{"course":{"slug":"ai-video","title":"Video with AI","url":"https://addaly.com/learn/ai-video","level":"intermediate","track":"create"},"lesson":{"slug":"motion-strength-and-brushes","title":"Taking motion out of language","url":"https://addaly.com/learn/ai-video/motion-strength-and-brushes","position":25,"of":66,"minutes":9,"answer":"Language is a poor way to specify a curve, so the controls that matter most are the ones that bypass it (a motion strength slider, a painted region.","keyIdea":"Language is a poor way to specify a curve, so the controls that matter most are the ones that bypass it (a motion strength slider, a painted region, a drawn trajectory) and each of them fails in a way the prompt equivalent does not."},"body":{"format":"markdown","text":"## Why these controls exist\n\nEvery interface in this category has, at some point, added a control that is not a text box. That is not product decoration. It is an admission that a sentence cannot specify how fast, how far, and in what curve.\n\nThe three you will meet:\n\n1. **Motion strength**, a single slider.\n2. **Motion brush**, where you paint the part of the frame that should move.\n3. **Trajectory or drag control**, where you draw a path an object should follow.\n\n## Motion strength, and why the good default is low\n\nA single number controlling how much the clip moves overall. Names vary: motion, dynamism, motion bucket, amount.\n\nThe relationship with quality is direct and mechanical. More motion means more change per frame, and every frame of change is another opportunity for drift, morphing limbs, background churn and identity slip. High motion does not just look busier; it is more likely to break.\n\nThe professional default is lower than feels exciting. A near-still frame with a slow drift, a small parallax and real ambient sound reads as footage. A high-motion clip with a morphing hand reads as generated, immediately, to everybody.\n\nThat is not timidity. It is choosing the version that survives being watched twice, which is the only test that matters for anything a client pays for.\n\nA useful calibration: find the motion value at which your model starts producing visible artefacts on your kind of content, then work one or two steps below it. That number is yours and it is worth twenty minutes of a free allowance to find.\n\n## Motion brush\n\nPaint a mask over the region that should move, leave the rest alone. Runway popularised it and similar tools exist elsewhere.\n\nWhat it is good for is specific and valuable: **motion that is local**. Steam rising from a cup while the room stays still. Hair moving while the face does not. Water in the bottom third. A flag. A single figure in a crowd.\n\nWhat it does not do is make the unpainted region truly static. It biases rather than freezes, so expect some residual drift outside the mask, and check for it rather than assuming.\n\nThe technique that gets the most from it: paint less than you think. A small moving region against a still frame is more convincing than a large one, because the stillness is what makes the motion read as real. This is the same principle as a subtle grade: the restraint is where the credibility lives.\n\n## Trajectory and drag controls\n\nDraw an arrow, or drag a point from where a thing is to where it should go. The tool converts that into a motion-conditioning signal.\n\nThis is the most direct control available and it is also the most brittle. Three honest limits:\n\n- **It controls position, not pose.** Dragging a person across frame moves them; it does not make them walk convincingly while doing so. You often get a sliding figure.\n- **It fights physics.** A drawn path that no real object would take produces exactly the impossible-looking result you asked for.\n- **It competes with the prompt.** If the text says one thing and the arrow says another, you get an averaged mess. Make them agree, or leave the motion out of the text entirely when you are drawing it.\n\nUse it for objects rather than people, and for simple translations rather than complex actions.\n\n## Camera controls as a separate axis\n\nSome tools separate camera motion from subject motion: sliders or gizmos for pan, tilt, zoom, roll and orbit. Where this exists, use it in preference to describing the move in words, for the obvious reason: a number is unambiguous and a sentence is not.\n\nIt also lets you do the thing language cannot, which is ask for a camera move of a specific magnitude. \"Slow push in\" is a genre. \"Dolly in 15%\" is an instruction.\n\n## Choosing between the three\n\nA decision rule:\n\n- **Whole-frame ambient motion**: slider, set low.\n- **One region moves, the rest holds**: brush.\n- **A specific object travels a specific path**: trajectory, and expect to fix the result in the edit.\n- **The camera moves**: camera controls if they exist, otherwise language.\n\n## The free path\n\nMotion strength exists everywhere, including free tiers. Brushes and trajectories are mostly features of paid hosted tools, but the open-weights equivalent is real: ComfyUI workflows expose motion masks and trajectory conditioning as nodes, free, if you can run them. On a phone with only a free hosted allowance, the slider is your whole toolkit, which is fine, because turning it down is the highest-value thing on this page.