The four subjects that are not opaque

Different problems that look like one problem

Hair, fur, glass and smoke all resist the ordinary cut-out, and for three different reasons. Naming the reason tells you the method.

• Hair and fur are opaque, and thin: each strand covers part of a pixel. The problem is fine partial coverage.
• Glass is transparent and refracting: it shows the background through it, distorted, plus reflections on top.
• Smoke, steam, flame and water spray are additive or subtractive light, not coverage at all.

Hair, in three passes

The method that works on a real portrait, and it is deliberately not one operation.

Pass 1: the body. Quick Selection or a segmentation model for the head and shoulders, then Quick Mask and a hard brush to fix the solid outline. Ignore the hair completely.

Pass 2: the wispy zone. Select and Mask, and scrub the Refine Edge brush only along the transition band. Turn on Decontaminate Colours. Output to a new layer with a mask. Do not touch the smooth parts of the outline; the refine brush makes clean edges worse.

Pass 3: the strands that are still missing. Some fly-away hairs will not survive any method. The professional answer is to paint them, with a small hard brush at one to two pixels, sampling the hair’s colour, on a layer under the subject, following the direction the real strands went. Five or six strokes. Photoshop and Krita both have brush presets that scatter multiple hair-like strokes at once; Krita’s are better and free.

Where the background behind the hair is busy, the alternative is not to cut out at all: keep a soft halo of the original background and blur it into the new one. Against a dark, plain new background this is invisible and takes two minutes.

Fur: the same, with more of it

Fur differs in that there is far more transition zone: an animal is mostly edge. Two things help:

• Build the mask from the channel with the best separation, not from a selection tool. A ginger cat against a green lawn separates beautifully in the red channel.
• Accept a slightly soft outline. Fur is soft, and a crisp one is what looks wrong.

Glass, and why a mask is the wrong tool

A wine glass, a bottle, a window, a pair of spectacles. The correct answer is almost never a cut-out.

The blend-mode method, which is what product retouchers actually use:

1. Shoot the glass against white and again against black if you can. Two frames, same position, tripod.
2. The black frame holds the reflections and highlights: everything the glass adds. Put it on top and set it to Screen, which makes its black areas invisible.
3. The white frame holds the shadows and absorption: everything the glass removes. Put it below and set it to Multiply, which makes its white areas invisible.
4. The new background goes underneath both. The glass now transmits it: the multiply layer darkens it where the glass is thick, the screen layer adds the highlights on top.

This is the technique, it is forty years old, and it produces results no mask achieves, because it reproduces what glass physically does to light rather than pretending glass is a shape.

With only one frame, a cut-down version works: mask the glass’s silhouette, set the glass layer to a blend mode that lets the new background through, then paint the specular highlights back on a Screen layer above. It is a compromise and it is usually good enough for a listing.

Smoke, steam and flame

These are light sources, not objects. Shot against black, smoke is pure addition: the black background contributes nothing and the smoke contributes everything.

So: put the smoke plate over your scene, set it to Screen, and the black vanishes with no mask at all. Adjust with a Levels layer on the smoke to control how much shows. For dark smoke against a bright sky, shoot it against white and use Multiply instead.

This is why every stock library sells smoke, fire, dust, rain, light leaks and bokeh overlays photographed against black. They are designed to be screened, and a mask would destroy exactly the soft variable density that makes them look real.

Free plates: Pexels and Pixabay carry usable smoke and bokeh footage stills under permissive licences. Check the specific licence for commercial use rather than assuming.

Water, net, lace and mesh

A hybrid case. Netting, lace, mosquito mesh and chain-link are geometrically regular and partially transparent. The route that works is to build a mask from the channel with the strongest contrast, push it hard with Levels to get the pattern clean, and then reduce its opacity to reinstate the partial transparency rather than trying to capture the partial values directly. The regularity that makes them hard to select is also what makes a pushed mask acceptable.

The summary

There is no single method for non-opaque subjects, and the tools that claim there is are selling the easy eighty per cent.

The decision, each time:

• Can I shoot this again, better? Almost always the cheapest fix.
• Is this coverage (hair, fur): then matting and decontamination.
• Is this transmission (glass, water): then blend modes and, ideally, two frames.
• Is this emission (smoke, flame, flare): then Screen, with no mask.
• Is none of it working: then say so before the deadline rather than after.

And the limitation nobody can engineer round: a single photograph of a transparent object against a complex background does not contain enough information to separate the object from what is behind it. The information was destroyed at capture, when three numbers per pixel were recorded for something that needed six. Every technique in this lesson is a way of supplying the missing information from somewhere else: a second frame, a known background, a physical model, or your hand.