Two exposures, one frame
On film, a double exposure is two shots on the same frame: wherever the first one left the film dark, the second has room to show. The look people search for now — a profile filled with a forest or a skyline — comes from that, and it depends on the same thing: a subject that is dark against a light background, and a second picture that fills the dark. This page reproduces it with the arithmetic rather than a filter: a blend mode decides how the two pictures combine, and an optional mask decides where.
What each control does to the pixels
- Blend mode. Seven modes from the W3C Compositing and Blending formulas. Screen is the classic; multiply darkens; overlay and soft light add contrast; difference shows only what differs.
- Where the texture goes. Everywhere, or masked by the portrait’s own brightness — the dark parts or the bright parts — with a soft edge measured from a blurred copy, so the boundary fades.
- Texture position. Covers the portrait to begin with, then moves, scales, turns and mirrors.
- Colour. The portrait can be turned black and white first, and the result toned toward a colour — black stays black, mid-tones take the colour, white stays white.
A worked example
A 3000 × 4000 profile against a white wall and a 6000 × 4000 forest. The forest is scaled to cover the portrait — ×1.0, because it is already as tall — and centred, so its middle 3000 columns sit behind the face. With Classic (screen, portrait in black and white), the wall stays white because screen of white and anything is white; the dark hair and jacket become forest; the mid-grey skin shows the forest faintly. Moving the texture 12 % across brings the tree line onto the forehead. The file is saved at 3000 × 4000.
Privacy, specifically
Portraits are personal. Both pictures are blended inside this tab, and the result is drawn fresh, so camera details and location stay behind. Nothing is sent or kept.
The mask comes from light and dark, not from recognising a person, so a subject must be darker (or lighter) than its background for the masked looks to work. Separating a person from a busy background is not done here.
Snags with double exposure photos
What kind of portrait works best?
A clear outline against a plain, light background — a profile against a white wall or a bright sky is the classic. Screen keeps the light background white and lets the texture show only where the portrait is dark, so a busy background fills with texture too and the outline is lost.
The texture covers the whole picture. How do I keep it inside the person?
Use the Silhouette look, or set Where the texture goes to Dark parts. The mask is measured from the portrait's own brightness: dark areas take the texture, light areas stay as they are, and Soft edge fades the boundary. It works from light and dark, not from recognising a person, so it needs a background lighter than the subject.
What is the difference between screen and multiply?
Screen only ever lightens: wherever either picture is white the result is white, and where one is black the other shows through. Multiply only ever darkens: white lets the other picture through, black stays black. Screen suits a dark subject on a light background; multiply suits a light subject on a dark one.
How do I get the forest to sit on the face rather than the shoulder?
Move the texture with Across and Down, enlarge it with Size, and turn it with Turn. Mirroring flips it left to right. The portrait never moves; only the texture does.
Why does it look the same in every browser?
The blend is worked out pixel by pixel in this page's own code, using the formulas from the W3C Compositing and Blending specification, rather than handed to each browser's drawing engine. The same two pictures and settings give the same file anywhere.
