How PSD layer writing actually works

A PSD is four sections and a trap

The format is a 1990s-vintage container: a header, color/resource blocks, the LAYER AND MASK section, and a flattened composite image at the end. The trap is that last part: a valid PSD only NEEDS the flattened composite. Write your JPEG there, leave the layer section empty, and every viewer opens it happily as "Background". That is the one-layer fake, and it is why a .psd extension proves nothing.

What real layers require

Each layer is a record: bounds, blend mode, opacity, a Unicode name, and per-channel pixel data (compressed, usually RLE) — plus, optionally, a mask channel with its own bounds. Groups are bracket markers — hidden divider layers that viewers reconstruct into a tree. Getting Photoshop, Photopea, Affinity, GIMP, AND Krita to agree on your bytes means testing against all of their readers, because each tolerates different corners of Adobe's spec.

Masks: the difference between editable and baked

A cutout can be stored two ways: bake the transparency into the alpha channel (destructive: the hidden pixels are gone), or attach a MASK channel alongside untouched pixels (non-destructive — paint the mask white and the pixels come back). We write masks. It costs an extra channel per layer and is the single clearest tell of a converter that cares.

Writing it in a browser

The writer (built on the open-source ag-psd) assembles the byte stream client-side: your pixels never leave the machine to become a PSD. Text layers are the deep end. A type layer is not a raster layer with words on it — it carries Adobe's type-engine data structures, so the string, size and colour live in the file as text and Photoshop can put a cursor in them. Writing that correctly is a different job from writing a raster layer, and it is the reason our OCR-recovered text comes back as genuine editable type objects. Whether any given competitor's "text layers" are real type objects or rasterised regions is not something we can tell you from the outside — but you can tell in one click: open their output, click the text layer, and see whether a cursor appears.

Verify anyone's output — including ours

Open any converter's file in Photopea and count layers; then try toggling a mask. Or go full nerd: a real layered PSD's Layer section is typically the majority of the file size, while a fake is one composite image plus headers. Our companion piece (why most converters are fake) has the 30-second version of this test.