PSD / PSB Inspector

Because no browser has a PSD decoder, and drawing a layer would mean far more than decompressing it. Each channel is stored separately, compressed with either PackBits or zip, and a layer only looks the way Photoshop draws it once its opacity, its blend mode, its mask, its clipping relationship to the layer below and its position in the group stack are all applied. Doing that faithfully is a rendering engine, not a parser. What this page does instead is report the structure exactly, and show the one picture the file already contains: the thumbnail image resource, which is a complete JPEG that Photoshop wrote when it last saved the document.

Because the file stores layers bottom first, while the panel lists them top first. This is not merely cosmetic: layer groups are encoded as a stack of section-divider markers that only make sense in one direction. A folder record sits above its contents in the panel, which means after them in the file, and a bounding divider sits below the contents, which means before them. Reconstructing the folder tree therefore means walking the list from the top down, opening a group at each folder record and closing it at each bounding divider. That is what the indentation in the layer table comes from, and it is why a reader that treats the layer list as a flat sequence loses the structure entirely.

Both are in the file. Every layer record ends with a Pascal string holding the name in MacRoman, padded to a multiple of four bytes — that is the original 1990s field, and it cannot represent most of the world’s writing systems. Modern Photoshop still writes it, but the name it actually shows lives in an additional-information block keyed "luni", as a character count followed by UTF-16. A reader that stops at the Pascal string reports a mangled or truncated name and looks confidently correct while doing it. This page prefers the Unicode name and prints the MacRoman one underneath whenever the two disagree, so you can see which reader produced which.

No — it is a documented signal. The count is a signed 16-bit value, and a negative one means the first alpha channel of the merged image holds the transparency for that composite rather than being an ordinary extra channel. Photoshop writes it when the flattened image has real transparency to preserve. A reader that takes the absolute value gets the right number of layers, which is why the mistake usually goes unnoticed, but one that treats the value as unsigned reads a count of 65 534 and then tries to parse tens of thousands of layer records out of a few kilobytes.

One number and its consequences. The version word in the header is 1 for PSD and 2 for PSB, the large document format. A PSB may be up to 300 000 pixels a side instead of 30 000, and to address that much data three lengths widen from four bytes to eight: the layer and mask section length, the layer info length, and every per-channel length inside a layer record. Nothing else about the layout moves. That is precisely what makes it dangerous — a reader that ignores the version word does not fail cleanly, it reads a plausible-looking but wrong length and then walks off into the middle of the pixel data.

Almost certainly because it was saved without a flattened composite. The fifth section of the file is a single merged picture that Photoshop writes for the benefit of everything that cannot read layers: operating-system thumbnails, other applications placing the file, most libraries. It is optional, and turning off "maximize compatibility" leaves it out. Photoshop itself does not care, because it rebuilds the image from the layers, so the file looks perfect to the person who made it and empty to everyone else. This page reports whether the composite is present, how many bytes it holds and how many an uncompressed one would need, so the diagnosis takes seconds instead of an afternoon.