\n\n---\n\nToday: generate the same still twice, once at your tool's default motion and once at roughly half of it. Judge which one you would show a client."},"check":{"question":"An operator paints a motion brush over a cup of steaming tea and leaves the rest of the café frame unpainted. The tea moves well, but the background still drifts slightly. What does this tell you about the control?","options":[{"text":"The brush leaked at its edges, and a tighter mask with a harder edge would have held the background completely still.","correct":false,"tempting":"Treats residual drift as a masking error; tightening the mask does not convert a bias into a freeze."},{"text":"The mask biases where motion is concentrated rather than freezing everything outside it, so some residual movement remains.","correct":true},{"text":"Background drift comes from the autoencoder rather than from motion conditioning, so no brush setting could affect it.","correct":false,"tempting":"Assigns drift to the wrong stage; the encoder is lossy but it is the generation that moves the background."},{"text":"The unpainted region received a motion strength of zero, so the drift must be interpolation artefacts introduced after generation rather than movement in the clip itself.","correct":false,"tempting":"Assumes the mask sets a hard zero and then invents a post-processing explanation for what is plainly generated motion."}]},"prev":{"slug":"blocking-and-screen-direction","title":"Where people stand, and which way they face","url":"https://addaly.com/learn/ai-video/blocking-and-screen-direction"},"next":{"slug":"conditioned-motion-controls","title":"Depth, pose and flow as instructions","url":"https://addaly.com/learn/ai-video/conditioned-motion-controls"},"authorship":"Founder and series editor: Dr Mustafa Mun. Written by AI models to a written standard. Exam keys checked blind by a second model: 3,665 of 3,666 agreed. Academic review invited.","review":{"status":"Draft, open for academic review"},"cite":"Addaly (2026) 'Taking motion out of language', in Video with AI. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/ai-video/motion-strength-and-brushes (Accessed: 13 October 2026).","licence":{"name":"CC BY-NC-SA 4.0","url":"https://creativecommons.org/licenses/by-nc-sa/4.0/"},"reader":{"schema":1,"title":"Taking motion out of language","courseTitle":"Video with AI","module":{"index":3,"slug":"directing-the-shot","title":"Directing, not describing"},"answer":null,"keyIdea":"Language is a poor way to specify a curve, so the controls that matter most are the ones that bypass it (a motion strength slider, a painted region, a drawn trajectory) and each of them fails in a way the prompt equivalent does not.","hero":{"kind":"plate","image":null,"alt":null,"caption":null},"blocks":[{"t":"h2","id":"why-these-controls-exist","runs":[{"text":"Why these controls exist"}]},{"t":"p","runs":[{"text":"Every interface in this category has, at some point, added a control that is not a text box. That is not product decoration. It is an admission that a sentence cannot specify how fast, how far, and in what curve."}]},{"t":"p","runs":[{"text":"The three you will meet:"}]},{"t":"ol","start":1,"items":[[{"text":"Motion strength","b":true},{"text":", a single slider."}],[{"text":"Motion brush","b":true},{"text":", where you paint the part of the frame that should move."}],[{"text":"Trajectory or drag control","b":true},{"text":", where you draw a path an object should follow."}]]},{"t":"h2","id":"motion-strength-and-why-the-good-default-is-low","runs":[{"text":"Motion strength, and why the good default is low"}]},{"t":"p","runs":[{"text":"A single number controlling how much the clip moves overall. Names vary: motion, dynamism, motion bucket, amount."}]},{"t":"p","runs":[{"text":"The relationship with quality is direct and mechanical. More motion means more change per frame, and every frame of change is another opportunity for drift, morphing limbs, background churn and identity slip. High motion does not just look busier; it is more likely to break."}]},{"t":"p","runs":[{"text":"The professional default is lower than feels exciting. A near-still frame with a slow drift, a small parallax and real ambient sound reads as footage. A high-motion clip with a morphing hand reads as generated, immediately, to everybody."}]},{"t":"p","runs":[{"text":"That is not timidity. It is choosing the version that survives being watched twice, which is the only test that matters for anything a client pays for."}]},{"t":"p","runs":[{"text":"A useful calibration: find the motion value at which your model starts producing visible artefacts on your kind of content, then work one or two steps below it. That number is yours and it is worth twenty minutes of a free allowance to find."}]},{"t":"h2","id":"motion-brush","runs":[{"text":"Motion brush"}]},{"t":"p","runs":[{"text":"Paint a mask over the region that should move, leave the rest alone. Runway popularised it and similar tools exist elsewhere."}]},{"t":"p","runs":[{"text":"What it is good for is specific and valuable: "},{"text":"motion that is local","b":true},{"text":". Steam rising from a cup while the room stays still. Hair moving while the face does not. Water in the bottom third. A flag. A single figure in a crowd."