One file, forty mockups, one change

A container, not a layer

Convert a layer to a smart object and the original pixels move inside a container. What you see on the canvas is a rendering of those contents at the current transform. The contents themselves are untouched.

That gives you three things an ordinary layer cannot do.

Non-destructive transforms. Scale a smart object to 20 per cent, work for an hour, scale it back to 100 per cent, and it is sharp. A normal layer scaled down has thrown away the pixels; scaling back up interpolates from what survived. The smart object re-renders from the stored original each time.

Smart filters. Any filter applied to a smart object becomes a re-editable entry beneath it, with its own mask and its own blend mode. Double-click the filter’s name a week later and the dialogue opens with your settings. In GIMP and most free editors, a filter is applied and gone.

Contents you can replace. Right-click and Replace Contents, choose a different file, and everything (the transform, the smart filters, the mask, the warp) is re-applied to the new image.

The mockup workflow this enables

This is the practical payoff and it is worth doing once to see it.

You have a t-shirt photograph. You want to show a client’s design on it. Put a flat rectangle where the print should sit, convert it to a smart object, warp it to follow the fabric’s folds, set the layer to Multiply so the shirt’s shading shows through, and mask it to the shirt’s shape.

Now double-click the smart object’s thumbnail. It opens as its own document. Paste the design in, save, and close. The mockup updates with the design warped, shaded and masked exactly as you set it up.

Forty designs become forty double-clicks rather than forty warps. Every downloadable mockup PSD you have ever seen works this way, and building your own takes twenty minutes.

Embedded against linked

A smart object can hold a copy of the source file inside the document, or a link to a file on disk.

Embedded is self-contained. Send the PSD and everything travels with it. The file is large, and two documents using the same logo hold two independent copies that will drift apart.

Linked (Layer > Smart Objects > Convert to Linked) stores a path. Update the logo on disk and every document using it updates. The file is small. And the link breaks the moment somebody moves the folder, renames the file, or sends you the PSD without the assets. Photoshop shows a warning badge and asks you to relocate it.

The rule that avoids grief: linked for assets you control and reuse (a logo, a price panel, a product cut-out used across a campaign) kept in a folder beside the document; embedded for anything you are sending outside your machine. File > Package collects a document and its linked assets into one folder, which is what you send.

What the free tools have

Photopea supports smart objects, including opening the contents, editing and updating, and it reads Photoshop’s smart objects in a PSD. Smart filters are partially supported: some open as re-editable, others are baked. For mockups it works.

Krita has file layers: a layer that references an external image file and refreshes when that file changes on disk. That is exactly linked-smart-object behaviour, and Krita’s version is arguably cleaner. It does not do smart filters.

Affinity Photo has embedded documents and live filter layers, which cover the same ground under different names.

GIMP has no equivalent. This is the most significant functional gap in GIMP for this kind of work, and it is honest to say so: scaling a layer down and back up in GIMP loses detail, and a filter applied is a filter baked. The workaround is to keep a duplicate of the original layer, hidden, at full size, and redo rather than re-edit.

The costs

Size. Every embedded smart object is a full copy of its source. A document with eight embedded 40-megapixel photographs is several gigabytes before you have made an edit.

Speed. The renderer re-renders the contents through the transform and every smart filter whenever anything changes. A smart object with three heavy filters on a large image makes every brushstroke lag.

Copies do not share. Duplicating a smart object with Ctrl/Cmd+J gives you a linked pair: edit one, both change. New Smart Object via Copy gives you an independent one. Two people have burned an afternoon on that distinction; know which you made.

Portability. A PSD full of smart objects and smart filters opens correctly in Photoshop and Photopea, approximately in Affinity, and poorly in GIMP. If the recipient’s tool is unknown, ship a flattened TIFF alongside.

When to rasterise deliberately

Smart objects are not free and not always right. Rasterise when the layer is finished, when you need a pixel-level tool on it (the clone stamp will not paint on a smart object without rasterising) and when the file has become slow enough that the re-editability is costing more than it is worth.

The discipline is the same as everywhere else in this block: keep the version that still has the smart object, then rasterise in a copy.