A Shadow Is a Picture of the Surface It Lands On
Add a shadow that bends over the surfaces it falls across, instead of one pasted on as a flat copy of the silhouette. Project a shape onto a flat wall and you get the same shape back. Project it across a step, a fold of cloth, a curved bowl or a table edge and you get something else entirely — it stretches, kinks, breaks and climbs, and every one of those distortions is information about the surface rather than about the object.
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What it does
- Shadow bent by the surfaces it falls across
- Darkest and sharpest at the contact point
- Direction and softness matched to the existing light
- Moderate denoise, so the subject is regenerated
How to use Add AI Shadows
- 1
Use it where there is real geometry
A table edge, a step, a rug, a fold of fabric. If the surface behind and beneath your subject is a plain flat sweep, the deterministic shadow page does the job without regenerating anything.
- 2
Run the pass
A cast shadow is added that follows the shape of the surfaces, darkest at contact and lighter further out, matched to the light already in the photograph.
- 3
Check the contact point before anything else
Where the subject meets the surface, the shadow should be tight and dark. That single region decides whether the object is standing on something or hovering above it.
How it works
The photograph is scaled to about one and a fifth megapixels and repainted at a moderate denoise — enough freedom to add a shadow that interacts with the scene, held low enough that the rest of the frame mostly survives.
The instruction describes the shadow physically rather than stylistically: it bends where the surface bends, breaks at a step or an edge, stretches across a fold, and is darkest and sharpest where the subject touches the surface, becoming lighter and softer further away. Its direction and softness are to match the light already in the photograph.
The negative prompt names the alternative directly — a flat offset shadow, a uniform blur, a shadow floating away from the subject, a hard shadow in soft light — because the failure here is a shadow that is present and wrong rather than absent.
Shadows are shaped by the receiver
The intuition to correct is that a shadow is a property of the object. It is not. It is the object's silhouette, projected, and then deformed entirely by whatever it lands on.
On a flat wall, the projection is a scaled copy, which is why the flat-world approximation survives for so long in catalog images.
Across a step, the shadow breaks. Part of it lands on the tread and part on the riser, and the two halves are displaced from each other by an amount that describes the height of the step.
Over a fold, it ripples, and the ripples run with the fabric rather than with the silhouette.
Around a curve, it compresses towards the edge, because the surface is turning away from the light.
None of that is derivable from the shape of the object. All of it is a measurement of the surface. Which is exactly why a shadow that behaves correctly is such a strong cue that a composite is real, and why a shadow that ignores the surface is such a strong cue that it is not.
Contact first, everything else second
If you check one thing after each run, check where the object meets the ground.
The contact region is small, and it is the highest-information part of the whole shadow. It is very dark, because that narrow gap can see almost no part of the light source. It is tight, with a hard edge, because there is no distance for the penumbra to spread. And it is exactly where the eye looks to answer the question of whether the object is really there.
Everything further from contact is more forgiving. The shadow lightens, spreads, and blurs, and a viewer's tolerance for it being approximately right is high, because real shadows out there are approximate too.
So the review order is contact, then direction against the other shadows in the frame, then softness against the light source, then everything else.
The price, stated plainly
This page buys shadow behavior with subject fidelity, and that is a genuine cost rather than a technicality.
At this denoise the subject is being repainted. A label may shift slightly, a texture may resolve differently, a fine edge may move. On a photograph of a room or a scene, nobody will ever know. On a product image that a buyer is going to order from, it matters, and the deterministic page is the right answer instead.
That page keeps every pixel of your subject and gives you a shadow that knows nothing about the surface. This one gives you a shadow that reads correctly in a real scene and hands back a subject that is no longer exactly what you photographed.
Neither is the better tool. They are two ends of the same trade, and knowing which end your image needs is the only decision here.
Publication gate
This page ships once the workflow has been run against a subject on a table with a visible edge, a subject on folded fabric, a subject at the bottom of a step, and a plain cut-out with no surface at all, with the contact region and the shadow direction compared against every other shadow in the frame in each case.
Examples
Object on a table with a visible edge
The intended case. The shadow runs across the tabletop and breaks at the edge, which is the behavior no offset shadow can produce.
Subject cut out on plain white
The wrong page. There is no geometry to bend around and no light to match, so the pass regenerates your subject in exchange for a shadow the deterministic page would have given you exactly.
Frequently asked questions
Why is the subject regenerated at all?
Because the shadow is produced by repainting the frame rather than by compositing a shape onto it. There is no setting that repaints the surroundings and freezes the subject inside the same pass, which is why the other shadow page exists.
What is a contact shadow?
The small, dark, tight region right where two surfaces meet, caused by that gap receiving light from almost no direction. It is the strongest single cue that an object is resting on something, and its absence is what makes composites look like they are hovering.
How do I tell if the shadow direction is right?
Find another shadow in the photograph and compare. Every shadow in a scene lit by one source points consistently, and for a nearby lamp they converge on it. A new shadow that disagrees with an existing one marks the image as edited, even to people who cannot say why.
Can it shadow onto a background that is not visible?
No. It can only cast onto surfaces present in the photograph. If your subject was shot against a plain backdrop, there is nothing for a shadow to describe, and the tool has nothing to work with beyond inventing a floor.
Is the Add AI Shadows tool free, and do I need an account?
Free, and deliberately account-free. Prathom has no login anywhere on the site, so there is no usage counter attached to you and no upgrade prompt waiting at the end of the job. The practical limits are the workflow's own: an image up to 25 MB going in, one queued GPU job at a time, and a result that stays available for thirty minutes.