Why Skies Turn Into Bands Of Color

A gradual sunset sky is one of the hardest things to reproduce on a screen. Smooth gradients expose limitations that detailed scenes conceal completely.

Values are counted, not continuous

Digital pictures store a finite number of levels per channel. Eight bits gives a couple of hundred steps between black and white, which is coarse for a gentle ramp.

A gradient spread across a wide area may need more distinct values than are available. Adjacent regions then receive the same value and form visible bands.

Detailed scenes hide this because texture masks the steps. A clear sky has no texture to hide behind.

Vision is unusually good at finding edges

The visual system enhances contrast at boundaries, so a step that is mathematically small becomes perceptually obvious. Bands look sharper than their actual difference.

Sensitivity is highest in mid-tones and moderate brightness, which is where skies and studio backdrops sit. The worst case and the common case coincide.

This is why banding seems to appear only in certain images. The steps are everywhere and only visible where nothing masks them.

Compression discards small differences deliberately

Quantization rounds fine detail away, and the difference between adjacent levels of a gradient is exactly the kind of fine detail it targets.

Worse, compression operates on blocks, so rounding can align with block boundaries and produce banding with visibly straight, rectangular edges.

Increasing the bitrate helps but does not solve it, because the underlying value count is unchanged. The encoder can only preserve steps that exist.

Extra precision prevents the problem upstream

Capturing and processing in ten bits or more provides many times as many levels. The steps become smaller than the eye can resolve.

Precision matters most during processing, since color adjustments stretch parts of the range. Stretching an eight-bit gradient spreads its existing steps further apart and creates banding that was not visible before.

Working at high precision and reducing only at the final export keeps intermediate stages from manufacturing bands.

Controlled noise conceals the steps

Adding a small random variation before reducing precision breaks up the boundaries. Instead of a clean edge, the transition becomes a scattered mixture of two levels.

The eye averages that scatter and perceives a smooth ramp. The picture is technically less accurate and looks considerably better.

Natural grain does the same thing incidentally, which is one reason film-originated material rarely bands. Aggressive noise reduction removes that protection and can introduce banding into footage that had none.