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Chroma-Key / Green-Screen Preview

See your key live, and measure the two things you cannot judge by eye.

Keyed previewKeyed in your browser — never uploaded

Camera not started

Key colour

Raise it until the background clears. If it starts eating into you before the background is gone, the backdrop is unevenly lit — no tolerance setting fixes that.

How this test works

Your camera feeds a live keyed preview: every pixel within a chosen distance of the key colour is made transparent, and you drag the tolerance until the background clears. Alongside the preview, two numbers are measured that are hard to judge by eye. Background evenness is the brightness variation across the keyed area, and it is checked first — an unevenly lit backdrop cannot be keyed at any tolerance, because raising it enough to catch the dark corner starts eating the subject. Spill is the share of kept pixels whose green channel sits clearly above both others: the fringe around hair and shoulders that separates a key that looks deliberate from one that looks cheap. Reporting spill first on an unevenly lit backdrop would send you adjusting your position when the fix is a second light, so the order of those two checks is deliberate.

What the results mean

Background removed
The share of the frame the key is currently taking out. Very low means the backdrop is not close enough to the key colour, or the tolerance is too low.
Spill onto the subject
Kept pixels carrying a cast from the backdrop. Above about 12% it is clearly visible as a coloured fringe. Spill is a lighting problem — distance from the backdrop fixes it, software cannot without eating into you.
Background evenness
Brightness variation across the keyed area. Above about 40 levels the backdrop is too uneven to key well at any tolerance, and no setting will rescue it.
Tolerance
How far a pixel may sit from the key colour and still be removed. The useful diagnostic is what happens as you raise it: if it starts removing parts of you before the background has cleared, the backdrop is the problem.

Common problems and fixes

The background will not clear without parts of you disappearing
That is uneven backdrop lighting, and it is the most common green screen problem. Two lights on the backdrop from either side, aimed to overlap, gives the flat surface a key needs. One light produces a hotspot in the middle and dark corners that no tolerance setting can reconcile.
There is a green fringe around your hair and shoulders
Move further from the backdrop — a metre or more. Light bouncing off it onto you is the cause, and distance is far more effective than any software spill suppressor, which works by desaturating the very edges you want to keep.
Creases in the backdrop show as dark lines
Iron or steam the fabric, or stretch it taut. Creases cast shadows, and shadows are exactly the uneven brightness that defeats a key. A slightly wrinkled backdrop lit very flatly still keys better than a smooth one lit badly.
Parts of your clothing vanish
Something you are wearing is too close to the key colour. Change it, or switch to the blue preset — this is why blue screens exist alongside green ones, and why presenters are told what not to wear.

Frequently asked questions

Why does my key look worse here than in my call software?

Because this preview uses a simple colour-distance key, while broadcast and call software use colour-difference keying and often a machine-learned matte. The preview is deliberately basic so that what you see reflects your lighting rather than an algorithm compensating for it. If it keys cleanly here, it will key cleanly anywhere.

Do I need a green screen at all?

For the background-blur effects in call software, no — those use a learned segmentation model and no backdrop. A physical screen is worth it when you need a hard, reliable edge: a replaced background that does not smear when you move, or anything that will be recorded and watched closely.

Green or blue?

Green is brighter to most sensors and needs less light, which is why it dominates. Blue is better when the subject is wearing green, when there is a lot of blonde hair, or under warm lighting that pushes everything towards green anyway. Both presets are here for that reason.

Why measure spill rather than just showing me the preview?

Because spill is the thing people cannot see until it is pointed out. Everyone notices whether the background disappeared; almost nobody spots a five per cent green cast along a shoulder on a small preview. Turning it into a number turns a vague sense that something looks off into a specific instruction about where to stand.

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