a coat, and summer clothing is what gives a winter edit away.

6. Say how defocused a new background is. Depth is what separates a room behind someone from a backdrop hung behind them.

7. Match the background's implied light to the subject's actual light. Two light directions in one frame is the classic composite tell.

8. Prefer locked-off shots for background swaps. A handheld camera asks the new background to have parallax it was never shot with.

9. Relocate tight shots, not wide ones. When the setting is most of the pixels, you are regenerating the shot and only keeping its motion.

10. Decide what happens to what the subject is touching. A separate object survives the relocation and can look wrong in the new room; a surface that was part of the old setting goes with it and needs a named replacement at the same height.

11. Treat the whole sequence. A relit clip carries a different grade and will not cut against untouched footage from the same shoot.