Flattening then saving over the layered master, which destroys every mask and adjustment in one keystroke.
Editing and retouching
Flatten
Also called flatten image, merge layers or flattening. Here is what it means, when it changes what you export, and what people get wrong.
Editing and retouchingRuns on your deviceUpdated 10 August 2026
The short answer
Flattening merges every layer in a document down into one. Anything transparent is composited against a background colour, usually white, and the separate layers, masks and blend modes are gone. It is the step that turns an editable master into a finished, shareable picture.
Updated
The short version
Flatten at a glance
| Quick fact | Detail |
|---|---|
| Also called | Flatten image, merge down |
| Result | A single opaque layer |
| Transparency becomes | The matte colour, normally white |
| Reversible | Only until you save over the master |
| Required for | JPEG export and most print output |
Slide the table sideways to see every column.
What happens to transparency when you flatten?
It is filled in. Every transparent pixel takes the matte colour, and every partly transparent pixel is mixed with it in proportion to its opacity. A pixel that was 40 per cent opaque against white does not stay 40 per cent opaque, it becomes a solid pixel containing 40 per cent of the subject colour and 60 per cent white.
That mixing is permanent, and it is where a lot of later trouble starts. If you flatten a cut-out onto white and someone afterwards places it on a dark banner, the soft edge pixels carry that white with them and read as a halo. Converting a PNG to JPEG in any tool, Photomop's converter included, performs exactly this flatten, which is why the transparency does not come back.
When is flattening the right move?
Whenever the file is leaving your hands as a finished picture. That covers exporting JPEG for a marketplace listing, supplying artwork to a printer, uploading to a platform that expects an opaque image, and cutting file size for the web.
Some destinations require it outright. Amazon's product image requirements state a pure white main image, Walmart Marketplace expects pure white (RGB 255,255,255), and Etsy does not support transparent PNGs, so transparent areas appear black. In each case you flatten onto white deliberately rather than letting a converter choose for you.
The counterpart is to keep the layered master. Save the PSD or TIFF first, then flatten and export a copy, so the next revision starts from the working file rather than from a finished JPEG.
Can you undo a flatten?
Only while the document is still open, through undo or the history panel. Once the flattened version has been saved over the original, the layers are not recoverable: the file no longer contains them, and nothing in a flat image records which pixels came from which layer. The habit that prevents this is Save As rather than Save at the export step. If the layers are already gone, the only route back is to rebuild from the source photographs.
What people get wrong about flatten
Each one is a real failure mode, not a style preference.
Flattening a cut-out onto white and reusing that file on a dark background, where the baked-in white edge shows as a halo.
Letting a JPEG conversion pick the fill colour by default, which can composite a transparent logo onto black rather than white.
Questions people ask
Flatten, answered
The follow-up questions people search for once they have the definition.
What is the difference between flatten and merge layers?
Merging combines the layers you have selected and leaves the rest of the stack alone. Flattening combines everything into a single layer and discards transparency by compositing it against a background colour. Merge is a tidying step, flatten is an export step.
Does flattening reduce image quality?
The flatten itself does not change the visible pixels, it just renders the stack once. Quality loss comes from what usually follows, such as JPEG compression, and from losing the ability to re-edit an adjustment rather than reapplying it to already-changed pixels.
Why did my transparent background turn white after flattening?
Because flattening has to put something in the empty area, and white is the usual default. If you want a different result, add a filled colour layer at the bottom of the stack before flattening so you control exactly what the transparency composites against.
Same cluster
More from the glossary
Neighbouring entries, so every term in the set is one hop from every other.
Focal length
Focal length, written in millimetres, describes how much of a scene a lens takes in and how strongly it stacks near and far objects together.
Flat lay
A flat lay is a photograph taken straight down onto items arranged on a flat surface.
Gamma
Gamma is the non-linear relationship between the numbers stored in an image and the amount of light a display emits.
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