Chroma Key: Green Screen to Transparent PNG

What it is for

Shooting in front of a green or blue screen makes cutting out a subject a matter of colour: everything that looks like the screen becomes transparent. This tool does that for one photo and returns an RGBA PNG plus the matte (white = kept, black = removed) as a separate grey image.

It is a colour-difference keyer: it measures how much more of the screen's dominant channel a pixel has than of the other two, compares that with the screen colour itself, and turns the ratio into transparency. Soft edges such as hair are then un-mixed — the screen colour is subtracted from them — and remaining green spill is limited. It is not an AI background remover: a photo without a coloured screen will not key well, and the tool warns you when the screen colour is weak.

Worked example

The first example uses a synthetic green-screen plate (192 × 128 pixels, generated by Lumina) with the screen colour on auto. The tool picked #00B140 from the image border and reported 70.42 % transparent pixels, 4.32 % soft edge and 25.26 % opaque. That matches the plate: a head-and-shoulders figure with anti-aliased edges on a flat screen.

The second example enters the colour by hand and cleans the matte: clip black 0.05, clip white 0.95, shrink 1 px and soften 0.8 px. The opaque share drops to 24.01 % because the shrink eats one pixel around the figure, and the soft edge grows to 5.49 % because of the blur.

Raising the screen gain to 1.3 on the first plate removes the screen more aggressively: 70.81 % transparent, 4.26 % soft, 24.93 % opaque. On a landscape photo without any screen, auto picked a grey-green #9CA791, keyed half of the picture into a soft edge (51.33 %) and the tool warned that the screen colour is not clearly green, blue or red.

How to read the result

• Screen colour is the colour that was keyed; with auto it is the median of the opaque border pixels.
• Transparent, soft edge and opaque add up to 100 %. A large soft-edge share on a studio shot usually means uneven lighting or a gain that is too low.
• The matte download shows exactly which pixels are kept; grey areas inside the subject are holes you can close with clip white.

Tips and common mistakes

• Light the screen evenly and keep the subject a metre or two away from it; shadows and green bounce light make the key harder.
• Raise clip black a little when the screen shows noise or creases, and lower clip white when the subject has see-through holes.
• Green clothing or green objects in front of a green screen will turn transparent. Use a blue screen for green subjects.
• Balance shifts the weight between the two other channels; move it when skin tones or blond hair key unevenly.
• Shrink by one or two pixels before softening to remove a thin outline of the screen.

FAQ

What happens to green spill on the subject?

Edge pixels are un-mixed so the screen colour no longer shines through, and the dominant screen channel is limited to the balanced mix of the other two. Set spill suppression to 0 to keep the original colours.

Does it work with blue screens?

Yes. The keyer uses the dominant channel of whatever screen colour you give or auto-picks, so blue and even red screens work the same way.

Can I put the result on a new background?

Download the PNG and place it anywhere, or use the lumina-compositor tool or Lumina's Compositor, whose green-screen preset keys and merges in one go.

How accurate is the transparency?

On a synthetic plate with known transparency, the recovered alpha differs by 0.0002 on average and at most about 0.016 — close to the 8-bit rounding limit. Real footage depends on lighting and compression.