RetouchPi Opens CMYK Files in the Browser: TIFF, PSD, JPEG

The samples are not pictures found online; a script built them from known pixels: the CMYK one is eight patches (white, cyan, magenta, yellow, black, blue and two mixes), the RGB pair is the same device values with and without an embedded profile, and the 16-bit one is a greyscale ramp. Known pixels are the only way to compute an error; otherwise "looks right" is all you ever get. The open is counted done when the tab label really changes to the new file, not when a budget expires - the first probe fell into exactly that trap and reported two fake 183-second numbers.

A browser canvas has exactly one road, sRGB, so four CMYK channels must become three first. This version uses the plainest formula there is: R = (255-C) x (255-K) / 255, and the same shape for G and B. Plain does not mean wrong - it is the answer least likely to surprise anyone when no press profile is in play, and the measurement says it is exactly what runs: on the two roads we decode ourselves, PSD and TIFF, the exported eight patches are identical to the formula, patch by patch, dE 0.

The JPEG road is different: a CMYK JPEG (its APP14 colour-transform marker) goes to the platform decoder and then onto the same canvas. The two roads disagree at rounding scale - patch 8's B channel is off by 1/255, a maximum dE of 0.65. What that buys the reader is one claim we can make honestly: changing the shell does not make the colour jump. What we cannot claim is "matches your print shop", because their side runs their own profile and RIP.

This is the most useful new fact of the round, and the method is A/B: one set of AdobeRGB(1998) device values saved twice as PSD, with and without the profile, exported and compared. The results differ - the profiled file matches the reference implementation (PIL ImageCms, AdobeRGB to sRGB) patch for patch, dE 0, while reading the raw values as sRGB would be off by up to dE 18.8, the mixed patch (120,200,80) landing at (51,201,67). On the PSD road, in other words, an embedded matrix/TRC-class RGB profile really is applied.

The other two roads need the sentence turned around. CMYK profiles: a TIFF carrying a generic CMYK profile comes out byte-identical to the profile-less one - the profile is ignored entirely, because this version has no CMYK colour management, only the plain formula. RGB profiles in TIFF: the same AdobeRGB profile packed into a TIFF is not read at all; values pass through. So "carrying a profile means being managed" is false here. What is true is narrower: RGB profiles on the PSD road are honoured; CMYK profiles, and any profile in a TIFF, are not.

No CMYK working mode: there is no CMYK slider, no four-channel editing road. It plates nothing and there is no press-proofing step. Exports are sRGB, every one of them - PNG, JPEG, WebP, PSD, TIFF, PDF, SVG. 16-bit files open, but they narrow to 8 bits on the way in; a deep-bit pipeline is not in this version.

These sentences sit on the first screen rather than the last because they decide who this page is for. Someone holding a CMYK original who wants to look at it, crop it, check its dimensions before sending it to the press: this is enough. Someone who needs to produce press files: this is not that stop, and saying so up front saves both sides an afternoon.

• Drop your own CMYK TIFF or PSD into the editor and read the dimensions and pixel count off the status bar against the original file.
• Export a PNG and put it next to the screen preview your press gave you. If the gap is acceptable, this stop is enough; if it is not, ask the shop which profile they run.
• Open a profiled RGB PSD twice: once as-is, once with the profile stripped by any tool. Different colours mean the profile is read on that road; identical ones mean it is not.
• After opening a 16-bit file, export an 8-bit PNG and open it back next to the editor's view. This version narrows on the way in, so the two should look identical - a visible difference would be the bug worth reporting.

• No claim of matching press output: their side runs their profile and RIP, ours gives an sRGB preview.
• No CMYK export: all eleven export targets are sRGB or vector masks; not one writes CMYK channels.
• No claim that every browser behaves alike: decoding is a platform gift, and the HEIC family of differences is written up on the capability page.
• No offline claim: the editor is a web page and there is no editor without a network. Ordinary retouching stays on your machine; AI generation and AI edits are the one step that touches a server.