}]},{"t":"p","runs":[{"text":"What it does not do is make the unpainted region truly static. It biases rather than freezes, so expect some residual drift outside the mask, and check for it rather than assuming."}]},{"t":"p","runs":[{"text":"The technique that gets the most from it: paint less than you think. A small moving region against a still frame is more convincing than a large one, because the stillness is what makes the motion read as real. This is the same principle as a subtle grade: the restraint is where the credibility lives."}]},{"t":"h2","id":"trajectory-and-drag-controls","runs":[{"text":"Trajectory and drag controls"}]},{"t":"p","runs":[{"text":"Draw an arrow, or drag a point from where a thing is to where it should go. The tool converts that into a motion-conditioning signal."}]},{"t":"p","runs":[{"text":"This is the most direct control available and it is also the most brittle. Three honest limits:"}]},{"t":"ul","items":[[{"text":"It controls position, not pose.","b":true},{"text":" Dragging a person across frame moves them; it does not make them walk convincingly while doing so. You often get a sliding figure."}],[{"text":"It fights physics.","b":true},{"text":" A drawn path that no real object would take produces exactly the impossible-looking result you asked for."}],[{"text":"It competes with the prompt.","b":true},{"text":" If the text says one thing and the arrow says another, you get an averaged mess. Make them agree, or leave the motion out of the text entirely when you are drawing it."}]]},{"t":"p","runs":[{"text":"Use it for objects rather than people, and for simple translations rather than complex actions."}]},{"t":"h2","id":"camera-controls-as-a-separate-axis","runs":[{"text":"Camera controls as a separate axis"}]},{"t":"p","runs":[{"text":"Some tools separate camera motion from subject motion: sliders or gizmos for pan, tilt, zoom, roll and orbit. Where this exists, use it in preference to describing the move in words, for the obvious reason: a number is unambiguous and a sentence is not."}]},{"t":"p","runs":[{"text":"It also lets you do the thing language cannot, which is ask for a camera move of a specific magnitude. “Slow push in” is a genre. “Dolly in 15%” is an instruction."}]},{"t":"h2","id":"choosing-between-the-three","runs":[{"text":"Choosing between the three"}]},{"t":"p","runs":[{"text":"A decision rule:"}]},{"t":"ul","items":[[{"text":"Whole-frame ambient motion","b":true},{"text":": slider, set low."}],[{"text":"One region moves, the rest holds","b":true},{"text":": brush."}],[{"text":"A specific object travels a specific path","b":true},{"text":": trajectory, and expect to fix the result in the edit."}],[{"text":"The camera moves","b":true},{"text":": camera controls if they exist, otherwise language."}]]},{"t":"h2","id":"the-free-path","runs":[{"text":"The free path"}],"role":"free"},{"t":"p","runs":[{"text":"Motion strength exists everywhere, including free tiers. Brushes and trajectories are mostly features of paid hosted tools, but the open-weights equivalent is real: ComfyUI workflows expose motion masks and trajectory conditioning as nodes, free, if you can run them. On a phone with only a free hosted allowance, the slider is your whole toolkit, which is fine, because turning it down is the highest-value thing on this page."}]},{"t":"rule"},{"t":"p","runs":[{"text":"Today: generate the same still twice, once at your tool’s default motion and once at roughly half of it. Judge which one you would show a client."}]}],"check":{"question":[{"text":"An operator paints a motion brush over a cup of steaming tea and leaves the rest of the café frame unpainted. The tea moves well, but the background still drifts slightly. What does this tell you about the control?"}],"options":[{"text":[{"text":"The brush leaked at its edges, and a tighter mask with a harder edge would have held the background completely still."}],"correct":false,"tempting":[{"text":"Treats residual drift as a masking error; tightening the mask does not convert a bias into a freeze."}]},{"text":[{"text":"The mask biases where motion is concentrated rather than freezing everything outside it, so some residual movement remains."}],"correct":true,"tempting":null},{"text":[{"text":"Background drift comes from the autoencoder rather than from motion conditioning, so no brush setting could affect it."}],"correct":false,"tempting":[{"text":"Assigns drift to the wrong stage; the encoder is lossy but it is the generation that moves the background."}]},{"text":[{"text":"The unpainted region received a motion strength of zero, so the drift must be interpolation artefacts introduced after generation rather than movement in the clip itself."}],"correct":false,"tempting":[{"text":"Assumes the mask sets a hard zero and then invents a post-processing explanation for what is plainly generated motion."}]}],"order":[3,0,2,1],"why":null},"prev":{"slug":"blocking-and-screen-direction","title":"Where people stand, and which way they face","minutes":9},"next":{"slug":"conditioned-motion-controls","title":"Depth, pose and flow as instructions","minutes":10}}}