Smooth Scrim Gradient

Smooth scrim gradient

A scrim is the block of gradient that sits between artwork and the page below it, so the image appears to dissolve into the background. Written the obvious way — Brush.verticalGradient(listOf(Color.Transparent, background)) — it produces two artefacts at once: a visible line where the fade begins, and a dirty band through the middle. Both are fixable, and neither is fixed by adding more alpha.

// adapted (the colour lerp is imported unqualified here)
fun smoothScrimBrush(
    from: Color,
    to: Color,
    startFraction: Float = 0f,
    endFraction: Float = 1f,
    startY: Float = 0f,
    endY: Float = Float.POSITIVE_INFINITY,
    steps: Int = 24,
): Brush =
    Brush.verticalGradient(
        colorStops =
            Array(steps + 1) { i ->
                val t = i / steps.toFloat()
                val position = startFraction + (endFraction - startFraction) * t
                position to lerp(from, to, t * t * (3f - 2f * t))
            },
        startY = startY,
        endY = endY,
    )

// the common case: ramp one colour from invisible to opaque
fun artworkScrimBrush(color: Color, steps: Int = 24): Brush =
    smoothScrimBrush(from = color.copy(alpha = 0f), to = color, steps = steps)

Traps

A linear ramp has a corner, and the corner is the edge you keep seeing. The eye is far more sensitive to a change in the rate of brightness than to brightness itself, so the discontinuity where a straight ramp leaves 0 (and where it arrives at 1) reads as a drawn line. Smoothstep, t² (3 − 2t), has zero slope at both t = 0 and t = 1, so there is no corner at either end. Ease both ends: easing only the start moves the seam to the bottom of the scrim instead of removing it.

Color.Transparent is black at alpha 0, so a coloured fade drags through black. Take a scrim that should fade a blue-grey page colour to nothing. Interpolating that colour against Color.Transparent walks the RGB channels toward (0, 0, 0) at the same time as the alpha, so at the halfway stop you get half-brightness colour at half alpha — which composites darker and less saturated than the correct half-alpha stop, and reads as a muddy band across the middle of the fade. Pass yourColor.copy(alpha = 0f) as the transparent end and the RGB never moves. (The originating code's comment records the underlying reason as the renderer interpolating gradient stops un-premultiplied; you do not need that to be true for the rule to hold — matching the RGB at both ends makes the ramp correct under either interpolation.)

Interpolate the colours yourself and hand over many stops. Two stops delegate the whole curve to the renderer: you get its interpolation, in its colour space, with its premultiplication choice, and those differ between backends. Emitting pre-interpolated stops fixes the stop values themselves and leaves the renderer only the short segments between them — and it is the only way to put a non-linear curve into a brush that has no easing parameter.

Two stops cannot express a curve. Even with smoothstep in hand, a gradient with a stop at each end is a straight line again; the curve only exists in the stops you emit. Enough stops turn the S-curve into a piecewise-linear approximation whose kinks fall below the visible threshold.

Choose the step count by looking, not by copying a number. Too few stops and the joins between segments show as faint bands; too many is wasted work in the shader. Render the scrim over the darkest background you ship (dark backgrounds are where 8-bit quantisation bites, because the colour delta per output level is smallest there), halve the count until banding appears, then go back one. The steps = 24 in the sample above is the value that survived that test where this was mined — a starting point, not a constant: yours depends on your ramp length and colour delta.

Confine the ramp with fractions instead of shrinking the box. A short scrim is a steep scrim, and steep is exactly what makes a fade read as an edge — so when a fade suddenly looks like a line, check whether someone capped its height. startFraction/endFraction (or startY/endY when you know the pixels) keep the box tall while confining where the ramp happens.

Tile mode matters on exactly one of the three routes. Brush.verticalGradient takes (colorStops, startY, endY, tileMode), and it is tempting to credit the default clamp with holding the flat regions above and below the ramp. Usually it is not doing that. On the default endY = Float.POSITIVE_INFINITY route the shader substitutes the box's own height for the infinite coordinate, so the ramp spans exactly the box, the gradient parameter never leaves [0, 1], and tile mode is never consulted. On the startFraction/endFraction route the ramp is confined by moving the stop positions inward, so the flat ends are held by the first and last stop — still not by the tile mode. Only an explicit startY/endY pair inside the box leaves anything outside the ramp for the tile mode to answer for; switching it to repeat or mirror changes that route and only that route.

Fade to the colour that is actually behind the scrim. The bottom of a scrim must equal the page background exactly, or you have moved the seam rather than removed it. When that background is derived from the artwork, the two must come from the same source — see the sibling skill artwork-palette-theming, and its fallback for items that have no artwork at all, deterministic-title-placeholder-painter.

Verifying it

Run these from the repository root. They are cheap and read-only.

1. No transparent stop inside a gradient call. Any hit is either a comment or a bug — read each one:

   grep -rn -A4 --include='*.kt' -E "verticalGradient\(|horizontalGradient\(|linearGradient\(" . \
     | grep -v '/build/' | grep "Color.Transparent"
   

2. Every gradient brush in the codebase, so you can check each is either eased or deliberately linear (a two-colour decorative gradient across a small tile does not need a curve; a scrim does):

   grep -rln --include='*.kt' -E "verticalGradient\(|horizontalGradient\(" . | grep -v '/build/'
   

3. Visually: put the scrim over a flat mid-tone fill instead of artwork. Artwork hides banding and hides seams; a flat fill shows both immediately. Then repeat on your darkest background.