Color Separation Overlay

When you calibrate an alternative process — cyanotype, Aquaprint, gum print, color charcoal — you need to test with varied images to adjust your correction curve. Before printing these images on expensive paper, you'd like to see them as they'll print: layer by layer, ink by ink. That's exactly what Color Separation Overlay offers. You load a photo, you see its CMYK decomposition (or CMYK plus extended inks), you adjust the opacity of each plate, and you preview the final rendering before shooting in a camera or exposing your print.

It's a decision tool. The negative generator gives you the recipe — the ink densities to print. Color Separation Overlay shows you the visual result: the image in the paper. Between the two, you decide whether your settings are sound or whether you need to restart testing.

#What you see on screen

You access this tool from the Scan page, in the step "Color Separation Analysis". You import a photo — local file, drag-and-drop, or thumbnail from your library. Calibration Flow automatically downscales it to 1500 pixels max (to stay fluid), then launches the CMYK separation and displays the simulated composite.

Five elements appear on screen:

1. The composite preview: your image as it will print, overlay of inks with their current opacities.
2. The ink pills: Cyan, Magenta, Yellow, Black (always present); Orange, Green, Violet in Full Gamut mode. Each pill colors itself with its pigment hue.
3. The "Intensity" slider: controls the opacity of the selected pill, from 0% (invisible) to 100% (opaque).
4. The "Black presence" section: a GCR slider (0–100%) that controls the strength of black generation in the separation.
5. Action buttons: Isolate the plate, View original, Reset pills, Change image.

Your typical gesture: click a pill to select it, then move the Intensity slider. The rendering recalculates frame by frame — it's reactive. Click the same pill a second time to toggle it instantly between 0% and 100%.

Isolate a plate: click "Isolate the plate (negative)"; the screen switches to B&W and displays ONLY the density of that ink. Dark gray = high density (that ink will be thick there), white = no ink. That's a plate "engraving" — useful for verifying that shadow zones have enough black, or that highlights stay light.

The pedagogical eyedropper: hover or touch the composite preview — a loupe reveals the RGB of the pixel under your finger, its HSL, its Lab, AND the calculated density of each ink that composes it. The eyedropper reads the SOURCE image you imported (not the simulation), letting you understand how a source color broke down into CMYK or extended GAMUT.

Before/after comparison: long press (450 ms) on the preview = the SOURCE image displays in place of the composite. Release = back to simulation. It lets you swap before-after visually without clicking anywhere — useful for judging whether your opacity adjustment improves or worsens the rendering.

The GCR (Black presence): the "Black presence" slider controls the percentage of black generated (0–100%). At 0%, no pure black — CMY only. At 100%, maximum black in shadows and midtones. Each slider move relaunches the WASM separation (debounce 110 ms) — you'll feel a micro-latency of about a second, then the composite updates.

Full Gamut (5 to 7 inks): if you switch to Full Gamut mode (Cyan-Magenta-Yellow-Black + Orange and/or Green and/or Violet), two additional sliders appear:

• Vampire Radius (15–45°): the attack radius of extended inks on pure primaries. A wide radius = Orange/Green/Violet eat more of Cyan/Magenta/Yellow hues.
• Vampire Strength (0–100%): the intensity of that attack. Four predefined presets — "Faithful", "Studio", "Vivid", "Spectrum" — change these two parameters in one click to offer you coherent regimes.

These two parameters are shared with the Full Gamut negative generator (same localStorage). If you adjust Vampire Radius here, then go to the generator, you'll see the same setting — no desynchronization.

Finally:

• "View original": overlay the imported source image at 100% to compare side by side.
• "Reset pills": set all opacity sliders back to 100%.
• "Change image": unload the current image and prompt you to load a new one (resets everything).

#Why it matters

For anyone printing in alternative processes. CMYK separation (or extended GAMUT) isn't unique — two separation engines can yield slightly different results. You can generate two negatives with different parameters (low vs high GCR, different Vampire Strength) and you want to quickly see which will produce the best rendering before printing. Color Separation Overlay shows you the two simulated renderings side by side (thanks to "Change image"), not just abstract density numbers.

To adjust your correction curve. You don't correct a target — you correct your calibration images. Each image receives your CMYK curve. You must verify that the curve respects the intent of each image type: a portrait must keep skin tone subtlety, a landscape must keep sky contrast. Color Separation Overlay, hooked to the real image, shows you exactly what rendering your curve will produce — without waiting for the print.

To understand where information is lost. A source RGB pixel isn't necessarily well reproduced in CMYK — a highly saturated hue might become slightly dull, or a very dark shadow might lose detail. The pedagogical eyedropper reveals this loss: you see the source RGB (ideal color), you read its calculated CMYK decomposition, and you judge whether you accept this simplification or whether you need to adjust the separation (increase the GCR, change the Vampire Radius, etc.).

To save precious paper. If you're printing on expensive fine art paper, you don't want to waste several trials before finding the right parameters. Color Separation Overlay lets you test multiple variations in minutes on screen, then print once with the best result. It drastically reduces waste.

#When you don't need it

You're only printing a calibration target. This tool simulates a real photographic image. A target — 24 gray patches, or 4 primary pills — doesn't need simulation; you measure ink densities directly. Color Separation Overlay would show you a target purely decomposed into raw CMYK, not a "realistic" rendering — which doesn't help you calibrate. Skip straight to the negative generator for your targets.

You're seeking perfect pixel accuracy in Full Gamut. When you switch to Full Gamut (Orange/Green/Violet enabled), extended inks do NOT reconstruct the source image pixel-for-pixel — it's a reduced color space. If you load an image with very pure hues (bright red), the CMYK+O+G+V decomposition won't give you that bright red exactly; it'll be slightly degraded. That's a choice: accept the loss to gain printing finesse. If you want maximum accuracy, stick to CMYK (4 inks), which reconstructs pixel-true nearly all RGB images.

You want to verify software decisions, not perceptual rendering. Color Separation Overlay is a simulator — it shows what you WILL SEE, not what you WILL MEASURE. If you want exact densities layer by layer (for logging, for comparing with another app, for validating a chemical formula), use the negative export and read the CSV or PNG files of isolated plates. Color Separation Overlay is for the eye, not for numbers.

#To remember

#The test

Load a photographic image (portrait, landscape, still life — something with color) into Color Separation Overlay. The app immediately displays the CMYK composite. Click the Black pill and reduce its opacity to 0% with the Intensity slider — the image becomes lighter (less black = more brightness). Then turn it back up to 100% — the image regains its shadows. Click "Isolate the plate (negative)" and select Black again — you now see ONLY the black "engraving", in B&W: the photo's dark zones are dark gray, the light zones are white. Click "Black presence" and reduce the GCR to 0% — the separation relaunches and you'll see the composite without black (CMY only), duller. This full cycle shows you've changed something with each gesture, and the simulator reacts in near real-time.