Add Stars

Recombine a starless image with a stars-only image — the inverse of removing stars. Choose a Screen or Add blend, dial the intensity, and apply the result to the starless view. Mask-aware, with a live preview.

1. Using the tool

Pick the two layers (from open views or files), choose how they combine, and watch the preview update live. Apply writes the blend onto the chosen Starless View.

Choosing the two layers

Starless View — the background layer, and the image the result is applied to. Pick an open view from the dropdown, or use Load from File.
Stars-Only View — the star layer to add back. Again, an open view or a file.
Refresh Views — rebuild the dropdowns after opening or closing images elsewhere in SASpro.

The two layers must be the same size. The dropdowns list every open view plus a “Load from File” entry; the file buttons set a layer directly from disk.

Blend controls

Control	What it does
Blend Type	Screen (non-linear, keeps bright stars from clipping) or Add (straight sum). See §2.1.
Blend Ratio (0–100)	How strongly the stars are mixed in: 0 leaves the starless image untouched, 100 is the full Screen/Add result.

Preview & apply

Zoom In / Out / Fit and the mouse wheel navigate the preview; scroll position is preserved as you adjust settings.
Apply commits a “Stars Added” step onto the Starless View's document (if the starless layer came from a file, it falls back to the active view); Cancel closes without changes.

2. Technical details

A per-pixel blend of the two layers, mixed by the ratio and confined to any active mask. Everything runs on float RGB in [0,1] (mono layers are promoted to RGB and collapsed back on output).

2.1 Screen vs Add

The two layers are first combined into a base result one of two ways:

\mathrm{Screen:}\quad \mathrm{base}=\mathrm{starless}+\mathrm{stars}-\mathrm{starless}\cdot\mathrm{stars}

\mathrm{Add:}\quad \mathrm{base}=\mathrm{starless}+\mathrm{stars}

Screen is the complement-product blend — it rolls off as values approach 1.0, so bright stars layer over the background without the hard clipping a straight sum produces. It's the exact inverse of the unscreen extraction in Remove Stars, so a round-trip (remove → add) is faithful. Add is the literal sum, useful when the stars layer is already scaled for it.

2.2 Ratio mix

The base is then mixed with the original starless image by the ratio r (slider / 100) and clipped:

\mathrm{out}=(1-r)\,\mathrm{starless}+r\,\mathrm{base}\,,\qquad r\in[0,1]

So the slider is a continuous fade from “no stars” (r = 0) to the full blend (r = 1), letting you tune exactly how prominent the stars are.

2.3 Masking

If the destination (Starless View) document has an active mask, the blend is confined to it — the starless background is kept everywhere the mask is zero:

\mathrm{out}=\mathrm{starless}\cdot(1-m)+\mathrm{out}\cdot m

2.4 Notes

Mono layers are treated as RGB internally; the committed result follows the source's mono/colour shape.
The result is applied to the selected Starless View (or the active view if the starless layer was loaded from a file), as an undoable “Stars Added” step.
This is the natural partner to Remove Stars: process the starless image however you like (stretch, denoise, sharpen), then add the untouched stars back on top.